WO2008023093A1

WO2008023093A1 - Method for coating a paper surface and a product thus obtained

Info

Publication number: WO2008023093A1
Application number: PCT/FI2007/000212
Authority: WO
Inventors: Isto Heiskanen; Kaj Backfolk
Original assignee: Stora Enso Oyj
Priority date: 2006-08-24
Filing date: 2007-08-24
Publication date: 2008-02-28
Also published as: FI20060755A; EP2061931A1; FI123458B; ES2443719T3; PL2061931T3; EP2061931B1; FI20060755A0

Abstract

The invention relates to electrically assisted coating of paper and cardboard products. More exactly, the invention relates to a method for transferring very small amounts of coating onto the product surface and especially to the use of very small amounts of solvent. A coated paper or cardboard thus obtained is also an object of the invention.

Description

Method for coating a paper surface and a product thus obtained

Field of the invention

The invention is part of the field of electrically assisted coating of paper and card- board products. More exactly, the invention relates to a method for transferring very small amounts of coating onto the surface of a product and to a product thus obtained.

Background of the invention

When using a coating agent dissolved in an abundant amount of solvent in the coat- ing of paper and cardboard products, smooth application can be achieved and the thickness of the coating layer can be adjusted from a thin layer of a few micrometers to thicker layers. In this case, a large amount of solvent penetrates into the fibre layer so that the component primarily intended for coating a surface is partly forced inside the fibre layer. The fibre layer is also penetrated by a solvent, which can also affect other fibres than those on the surface. It is more difficult to evaporate the solvent deeper from the fibre layer, which consumes energy and stresses the fibres. The solvent penetrating the surroundings and the interior of the fibre layer can also generate non-desirable phenomena such as reversible or irreversible swelling of the fibres, dissolution of the fibres, etc. On the other hand, also a non-swelling solvent can be used. In literature there have been presented results in which, when using a non-swelling solvent, improvements can be achieved in the surface properties of the paper with small amounts of chemicals. However, there is the drawback of poor sticking of the chemical to the web. With these methods, the coating is performed covering the whole width of the web.

Electrically assisted coating as such is known in the coating of processed paper products. Both dry and wet methods are used for transferring and adhering the coating to a desired surface. Because the product itself is typically non-conductive, in addition to the desired final properties of the coating agents and additives used, demands are also directed to their electrical properties. Thus, the complexity of coat- ing components known this far and the need to use polar organic solvents can be mentioned as drawbacks in the electrically assisted transfer.

When trying to reduce the use of a solvent, some advantages to be gained with the use of a large amount of solvent will be lost; for example, smooth application. It is known that the covering power can be improved in many applications by reducing the drop size. However, in a spray application, reducing the drop size causes the smallest drops to move with air currents. When increasing the running speeds, a small drop size cannot thus be easily used because of the big air currents. If a small drop size is used, a separate inverted vacuum funnel needs to be built, and even then the coating result will not be smooth enough, because it is not possible to make the air currents completely uniform. When the intention is to additionally prevent the spreading of "spray dust" to the environment by using an inverted funnel, the solution will fast grow to be unreasonably expensive.

On a laboratory scale, smooth application can be achieved when coating a stationary product, but the challenge is to achieve a sufficiently smooth application on a large scale and with high running speeds. The nozzle spacing has to be increased, which will lead to longer flying distances of the particles. This again will increase their sensitivity to disturbances in air currents.

Spray nozzles producing the small drop size are susceptible to blocking. In addition, it is problematic to achieve a small drop size with chemicals of high viscosity. Patents have been published relating to the control of spray mist (FI 111912B / DE10330801).

Because of the problems mentioned above, the spray coating unit will become com- plex and expensive.

In addition, because it has not been possible to solve the mentioned problems in a satisfactory way, it still is necessary to use, especially in the spray application, great amounts of liquid to achieve a smooth coating. It is usual that the amount of liquid solvent or carrier is multiple compared to the actual coating agent.

Thus there is a need for the development of a method, the embodiments of which will solve these problems and offer to the use a method for the application of various coating agents in an electrically assisted manner so that the coating agents will not be lost inside the structure, where they have no contribution to the properties of the product, coating agents will not get into to the environment or to discharge cur- rents of the coating unit, and that the fibres in the base layer will not be unnecessarily swelled by the excessive use of solvents. There is also a need to develop paper and cardboard products, in which the desired smoothness of the coating and the surface can be achieved with a small amount of coating. Summary of the invention

A paper or cardboard product, the coating of which has been performed by using electrically assisted transfer, is characterised in that the main part of the coating agent and/or its solvent or carrier will be retained on the surface, penetrating into the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating. This means that the main part of the coating will stay on the surface of the object to be coated, or that it will stop to the surface of the object and the coating. When the application is performed to the fibre layer, this means that the coating will be adsorbed and/or absorbed to the interface between the fibre layer and the coating, or to the essentially first fibre layer in relation to the nozzle.

The method of the invention for the electrically assisted coating of a paper or cardboard product is characterised in that the main part of the coating agent will be retained on the base layer at the depth of at most one fibre layer, essentially at the depth of at most 30 μm, more preferably at the depth of at most 20 μm, even more preferably at the depth of at most 10 μm, and most preferably at the depth of at most 5 μm in relation to the application surface of the coating.

Short description of the drawings

The paper or cardboard product and the method of the invention will next be explained in more detail by referring to the enclosed drawings, in which

Figure 1 illustrates a few fundamental possibilities for the use of an electrically assisted spray (alternatives A; charging of the web, and B; web between the spray and the electrode).

Figure 2 illustrates a principle view of a moistening equipment based on the electrical charging of the web.

Figure 3 illustrates the effect of electrically assisted spraying in the coating. In Figure 3 A, the electrically assisted system is not switched on. In Figure 3B₅ the system is in operation.

Figure 4 illustrates a sample surface, first as a colour picture and as a black-and- white enlargement of the same picture, in which, by adding optical clarification agent to the water, the stripedness of the application of spray water can be seen in an electrically assisted spray according to the reference example 4. Figure 5 illustrates a sample surface, in which the smoothness of the PVA coating agent and the optical clarification agent applied together with it by using the method of the invention according to example 4 can be seen in UV light

Figure 6 illustrates a cross-cut of the surface of the sample processed according to example 4, in which the retention of the optical clarification agent on the surface of the substrate (in the figure, the lower surface) added according to the method, can be clearly seen, according to the invention at most at the depth of 30 μm. The sample is a Trayforma 280 cardboard, the total thickness of which is approximately 350 μm, measured according to the ISO534 standard.

Figure 7 is a SEM view of the surface of the substrate coated according to example 4, which has been coated with PVA by an e-spin method. Bigger fibres are pulp fibres, and fine, cobweb-type spirals consist of the coating agent.

Figure 8 illustrates a cross-cut (8a) of the surface (Kp 4) to be achieved by an e-spin method, and the respective reference (8b). In Figure 8a it can be noticed how the coating remains exactly on the surface of the cardboard without swelling of the fibres. Swelled fibres are seen in Figure 8b, coated by a moistening method.

Figure 9 illustrates coating methods of different embodiments, realised with the technique of the invention. Shown here, spray can only mean the feed of a chemical.

Detailed description of the invention

Upon finishing paper it has now been observed that, when using swelling liquid, and when keeping the total amount of liquid sufficiently low, better results are achieved already with very small amounts of coating agent than if the total amount of liquid and the amount of chemical were bigger. The inventors have noticed that the amount of swelling liquid getting to the surface should not exceed 2 g/m² in the coating, preferably 1 g/m² at one coating time in order to achieve this advantageous effect. The swelling liquid is typically a solvent or carrier of the coating agent, usually water. The term "swelling" refers here to the swelling of all components in the paper, for example, filling agents, pigments, binding agents, etc., but especially to the swelling of fibres, as in Figure 8b. When using water there is the advantage that the bonding between the chemical and fibre (web) is sufficiently good.

When using essentially smaller amounts of coating (as, for example, in the traditional spray application) in non-contacting coating, a considerably smaller average particle size can be achieved. With a smaller particle size it is essentially easier to achieve better results in the controlling of the mist (among others, by electrically assisted control of particles) and in the uniformity of the application with large web speeds. Surprisingly enough, it has now been noticed that by reducing the amount of the coating it is possible to achieve a better smoothness of the coating, which is contrary to the traditional professional know-how. Without committing oneself to any theory, it is believed that this is at least partly due to the increase in the charging density (ion concentration) caused by drying, which occurs during the flight of an at least partly charged drop (or fibre). This also has the consequence that the charged drops will not collide with each other during the flight, so that the re- agglomeration to larger drops can be avoided.

The use of very small amounts of coating together with the traditional charging of the web makes it possible to charge the material to be sprayed to be of the opposite sign without problems, which would not be done as easily with larger amounts of material. When also the control improves, this makes possible a larger distance be- tween the web and the nozzle, which again is important for the smoothness of the coating on a larger scale, and for ensuring the runnability of the web.

The inventors have surprisingly noticed that the small drop size is especially significant in electrically assisted coating. A phenomenon related to it is, among others, the considerable increase in surface area obtained in connection with small particles to be sprayed, which again makes possible the evaporation during the flight, which has been noticed to be of advantage. Experimentally it was observed that at least one dimension of the flying drop or fibre is advantageously on the nano scale; in other words, the thickness of the drop or the diameter of the fibre when it adheres to the surface to be coated. Achieving a smaller drop size can further be improved by using surface-active agents and/or agents affecting the charging density or similar, intended especially for this purpose. Further, because achieving a small drop size is important, the temperature of the liquid can be raised, when needed, so that a smaller drop size can often be achieved. By diluting the chemical used it is possible to get a smaller drop size and, due to the small drop size, it is possible to make use of the evaporation of the diluting agent from the drop during the air current so that the amount of liquid getting to the web will, nevertheless, remain relatively low.

In the coating of paper, a very good covering power, smoothness and uniformity is required of the coating. When using the traditional spraying method, this can be achieved with large amounts of coating and by changing the drop size. Within the scope of the present invention, it has been tried to reduce the large loss of mist with the air currents caused by the small drop size by using electrostatic arrangements, which can be used both to form the drops and to guide the particles onto the surface.

According to the invention, it is thus possible to finish paper and/or cardboard in a uniform and controlled manner with a small amount of chemicals and with a large production speed, and thus to achieve essential advantages.

It is characteristic of a paper or cardboard product, in the coating of which electrically assisted transfer has been used, that the main part of the coating agent and/or its solvent or carrier will remain on the surface or penetrate in the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating. Preferably, no or quite a marginal amount of coating agent will penetrate into the fibre itself, and the solvent or carrier will be absorbed only to the fibres of the first layer, especially to their surface. Also in the spaces between the fibres, retention of the coating agent near the application surface, at the said depth of at most 30 μm, more preferably at most 20 μm, even more preferably at most 10 μm, and most preferably at most 5 μm, is an objective of the coating according to the invention. Some insignificant remainders can still be found at the depth of 40 μm. In this case it is typical that over 90% of the coating agent found in the product will essentially remain on the surface of the product or will penetrate in the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating. In advantageous em- bodiments it is possible that even as much as over 99% of the coating agent will substantially remain on the product surface or penetrate in the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating. The expression " essentially on the surface" thus refers to the application surface or a depth of at most 30 μm, as is determined by means of the penetration depths. "The coating agent found in the product" refers here to a coating, which has adhered to the product after having been processed according to the invention and, optionally, after further processing stages typical of the field. The number emphasises the lack of migration typical of the method from the spreading surface to the fibre spaces and to the interior. Thus, the percentage does not include the loss during the process, or the coating agent got into the side or discharge currents.

In addition to dry powder, in coating applications representing a related technical field, it is possible to transfer onto surfaces also liquid particles and aerosols in an electrically assisted manner. For example, in electrical spray painting, paint typically sprayed under pressure is transferred onto the surface to be coated as drops, assisted by an electric field. The objective is to form a uniform layer as the drops a combine to a film on the substrate to be coated. Control mechanisms of the electric field are used in these embodiments to facilitate the control of the material.

In the method of the invention, an e-spray or e-spin method is used as the electrically assisted transfer. E-spraying i.e. electrostatic spraying, i.e. electric spray devi- ates from, e.g. spray painting in that the drop is formed by an electric field and not by the kinetic or viscose forces of the material. The extremely fine nano-aerosol formed in the electric field offers advantages in the coating.

E-spin, i.e. electrostatic defibration deviates from e-spray in that in this method an electric field is used for forming fine, continuous fibres. The diameter of single fi- bres is typically less than 5 μm, but especially in the range of less than 400nm. Typically, at least part of the material forms fibres, but there may also occur drops and/or a combination of these, chained drops.

In a product coated by the e-spin method, the coating agent forms a reticular layer to the surface of the product, the layer being in contact with the fibres essentially principally from the fibre tops. It can be seen from the SEM view in Figure 7 how the coating agent has been spun onto the core surface as fibres and drops. The fibres of the coating agent set to the tops of the topography of the surface to be coated, where they will adhere to the base layer. On the other hand, the coating agent will not be directed to the "holes" of the core surface structure. Without committing oneself to any theories, this is believed to be due to the relatively small amount of solvent/carrier that is applied in the method, and the charging distribution. Respectively, the charging distribution guides the drops or pearls to the tops of the surface structure in the e-spray coating of the invention. The coating agent layer does thus not imitate the topography of the fibre surface, at least not on the scale level of pulp fibres, but it forms a separate layer on the level of the tops. It also has to be noted that in the products according to the invention the coating agent does not typically form a uniform film onto the surface, but individual particles spread uniformly to the tops or a non- woven type net.

Pressurisation as assistance of e-spraying refers here to that even if a large part of the drop and/or fibre formation occurs by means of electric powers, the forming of drops and/or fibres can be assisted by means of a liquid to be sprayed or by air pressure.

When the e-spin method is used as the electrically assisted transfer, the typical amount of coating agent on the surface of the product after one coating step is 0.0001 - 1.0 g/m², preferably 0.0001 - 0.5 g/m². On the surface of the product at least part of the coating agent is observed as fibres, drops, or a combination of these. Another advantageous method for electrically assisted transfer is electrical spraying, i.e. the e-spray method. When using this application method, the amount of coating agent is present on the surface of the product after one coating step is 0.00001 - 1.0 g/m², preferably 0.00001 - 0.1 g/m². In this case, drops or pearls of the coating agent can be observed on the surface of the product.

A fibre layer manufactured as a continuous web is typically functioning as the core in paper and cardboard products, either side or both sides of which can have been coated or layered with different functional layers, for example, moisture, oxygen or aroma barrier layers, and or alternatively, layers influencing other properties, such as opacity, gloss, printability, etc. The fibre layer usually contains pulp fibres, but also other fibre layers are known in the field. When a coating according to the invention is made to a fibre layer, the coating agent and its possible solvent or carrier penetrate at most to the depth of one single fibre, i.e. to the depth of about 30 μm in the layer in relation to the spreading, i.e. application direction of the coating. If again the application of the invention is used for a different material with a layered structure, the coating will either be retained in an interface or interfaces between the layers. However, it is common for both the application methods that substantially no coating agent or its solvent or carrier is observed in the fibre layer at the depth of over 50 μm from the application surface. In other words, the fibre layer contains substantially no coating agent at the depth of over 50 μm from the application surface of the coating. At this depth there also is substantially no solvent or carrier originating from the treatment according to invention.

In this connection, surface refers to the average surface alignment of the fibres in the core. One skilled in the art will understand that the paper surface is irregular and rough, when examined with a certain resolution, as is presented, for example, in the work Paper Physics, pages 89-115, Kaarlo Niskanen, Jyvaskyla, Finland (1998). In the embodiments according to the invention the thickness of the product to be coated is, however, such that in relation to the product's overall thickness it will be sensibly possible to define the depths of the invention, into which the coating agent will penetrate and, on the other hand, will not penetrate. Thus, the selection of the measuring point is not relevant and, in practice, several points can be chosen so that, for example, by using their average it is possible to exclude random extremities.

In a product according to the invention it is possible to have a coating agent applied on only a part of the product surface. This kind of partial application can be made in the direction of travel of the web, i.e. as a continuous application in the longitudinal direction to part of the web width, in which case, for example, different products or a different product with different areas can be obtained after the slitting machines. Partial application can also be made by matching the dosage of the coating agent as pulses in the longitudinal direction, by setting an obstacle between the nozzle and the web, or by some other method known to one skilled in the art. Also a combination of these is possible, i.e. intermittent application to only part of the web width.

The method for electrically assisted coating of a paper or cardboard product is characterised in that the main part of the coating agent will be retained on the surface or penetrate in the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating. The e-spin or e-spray method is used as electrically assisted transfer, as is presented above in connection with the product. In addition to the electrically assisted transfer, also pressurisation can be used to assist in the e- pray method.

Described as method steps, the method of the invention has the following steps in the said order:

- A coating agent, or its solvent or dispersion will be formed as drops, fibres, or both as drops and fibres by using a nozzle in an electrically assisted manner,

- the formed drops, fibres, or the mixture of these will be guided in an electri- cally assisted manner onto the surface of the substrate to be coated, to which they will adhere.

Depending on the character of the coating agent, it can be either dissolved or dispersed before the step, in which it will be formed as drops and/or fibres. Among the coating agents there also are such agents that can, for example, be hot melt proc- essed or otherwise taken to the nozzles without a solvent or carrier, in which case no dissolution or dispersion step will be needed. The formation of drops and/or de- fibration is dependent on different factors with different coating agents, but some essential properties are, for example, solubility, plasticity, viscosity, electrical properties, etc. Depending on the method, drops and/or fibres will be formed of it by a nozzle in an electrically assisted manner. The electrically assisted method will also be used when controlling the formed particles.

In the method, as the coating agent leaves the nozzle, the said coating agent is in liquid form. Thus, it can also be a solvent or dispersion of the coating agent. Dispersion is here understood in a wide form, as is defined e.g. in the work Hawley's Condensed Chemical Dictionary, Richard J. Lewis, John Wiley & sons, (1997), page 417, including two-phase liquid coatings. In connection with dispersions, a carrier thus refers to the continuous bulk phase of a two-phase system. After this, the solvent or carrier can evaporate completely or preferably partly during the flight; in other words, before the coating agent contacts the surface of the product to be coated. In this case, upon contacting the surface of the substrate to be coated, the ratio of the amount of the coating agent to the amount of the solvent or carrier is typically smaller than in traditional methods. In the method of the invention, a solvent or carrier is present when the drops or spun filaments contact the surface, in which case the solvent or carrier can participate in the formation of the film and adsorption on the surface, and then evaporate.

Flight refers to the travelling of the drops or spun filaments guided by pressure and/or electric current from the nozzle to the surface of the object to be coated. Different phenomena can occur during the flight, for example, the morphological for- mation of drops or spun filaments, evaporation of solvent or carrier, or their distribution into smaller particles, changes in charging density, etc. In the method of the invention, it has been observed that a relatively long flight distance has certain advantages. In an electrically assisted transfer, according to the invention, the flight distance between the nozzle and the object to be coated is over 10m, typically over 40mm. In some embodiments the flight distance can be over 90mm or even over 500mm. The significance of the control of the flight distance is especially emphasised as the application is made in accordance with the invention when the object moves with a speed of at least 50 m/min. Speeds of over 500 m/min are usual, and the method has been shown to be controllable even when the speed is over 700 m/min.

The nature of the solvent of the solution or the carrier of the dispersion according to the method of the invention can be fibre-swelling, especially water. Water is known to absorb to the fibres and remain there, swelling them. The advantages of the method of the invention become well apparent when using this liquid, which can il- lustratively be seen by comparing Figures 8a and 8b with each other. The scope of the invention also includes an embodiment, in which the solvent of the solution or the carrier of the dispersion is non-swelling, when considering the fibres.

The method of the invention can be applied so that the coating is scheduled to be performed in the paper machine, especially in its coating section. Alternatively, the coating can be performed as a separate process. Nevertheless,, the coating is most preferably performed before calendering. When needed, also piezoelectric ultrasonic atomisers can be used as a technique assisting in the improvement of the spraying.

One embodiment of the invention is the use of a spray chemical treatment so that surface gluing can be omitted. In this case, the surface strength increases and dust- ing decreases.

One advantageous embodiment is to reduce the unequal sidedness of paper/cardboard by reducing the porosity/absorbancy of paper/cardboard in a controlled manner from one side and/or both sides.

Because on the basis of tests conducted, it was possible to equalise the absorption of the printing ink into the cardboard, it is obvious that an advantage is also achieved, if the method of the invention is used before the mineral and/or other coating of the cardboard. In this case, a better smoothness of the mineral and/or other coating and reduced absorption of the coating into the structure of the paper/cardboard can be achieved.

A second advantageous embodiment is the manufacture of cardboard for electrophotography. When using the method of the invention it is not necessary to alter the whole coating recipe, because the desired effect can be obtained by altering the surface properties with a chemical.

Still another advantageous embodiment is to improve the adhesion of the plastic surface of a layered cardboard by primer treatment (autoclave, oven dish PET and normal PE plastic coating).

One embodiment for the method of the invention is to get small water drops to a paper web by using electric charging (Figure 2). The method reduces significantly the amount of mist spreading to the environment when using water and high- pressure nozzles (i.e. small particle size).

Examples

Example 1. Spray coating on a laboratory scale.

Uncoated cardboard (3-layer trayforma cardboard 280 gsm, 100% bleached pulp) was used as the base board. Different test points were chosen from the treated area, of which the chosen parameters were defined. The points were the following: Kp 0 i.e. the reference, which was cardboard without treatment; Kp 1, in which the applied water amount was 0.2 g/m²; Kp 2, in which the applied hydrophobic corn starch 0.2 g/m²; Kp 3, in which 90% of hydrophobic corn starch and 10% of starch crosslinking agent were applied, in total 0.2 g/m².

The laboratory coating was performed as spray application, in which the adding of spray was made to ensure even application by running the sheet several times through a spray nozzle. In this way it was simultaneously ensured that the amount of water added at one coating step was clearly under 1 g/m². The sheet had time to dry between the spray runs, and thus the adhering of the chemical to the surface of the cardboard was achieved. In this case it was seen to that the chemical would remain on the surface of the cardboard. However, also in these tests, the surface structure of the cardboard had slightly swelled, which can be seen as a roughness difference between the reference (Kp 0) and the water-treated test point (Kp 1), which is shown as defined according to the Bendtsen method. Without committing oneself to any theory, I assume this is due to the relatively large drop size of the drops formed by the spray nozzle used, in which case: the amount of solvent remaining under a single drop is bigger than 1 g/m².

Table 1. The obtained results on a laboratory scale

in which: IGT pick ps, pi m/s SCAN-P 63 :90

Z strength kPa SCAN-P 80:98

Porosity Bendtsen, pi ml/min SCAN-P 26:78

IGT flexo printed strips: IGT FI, internal method

Density Gretag DI86 densitometer

Mottling 100% Measurements are performed by using a computer and a UMAX scanner with PapEye program (the program is sold by Tapio Technology in Finland).

Visually the print quality had distinct differences. Likewise, there was a visible improvement in the density of the printing ink (darkness). The surface strength had clearly increased. The level of surface gluing was not quite achieved, but nevertheless, the change was considerably big compared with the dosage in relation to amount.

Reference example 2. Coating by using a large amount of water.

One wanted to repeat the results of the laboratory scale by using a large amount of water (i.e. "traditional spray application") with a pilot paper machine. The points were similar to those in the laboratory scale test: Kp 1, in which the applied water amount was 0.2 g/m²; Kp 2, in which the applied hydrophobic corn starch 0.2 g/m²; Kp 3, in which 90% of hydrophobic corn starch and 10% of starch crosslinking agent were applied, in total 0.2 g/m². To ensure even application of the chemical, the total amount of liquid used at one coating step was 5-7 g/m² (the concentration of the chemical was adjusted so that 0.2 g/m² of coating was achieved onto the surface of the cardboard). Table 2 shows the results of the application of chemicals with a spray unit (0.2 dry special starch and approximately 5-7 g/m² liquid). Table 2. The obtained results on a pilot scale.

A spray made onto a hot and rather moist web with a cup cardboard recipe in a pilot paper machine improved the surface strengths relatively much. The density of the IGT flexo printing and the mottling improvements were essentially smaller than anticipated by the results obtained in the laboratory, and visually there was no distinct difference in the smoothness of the paper surface.

The reasons for the clearly poorer results are the distinctly more uneven application in the pilot and the chemical penetrating deeper, due to the larger total amount of water used.

Example 3. Spray application assisted with a commercial web charging system.

Traditional air-disintegrated spray nozzles were used to apply water and optical clarification agent onto a cardboard surface. The web was charged by an arrangement according to Figure 2b, the running speed of the cardboard web being 400 m/min.

15, 7.5 and 1.0 g/m² of liquid were dosed. Contrary to expectations, the clearly smoothest surface coverage (small scale and large scale) was achieved with the dosage amounts of 1.0 g/m² (the next smoothest was 15 g/m²). A precondition for good results with these nozzles has been the larger amount of disintegration air than rec- ommended (in this test 180 1/min) and the electric charge of the web.

When the electric charge of the web is missing, a large amount of disintegration air caused an unsolvable problem, as a smaller share of the spray mist got to the web, due to the smaller drop size and larger air currents. The arrival of the spray mist to the web can further be intensified by charging the spray mist differently from the web. This procedure will become possible, if very small amounts of liquid are used.

Also spraying a solution of 10% CMC was tested with an air-disintegrating nozzle. The air-disintegrated nozzle (with the air amount of 180 1/min) could visually disintegrate the mist well, but distinct drops and wakes could be observed in the web indicating that the drop size achieved with this nozzle type was not small enough. However, the charging of the web did have a distinct effect, reducing the amount of small-sized mist spreading to the environment.

When the drop size is too large, the situation can be improved by using surface- active agents reducing the drop size in the liquid. When the drop size is too large, the situation can be improved by diluting the liquid further and by increasing the temperature of the liquid. Thus it is possible to make a smaller drop size to exit from the nozzle and, due to the higher temperature, part of the liquid will evaporate before contacting the web.

Reference example 4. Spray application of a model agent assisted with a commercial web charging system.

In this example, commercial web spraying and charging systems meant for the moistening of paper were used to control a spray-spreadable water solution of an optical clarification agent.

When using small amounts of water, small drop size had to be used when striving for smoothness. As the drop size reduced, air currents affected the smallest drops, and it was difficult to make them to remain on the web.

When circumstances were changed, i.e. upon charging the web, the smallest drops were made to seek their way onto the charged surface and thus it was possible to reduce the fuming. The charging of the web can be performed with two different principles (Figure 2).

In this test arrangement, the principle B (Figure 2) was used for charging the web. The paper web was a wood-containing printing paper, and the running speed was tested up to 1000 m/min.

By adding an optical clarification agent to the water, the spreading of the spray water onto the charged web can be seen, which result has been shown in Figure 4. It can be seen in the Figure that the spreading is uneven in relation to the lateral direction of the web; in other words, stripes can be seen in the longitudinal direction and, in addition, the web contains a lot of stains of optical clarification agent, left by big drops. With commercial devices, the desired smoothness was not obtained even with the optical clarification agent functioning as the model agent, and it was not possible to function with the amount of water according to the invention.

Example 4. E-spray and e-spin as electrically assisted transfer, in addition pressurisation as help.

The base board was uncoated pulp board (3-layer Trayforma cardboard 280 gsm, 100% bleached pulp). There were 85 sample points in all, of which the following ones were selected for examination: KpO i.e. reference, which was a cardboard without processing; Kp 1, in which the applied PVA amount was 0.09 g/m2; Kp 2, in which the applied PVA was 0.13 g/m2; Kp 3, in which PVA 0.22 g/m2; Kp 4, in which PVA 0.31 g/m2; Kp 5, in which PVA 0.6 g/m2. In Figure 5, the marking "- >23.<." refers to the numbering arrangement of the 85 sample points.

The coating was made as spray application, in which the viscosity of the chemical and the dry substance were optimised for the coating. In this case it was seen to the retention of the chemical onto the surface of the cardboard and, however, the complete drying of the chemical was prevented before penetrating into the substrate. The product was not calendered after the coating, because it was desired to demonstrate the behaviour according to the invention (i.e. fibre swelling does not occur, i.e. smoothness does not deteriorate but improves).

Table 3. The obtained laboratory measurement results

IGT-pick ps, pi m/s SCAN-P 63:90

Porosity Bendtsen, pi ml/min SCAN-P 26:78

IGT-flexo printed strips: IGT Fl, internal method

Density Gretag Dl 86 densitometer

With Mottling Handy Measure equipment, in the wavelength range of 1-8 mm, internal method (the equipment calibrated to correspond to the STFI method).

UCA uncovered area with Handy Measure equipment, internal method.

The smoothness of the application can be evaluated in Figure 5. The speed of the coated cardboard web was 600 m/min. The width of the presented sample is 70 cm. Optical clarification agent was added to the PCA solution, and the sample was illuminated with ultraviolet light. Visually, the printing quality improved substantially, due to the strongly reduced amount of uncovered area (UCA). Likewise, the density of the printing ink (darkness) had a visibly substantial improvement. The surface strength had increased slightly. Without committing oneself to any theory, we believe that it is significant for the surprisingly good results obtained that during the flight distance a large part of the solvent or carrier is removed and the coating dries. In the case of the example, the drying of even a slight amount of PVA during the flight distance will substantially increase the viscosity of PVA. If the dry substance increases from 10% to 15%, the viscosity will increase from 1000 mPa to 10 000 mPa. Drying occurs fast from the surface of the drop. This reduces the swelling and opening of the base web, but there is still enough moisture/solvent to achieve a sufficient bonding between the web and the coating. In addition, without committing oneself to any theory, we be- lieve that the drying of the drop/fibre during the flight will promote successful deposition through the increase in ion concentration (charging density).

Claims

1. Paper or cardboard product, in the coating of which electrically assisted transfer has been used, characterised in that the main part of the coating agent and/or its solvent or carrier is retained on the surface or it penetrates in the fibre layer at most to the depth of 30 μm, preferably at most 20 μm, more preferably at most 10 μm, most preferably at most 5 μm in relation to the application surface of the coating.

2. Product according to claim 1, characterised in that over 90% of the coating agent found in the product is retained substantially on the surface of the product or penetrates in the fibre layer at most to the depth of 30 μm in relation to the applica- tion surface of the coating.

3. Product according to claim 2, characterised in that over 99% of the coating agent found in the product is retained substantially on the surface of the product or penetrates in the fibre layer at most to the depth of 30 μm in relation to the application surface of the coating.

4. Product according to claim 2, characterised in that the fibre layer does not substantially contain any coating agent at the depth of over 50 μm in relation to the application surface of the coating.

5. Product according to claim 2, characterised in that at the depth of over 50 μm in relation to the application surface of the coating, the fibre layer does not substan- tially contain any coating agent and/or its solvent or carrier.

6. Product according to one of the preceding claims, characterised in that the coating agent forms a reticular layer onto the surface of the product, the layer being in contact with the fibres essentially from the fibre tops.

7. Product according to one of the preceding claims, characterised in that coat- ing agent layer does not reproduce the topography of the fibre surface.

8. Product according to claim 1, characterised in that the coating agent is only applied to part of the product surface.

9. Product according to claim 1, characterised in that an e-spin method is used as the electrically assisted transfer.

10. Product according to claim 9, characterised in that the thickness of the coating agent on the surface of the product is 0.0001 - 1.0 g/m², preferably 0.0001 - 0.5 g/m².

11. Product according to claim 1, characterised in that an e-spray method is used as the electrically assisted transfer.

12. Product according to claim 11₅ characterised in that the thickness of the coating agent on the surface of the product is 0.00001 - 1.0 g/m², preferably 0.00001 - 0.1 g/m².

13. Method for electrically assisted coating of a paper or cardboard product, char- acterised in that the main part of the coating agent and/or its solvent or carrier is retained on the surface or it penetrates in the fibre layer at most to the depth of 30 μm, preferably at most 20 μm, more preferably at most 10 μm, most preferably at most 5 μm in relation to the application surface of the coating.

14. Method according to claim 13, characterised in that the method contains at least the steps (ii) and (iii) in he said order:

(i) the coating agent will optionally be made to a solvent or dispersion by dissolving or dispersing it to the solvent or carrier;

(ii) the coating agent as such or its solvent or dispersion obtained in step (i) is formed to drops and/or fibres by using a nozzle in an electrically assisted manner,

(iii) the formed drops and/or fibres are directed in an electrically assisted manner onto the surface of the substrate to be coated, to which they will adhere.

15. Method according to claim 13 or 14, characterised in that as the coating agent leaves the nozzle, it is in a liquid form and/or in the form of a solvent or dispersion.

16. Method according to any of the claims 13 - 15, characterised in that the solvent of the said solution or the carrier of the dispersion is water.

17. Method according to claim 13, characterised in that an e-spin or e-spray method is used as the electrically assisted transfer.

18. Method according to claim 13, characterised in that in the electrically assisted transfer, the flight distance between the nozzle and the object to be coated is over 10 mm and preferably over 90 mm.

19. Method according to claim 13 or 17, characterised in that in addition to elec- trically assisted transfer, also pressurisation is used.

20. Method according to claim 13, characterised in that the application is made as the object to be coated moves with a speed of at least 50 m/min, preferably 500 m/min, and still more preferably over 700 m/min.

21. Method according to claim 13, characterised in that the coating is performed before calendering.