FI123458B - Process for coating paper or cardboard product and product obtained thereby - Google Patents

Description

FIELD OF THE INVENTION Field of the Invention

The invention is within the field of electroplating of paper and board products. More particularly, the invention relates to a process for transferring very small amounts of coatings to the surface of a product and to the product thus obtained.

Background of the Invention

The coating of paper and board products using a coating solvent-rich coating achieves uniform application and the thickness of the coating layer can be adjusted from a thin layer of a few micrometers to thicker layers. However, in this case, the abundant amount of solvent must be removed by drying the coated product or blank, whereby the solvent typically evaporates under the effect of the supplied energy. A large amount of solvent is thus infiltrated into the fiber layer, whereby the component primarily intended for surface coating is partially exposed to the interior of the fiber layer. The fiber layer also penetrates the solvent, which can also affect non-surface fibers. From the deeper layer of the fiber, the solvent is more difficult to evaporate. It can also cause undesirable effects such as reversible or irreversible swelling of the fibers, dissolution of the fibers, etc. With these methods, the coating is carried out over the entire width of the web.

20 Electro-assisted coating is a known method for coating paper products. There are both dry and wet methods of transferring and applying the coating to the desired surface. Because the product itself is typically non-conductive, "in addition to the desired end properties of the coatings and excipients used, there are requirements for their electrical properties. Thus, one disadvantage of electrospray transfer 25 is the complexity of hitherto known coating components and the need to use polar organic solvents.

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Q- In seeking to reduce solvent use, some of the benefits of using a large amount of solvent, such as even application, are lost. It is known that in many applications, the coverage-o ability can be improved by reducing the droplet size. However, reducing the size of the droplet 30 in the spray application causes the smallest droplets to move with the air flows. Therefore, due to the high air currents, the small droplet size cannot be easily used when increasing travel speeds. If a small droplet size is used, the construction of a separate underpressure hood is required, and yet the coating 2 result is not smooth enough, since the air flows are never completely smooth. In addition, the hood is designed to prevent 'spray dust' from spreading into the environment, and the solution quickly becomes cost-insensitive.

On a laboratory scale, uniform application is achieved when coating a stationary object, but the challenge is to obtain a sufficiently uniform application on a large scale and at high travel speeds. The nozzle spacing needs to be increased, which results in longer particle flight distances. This in turn increases their susceptibility to airflow disturbances.

Spray nozzles providing a small droplet size are prone to clogging. Further, with high-viscosity chemicals, getting a small droplet size is problematic. Patents (FI 111912B / DE10330801) have been published in connection with spray mist control.

Due to the above problems, the spray coating unit becomes complex and expensive.

Furthermore, since the aforementioned problems have not been satisfactorily solved, especially in the spray application, large volumes of liquid still need to be used in order to achieve a uniform coating.

Thus, there is a need to develop an invention that solves these problems and provides a method of electro-assisting the application of a variety of coating materials so that they do not penetrate deep into the fiber layer. There is also a need to develop paper and board products in which the coating materials are retained on the surface or substantially on the first fiber layer relative to the coating direction.

The paper or board product according to the invention, the coating of which is carried out by means of electro-assisted transfer, is characterized by what is stated in the characterizing part of claim 1.

The method of the invention for electroplating a paper or board product is characterized by what is said in the characterization section of claim 11, wherein the paper or board product is coated with an electrospun transfer, the bulk of the coating material and its solvent being retained. swells in the fiber layer to a maximum depth of 30 µm relative to the coating application surface. This means that most of the coating material is retained on the surface of the object to be coated or stops at the interface between the object and the coating. When applied to the fiber layer, this means that the coating adsorbs substantially to the first fiber layer relative to the interface between the fiber layer and the coating or from the nozzle.

In a method of electroplating a paper or board product, most of the coating material penetrates into the fiber layer to a thickness of up to one fiber layer, substantially to a depth of 30 µm relative to the coating application surface.

The paper and cardboard product and method of the invention will now be described in more detail using the technical solutions described in the accompanying drawings.

Figure 1 illustrates the basic possibilities of using electrospray spray (alternatives A; web charge B; electrode between spray and web).

Figure 2 shows a principle view of a wetting apparatus based on the electric charge of the web.

Figure 3 shows the effect of electrospray (ESA) spraying on coating. In Fig. 3A, the electro-assisted system is not connected. In Figure 3B, the system is on.

Figure 4 illustrates the aerosolized spray nozzles used in the experimental setup of the examples described below.

Figure 5 shows a sample surface showing the uniformity of spray water distribution in an electrospray spray by adding an optical brightener to water according to Comparative Example A;

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i 25 During surface treatment of paper, it has been found that, when using a fiber swelling fluid, keeping the total fluid volume low enough results in better results even with very small amounts of chemicals than if the total fluid amount is higher and § the amount of chemicals is higher. The inventors have found that the amount of swelling fluid should not exceed 1 g / m in one coat in order to achieve this advantageous effect. The swelling fluid is typically a solvent or carrier for the coating material. On the other hand, a non-swelling solvent may be used. In the literature, results have been reported in which, with the use of a non-swelling solvent, small improvements in the surface properties of the paper can be achieved. The disadvantage is the poor adhesion of the chemical to the web.

However, the advantage of using water is that the adhesion between the chemical and the fiber (web) is good enough.

By using substantially smaller amounts of liquid in non-contact coating (such as conventional spray application), a significantly smaller average particle size can be achieved. With a smaller particle size, it is substantially easier to achieve better results in fog control (e.g., electrically assisted particle control) and uniform application at high web speeds. 10 It has now surprisingly been found that by reducing the amount of coating, better uniformity of coating can be achieved, which is contrary to traditional art.

The use of very small amounts of liquid with conventional web charging allows the sprayed liquid to be charged to the opposite sign without problems, which would not be possible with larger volumes of liquid. This allows for a greater distance between the mouth and the web, which in turn is important for the larger scale uniformity of the coating and to ensure the runnability of the web. The achievement of a smaller droplet size can be further improved by the use of surfactants suitable for this purpose or specifically intended for this purpose. Further, since it is important to achieve a small droplet size, the temperature of the liquid can be increased as required, often resulting in a lower droplet size. By diluting the chemical used, a smaller droplet size is obtained and, due to the small droplet size, the evaporation of the diluent from the droplet during the airflow can be utilized, but the amount of liquid entering the web remains low.

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Thus, according to the invention, paper and / or board can be surface-treated with a small amount of chemicals evenly and in a controlled manner at a high production rate, thereby achieving

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^ essential benefits.

| A paper or board product coated with an electro-assisted transfer is characterized in that the majority of the coating material and its solvent are retained at the interface or penetrate into the fibrous layer up to a depth of 30 µm relative to the coating application surface. Preferably, the coating material does not penetrate into the fiber itself, or at all, and the solvent is absorbed only into the top layer fibers. Also in the fiber spacing, the retention of the coating material near the application surface at said depth of less than 30 µm, more preferably less than 20 µm or even less than 10 µm, is an object of the coating of the invention. Some insignificant remains may still be found at a depth of 40 pm. Here, it is typical that more than 90% of the coating material retains substantially on the surface of the product. In preferred embodiments, it is possible that up to 99% of the coating material is substantially retained on the surface of the article.

Electrospray transfer refers to electrospray or electrospun processes. Traditionally, there are three different types of spray equipment: high pressure air diffuser, high pressure mechanical and low pressure mechanical. With a low pressure mechanical spray, the droplet size is coarse and at the same time the amount of particles carried by the airflow 10 is smaller. (Davies, B. 1977, Starch spraying. Pap. Technol. Ind. Vol. 18, no. 6, 186-187). This system is mainly suitable for coarse and large volume application.

In the case of the high pressure air diffuser type, the liquid is decomposed by air to high pressures. Air dispersed is versatile in all viscosity ranges. 15 Fine droplet size is achieved, but this causes large losses as the mist passes with the dispersion air. Because of this, high-pressure air-dispersed and high-pressure mechanical nozzles are often equipped with an electrostatic system that seeks to separate the liquid from the air transporting it by directing it to the surface of the web.

20 For paper coating, the coating requires a very good coverage. The spray method achieves this with small droplets and large volumes of liquid. The large mist loss caused by the small droplet size with the air currents has been reduced by electrostatic systems.

“When using electrospun transfer, the typical thickness of the o 25 coating material on the surface of the product is 0.0001-0.5 g / m2. Another preferred method of electrospray transfer co is electrospray or electrospray.

When applying this application method, the coating material has a thickness of 0.00001-0.1 g / m 2 on the surface of the product.

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Typically, the backing layer in paper and board products is a continuous layer of fiber 30, on which one or both sides may have various functional layers, for example moisture, oxygen or aromatic barrier layers, or alternatively other properties such as opacity, layers affecting gloss, printability, etc. When the coating according to the invention is applied to the fiber layer, the coating material and its possible solvent penetrate to a depth of at most one single fiber, that is to say about 30 µιη in the layer relative to the direction of application or application of the coating. On the other hand, if the application according to the invention is applied to a layer-like, different material, the coating material will be trapped either at one of the interfacial interfaces or interfaces. What is common to both applications, however, is that the coating material or its solvent is not substantially detectable in the fiber layer at depths greater than 50 µm relative to the application surface. In other words, the fiber layer at a depth of more than 50 µm relative to the coating application surface is substantially free of coating material.

In the product according to the invention, it is possible to have the coating material applied to only a part of the surface of the product. Such partial spreading can be done in the longitudinal direction of the web, that is, in the longitudinal direction, as a continuous spread over a part of the web width, whereby, after the winders, different products or products with different areas are obtained. Partial application may also be accomplished by sequentially applying the coating agent in longitudinal pulses, placing a barrier between the nozzles and the web, or by any other method known to those skilled in the art. A combination of the above is also possible, ie periodic spreading only for part of the track width.

The process for electro-assisted coating of a paper or board product according to the invention is characterized in that the majority of the coating material is retained at the interface or penetrates into the fiber layer up to a depth of 30 µm relative to the application surface of the coating 20. The electro-assisted transfer uses an electrospinning or electrospray method as described above for the product. In addition to electro-assisted transfer, pressurization may be used to assist spray application.

In the process, when the coating agent leaves the nozzle, said coating agent is in the form of a solution or dispersion. Thereafter, the solvent may evaporate completely or partially during the flight, i.e., before the coating material contacts the surface of the object to be coated. Thus, when the coating material encounters the target surface, its solvent content is very low, significantly less than 20%, even less than 1%. Alternatively, the solvent is present when the droplets or spun fibers hit the surface, whereby the solvent x can participate in film formation and adsorption on the surface and subsequently evaporate.

m m o Flight means the transport of droplets or spun threads, controlled by pressure and / or electric current, from the nozzle to the surface of the object to be coated. Lennon

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Various phenomena may occur during the course of the process, for example morphological deformation of droplets or spun fibers, agglomeration, evaporation of the solvent, etc. In the method according to the invention 7 certain advantages have been observed over a relatively long flight. According to the invention, the electric distance between the angry and the object to be covered during the transfer is more than 10 mm. In some embodiments, the flight distance may be greater than 90 mm or even greater than 500 mm. In particular, the importance of flight control is emphasized when the application is carried out when the target to be coated according to the invention moves at a speed of at least 200 m / min. Speeds above 500 m / min are common and the method is expected to be controllable up to 700 m / min.

The solvent or dispersion carrier of said solution of the process of the invention is a fiber swelling solvent, particularly preferably water. Water is known to be absorbed by the fibers and retained by swelling. The advantages of the process of the invention are well apparent when using this solvent.

The method according to the invention can be applied in such a way that the coating on the paper machine takes place on its coating part. Alternatively, the coating may be performed as a separate process. Most preferably, however, the coating is performed prior to calendering.

15 If necessary, piezoelectric Ultrasonic atomizers can also be used as assistive techniques for improving nebulization.

One embodiment of the invention is the use of a spray chemical treatment so that surface sizing can be omitted. This increases the surface strength and reduces dusting.

Another preferred embodiment is the manufacture of digital board. Using the process of the invention, there is no need to change the entire coating recipe since the desired effect is obtained by changing the surface properties with a chemical.

Another preferred embodiment is the improvement of the adhesion of the plastic surface of the laminated board by a primer treatment (autoclave, PET furnace and normal c3 PE molding).

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Electric charge has also been used to obtain small droplets of water in the paper web (Figure 2). The method significantly reduces the amount of EE that is released into the environment when using water and high pressure spray nozzles (i.e., small particle size).

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eXAMPLES

Example 1. Spray coating on a laboratory scale.

Uncoated cellulose board (3 layers tray 280 gsm, 100% bleached pulp). Different test points were selected from the treated area, from which the selected parameters were determined. The points were as follows: Kp 0, or the reference, which was the cardboard without treatment; Kp 1 with 0.2 g / m 2 applied water; Kp 2 with 0.2 g / m2 hydrophobic corn starch applied; Kp 3 with 90% hydrophobic maize starch and 10% starch crosslinker applied, total 0.2 g / m 2. The spray nozzles used are shown in Figure 4.

10 Laboratory Coating Spray Application In which, to ensure uniform distribution of the chemical, a spray application was made by passing the sheet several times through the spray nozzle. Between the spray runs, the sheet was allowed to dry and this resulted in sticking to the board. In this case, care was taken to keep the chemical on the cardboard surface. However, even in these tests, the surface structure of the paperboard had slightly penetrated the swell-15 soil, which is reflected by the roughness difference between the reference (Kp 0) and the water-treated test point (Kpl) as determined by the Bendtsen method.

Table 1. Laboratory Scale Results IGT Flexo Printing__Kp 0__Kp 1__Kp 2__Kp 3

Densiteetti__E0__ £ 0__1_J__1,1

Mottling 100% __ 6.2 6.9__4.2__4.0 IGT-pick ps, pi (surface strength) m / s__IA__ £ 5__2J__2.6

Bendtsen roughness pi / ml / min__350__425__320__320 Z-strength (z-directional tensile strength of the entire board) kPa__450__ £ 70__ £ 70__480 "where: o

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cö IGT-pick ps, pi m / s SCAN-P 63:90 ιό 20 Z-strength kPa SCAN-P 80:98

Porosity Bendtsen, pi ml / min SCAN-P 26:78 * IGT Flexo Printed Strips: IGT F1, internal method [£ Density Gretag Dl86 densitometer h *.

o Mottling 100% Measurements are made using a computer and the UMAX 25 scanner. With PapEye (sold by Tapio Technology in Finland).

Visually, there were clear differences in print. Similarly, the ink density (darkness) showed an appreciable improvement. The surface strength had clearly increased. Just 9 was not at the same level as surface sizing, but the change was still considerable compared to the amount of dosing.

Example 2. Coating according to the invention on a pilot scale.

It was desired to reproduce the results of the laboratory scale on a pilot paper machine. The points were 5 similar to the laboratory scale experiment: Kp 1 with 0.2 g / m2 applied water; Kp 2 with 0.2 g / m2 hydrophobic corn starch applied; Kp 3 with 90% hydrophobic maize starch and 10% starch crosslinker applied, total 0.2 g / m 2. To ensure uniform application of the chemical, a total liquid volume of 5-7 g / m 2 was used (the concentration of the chemical was adjusted to obtain a coating of 0.2 g / m 2 on the carton surface). Table 2 shows the results of application of chemicals by spray unit (0.2 dry specialty stars and about 5-7 g / m2 wet liquid).

Table 2. Results achieved on pilot scale IGT flex printing___Kp 1__Kp 2__Kp 3

Density ___ 1.04 1.09 1.09

Mottling 100% ___ 8J5__7J5__7.8 IGT-pick ps, pi (surface strength) m / s 2.7__3A__3.3

In a Pilot paper machine, a spray applied to a hot and fairly moist web 15 using a cupboard recipe moderately improved surface strength. The density and mottling improvements of the IGT flex printing were substantially smaller than the results predicted by the laboratory and there was no discernible difference in the visual surface uniformity.

The reason for the significantly worse results are clearly uneven distribution in the pilot and '20 deeper penetrating chemicals.

g Example 3. Spray application assisted by a commercial web charging system.

| Water spray and m optical brightener were applied to the carton surface using conventional air-sprayed spray nozzles. The web was charged with an arrangement similar to Fig. 2B at a running speed of 400m / min for a paperboard web.

co o cv The liquid was dosed at 15, 7.5 and 1.0 g / m 2. Contrary to expectations, a clearly even surface coverage (small scale and large scale) was achieved with a dosing volume of 1.0 g / m 2 (the next most even being 15 g / m 2). As a prerequisite for good results, these 10 nozzles have had a higher than recommended amount of disintegration air (180 rpm in this test) and electrical charge of the web.

In the absence of electrical charge of the web, the large amount of scavenging air caused a smaller problem with a smaller proportion of spray mist entering the web due to a smaller droplet size of 5 and higher air flows.

The discharge of the spray spray into the web can be further enhanced by charging the spray spray with a different charge than the film. This procedure becomes possible if very small amounts of liquid are used.

An aerosol nozzle was also used to spray 10% CMC solution. The air-10 dispersed maximum (180 rpm airflow) was able to visually disperse the mist well, but there were clear drops and old age on the cardboard web, indicating that the droplet size achieved with this nozzle type was not small enough. However, the charge of the web had a clear effect in reducing the amount of small-scale mist spreading into the environment.

When the droplet size is too large, surfactants that reduce droplet size can be improved within the liquid. If the droplet size is too large, the situation can be improved by further diluting the liquid and raising the temperature of the liquid. This causes the annoyed to leave a smaller droplet size and, due to the higher temperature, a part of the liquid is evaporated before coming into contact with the web 20.

Comparative Example A

Spray application assisted by a commercial web spraying and charging system co o When using a small amount of water it was necessary to use a small droplet size to ensure uniformity. As the droplet size decreased, the airflows affected the smallest droplets and were difficult to cause to remain in the web. When the web was charged, the smallest droplets were caused to contact the charged surface and thus the fogging was substantially reduced. The booking of the web can in principle be made on two different principles (Figure 2).

o o In this experimental setup, principle B was used to charge the web (Figure 2). Paper-

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w 30 slurry was wood-containing printing paper and tested at running speeds up to 1000 m / min.

By adding an optical brightener to the water, the uniformity of the spray water distribution in the electrospray spray is seen, as shown in Figure x

Claims (18)

A paper or board product for which an electro-assisted transfer has been used for coating, characterized in that the majority of the coating material and its solvent or carrier are retained at the interface and a part penetrates into the fiber layer up to a depth of 30 µm to the coating application surface. Product according to claim 1, characterized in that more than 90% of the coating is substantially retained on the surface of the product. Product according to claim 2, characterized in that more than 99% of the coating is substantially retained on the surface of the product. Product according to Claim 1, characterized in that the coating material is applied only to a part of the surface of the product. Product according to claim 1, characterized in that the electro-spinning method is used for the electro-assisted transfer. Product according to Claim 5, characterized in that the coating material is present on the surface of the article at a thickness of 0.0001-0.5 g / m. Product according to Claim 1, characterized in that the electrospray method is used for the electrospray transmission. Product according to Claim 7, characterized in that the coating material is present on the surface of the product in a thickness of 0.00001 to 0.1 g / m2. Product according to Claim 1, characterized in that an electrospray method is used for electrically assisted transfer of £ 2. δ c \ j co Product according to Claim 9, characterized in that the coating material is present on the surface of the product at a thickness of 0.00001-0.1 g / m. 11. A process for electroplating a paper or board product 25 characterized in that it comprises steps (i), (ii) and (iii), respectively: m (i) leaving the nozzle in the form of a solution or dispersion, O (ii) forming droplets or spun threads which, under the control of pressure and / or electric current, are infiltrated on the surface of the object to be coated, (iii) the bulk of the coating material and its solvent or carrier Process according to claim 11, characterized in that the solvent or dispersion carrier for said Liu-5 solution is water. The process according to claim 11, characterized in that, when the coating material contacts the target surface, its solvent content is less than 1%. 14. A method according to claim 11, characterized in that the electrowinning process is carried out using an electrospinning or electrospray method. Method according to Claim 11, characterized in that, in the case of electric aids, the distance between the object angled during the transfer and the object to be coated is more than 10 mm. Method according to Claim 14, characterized in that, in addition to the electro-open transfer, pressurization has been used as an aid for spray injection. Method according to Claim 11, characterized in that the application is carried out when the object to be coated is moving at a speed of at least 200 m / min. The method according to claim 11, characterized in that the coating is carried out before calendering. co δ CvJ co cp lo X X Q. LO LO 1 ^ O CO o o CvJ