FI95946C - The apparatus and method for coating paper - Google Patents

Description

95946

HARDWARE AND METHOD OF COATING PAPER

The invention relates to a method and apparatus for coating paper, board or the like.

The aim of coating paper and board is to improve certain properties of these products, in particular their printability and surface smoothness. As coating materials, aqueous slurries of suitable mineral pigments, e.g. kaolin, talc, carbonates, chalk, etc., are used. in the form of pastes containing e.g. binders, according to the intended use of the final product. Paper most commonly requires coatings on both sides, which can be performed either simultaneously or in sequential processes. Surface sizing can also be used in coating, e.g. in the production of copy papers, in which case only an adhesive is used as the coating material.

In many cases, the board requires only one-sided surface treatment. Coating is most commonly accomplished by passing the web to be treated through a coating station, the main features of which are described below. The coating process comprises two main steps, namely feeding the actual paste, i.e. applying it to the surface of the web in excess, and then scraping and smoothing the resulting paste layer into a uniform and as uniform a coating as possible.

There are several different types of coating methods and equipment, the suitability of which for different types of paper and board depends on the process variables and the desired quality characteristics of the final product. Such process variables include e.g. the amount of coating and the speed of the web during the coating operation. Quality characteristics are also affected by the type of paste and the number of coatings, which in turn impose requirements on the coating method / equipment.

According to the prior art, the application of the coating to the surface of the web can take place either as a brush roll application, in which the paste is applied to a paper web by rotating a roll dipped in the paste. into a molding chamber, one wall of which is a paper web supported by a counter roll, whereby the moving web pulls the coating onto its surface, which is scraped immediately, or using a special application scraper steel arrangement. The coating is leveled using a blade scraper, which is used to adjust the final amount and / or thickness of the coating. Between the beginning of the coating operation and the actual cabinet, the so-called a leveler-scraper that controls the flow of the coating.

All currently known coating methods are based on the fact that both the application of the coating and the adjustment of its amount with a blade or other scraper take place against one and the same counter roll. In this case, the possibilities to choose the size, shape and placement of the application and molding devices in the best possible way for the coating are considerably limited. In the prior art structure shown in Figure 1, the web 10 is fed to a roll 5 which acts as a counter roll for the coating station. Against this, a rotating applicator roll is arranged in the paste basin 12, which raises the coating paste to the nip between the rollers 5 and 11, where the web 10 obtains a coating layer 18 determined by the operating parameters of the coating station (compression between rolls 5 and 11, roll speed 11, etc.). 5, a beam 9 is further arranged against which a scraper blade 6 is attached, which removes excess from the coating 18 so that after the coating station 5/11 there is a desired amount of coating paste on the surface of the web in a uniform layer 187.

At present, two main methods are used at high speeds:. High speeds currently mean coating speeds in excess of 800 m / min. One of these methods is a roller application / blade patterning combination and the other a so-called short dwell time application. The former has been referred to as LDTA (Long Dwell Time Application).

The main difference between the methods is the time of application and the

II

3 95946 time difference between the planning time or delay. In the case of a short stay, this delay is very small because the scraper blade is placed on the other edge of the pasta feeding nozzle so that the pasta is actually fed directly behind the blade. The arrangement is basically similar to the apparatus 1, 2, 3 against the roll 5 in Figure 2 of the drawing.

In the LDTA method, the delay is longer because the application takes place using a roller or an application nozzle with a pre-forming blade, after which the formulation is treated with a separate blade. According to the method, an applicator nozzle with a pre-forming blade and an applicator roll dispense paste onto the surface of the paper, after which the scraper roller scrapes off the excess paste. The long delay is caused by the fact that the applicator and the scraper are placed against the same counter roll, so that they cannot be structurally placed close to each other. It has been found that in the long residence method, successive coatings without intermediate drying fail because the successive coatings tend to mix and contaminate the previous coating and the absorption of binders separately into the paper. The time between coating and scraping should not be too long to prevent coating binders from being absorbed into the web.

Regarding the applicability of the methods at different speeds, it can be stated that the roll application works well only at a speed of less than 1000 m / min, while the short-stay application method works well only if the web speed is above 1000 m / min. The applicator roller starts to cause problems at speeds above 1000 m / min. As the speed of rotation of the roll increases, the paste cannot be spread evenly due to splashes and flow disturbances, resulting in an increase in the number of coating defects. The short residence-application method, on the other hand, does not cause a sufficient amount of coating to adhere to the surface of the paper when the speed is below 1000 m / min.

It is an object of the invention to provide a coating method and apparatus which operates over a wider range of speeds and coating amounts than previously known methods and apparatus. In addition, the various variations of the apparatus according to the invention can be quickly implemented in connection with changes in the type of paper, whereby the coating operation with its methods, pastes and coating amounts can be better optimized without losing cost-effectiveness.

With the presently known coating devices, in which an application nozzle with a blade and a separate patterning blade are placed against one and the same counter roll, the problem has been the poor runnability of the coating device. Pleats are formed in the paper before the patterning blade, as a result of which the paper can be cut with a patterning blade, later with drying devices or a take-up reel. There can be several reasons for this. For example, a paper applied with an aqueous paste between two nips absorbs water from the paste, whereby the tension level of the paper decreases between the nips.

Conventionally, the coating of two coatings on the surface of the paper is carried out in successive stages, in which the coating means, two SDTA coating stations, are arranged in succession against the same counter roll, the so-called in the wet on wet method, or the coating station and scraper are first placed against the first counter roll, after which, after the drying step, a second layer of coating is placed on dry, once-dry paper in a separate coating station against the second counter roll. Since these are separate coating stations, there is only one counter roll in one coating station.

U.S. Pat. No. 5,112,653 describes a coating station or two stations against one and the same support roll, comprising an applicator 42 and first and second blade scrapers 44, 46. FI patent 71801 describes the adjustment of the amount of coating in one step either on a cloth brush 4 or between rollers 9 and 17. the nip.

: The publication deals with the drying of the coating. Two or more support rollers for forming coating nips are not shown.

FI patent application 915964 describes a method and apparatus for coating moving webs. There is an applicator opposite one of the counter rolls, after which the coating is scraped against the said counter roll in one or more stages. FI patent application 5 95946 930757 describes the coating of a paper web with a layer of coating material in which a spreading roller and a guide plate or plates for excess coating flows and one scraper are placed against one counter roll.

FI patent 71801 describes the coating of paper. The coating material is fed to one side of the web and a vacuum is applied to the opposite side of the web while the coating is dried by heating. The vacuum is said to improve the fluidity of the paper. The coating roll and the scraper are opposed by one and the same counter roll.

U.S. Patent 4,963,397 describes an applicator with respect to which a scraper unit is located downstream. Scraping before is a leveler that aims to create turbulence before scraping. The applicator roller and the scraper are facing the same counter roller. Wochenblatt 23 / 24.1987, pp. 1056-1062 Sommer et al. describes the so-called Combi blade design where the blade scraper against one and the same counter roll is interchangeable.

• · ·

The method and apparatus of the invention can better control changes during the coating operation due to the viscoelastic nature of the paper compared to conventional coating methods. As a crucial difference from the coating methods in use, the present invention uses at least two counter rolls in the same coating station, where at least one coating nip is an application unit and otherwise alternatively an application unit or a coating amount control unit, i.e. preferably one counter roll against each coating nip. At its simplest, the coating nip is a scraper with a counter roll on one side. In particular, it has been found that the application can be carried out in such a way that, in the case of two counter-rolls, it is preferably coated against each other and also scraped against at least the latter. It can also be more than two counter rolls, for example three. In this case, at least the first roll is coated and scraped, in which case the latter is either coated and / or scraped and at least scraped against the last roll.

95946 6 Coating and scraping can also take place against the same roll, after which, for example, only only one or preferably several counter-rolls are scraped. This allows a particularly wide range of applications for small and large amounts of coatings with the same equipment, as the wetting / viscoelastic change of the paper caused by the coating can now be controlled with a completely new control variable (tensile difference between coating nips). By adjusting the angular speeds of the adjacent counter rolls, a smooth flow of paper is achieved without creases or bags before the scraper or coating device. This makes it possible to take into account the loosening caused by the wetting of the paper web by different pulling of the counter rolls.

It is also possible to apply the invention to coating devices already in use, which require quite minor structural changes. Variations and combinations of different pastas are easier to implement.

The method according to the invention is characterized by what is • ·. set out in the characterizing parts of claims 1 and 16 and for the apparatus what is set forth in the characterizing part of claim 11.

The invention and its background will be described in more detail below with reference to the structural examples shown in the figures of the accompanying drawing.

Figure 1 is a schematic side view of a prior art coating station using an applicator roll and a blade scraper. Figure 2 shows a schematic side view of a simple coating station according to the invention.

Figure 3 shows one embodiment of a coating station according to the invention.

Figures 4a and b show a schematic side view of two different solutions of an embodiment of a coating station according to the invention.

Figure 5 shows an embodiment of a coating station li 7 95946 according to the invention.

Figure 6 shows a further embodiment of a coating station with more than two counter rolls.

Figure 7 shows the adjustment of the amount of coating with two scrapers after application.

Figure 8 shows a diagram of the amount of coating as a function of web speed.

In the following, the following names are used for the coating station, the coating nip, the application unit, the coating amount control unit and the counter rolls. By coating station is meant a device used for coating paper which provides one or more applications of paste and one or more adjustments to the amount of coating. The coating station consists of one or more coating nips, but on the other hand the coating station does not include other process steps essentially related to the coating (e.g. the actual drying). Mechanical / hydrodynamic forces are applied to the paper web in the coating nip. These forces are caused by the paper web passing through a nip with a counter roll on one side and alternatively either a pressure impulse caused by the application of the paste (application unit, e.g. in extruder application) or a mechanical element caused by mechanical stress (coating amount control unit). In the application unit, the coating paste is applied / applied to the surface of the paper web against the counter roll. In some cases, the application unit involves pre-adjustment of the amount of coating. The coating amount control unit forms a coating nip such that there is a counter roll on one side of the paper web and some mechanism used to adjust the coating on the other side. Most often, some mechanical scraper is used to adjust the amount of coating: but air drawing can also be used. The counter roll is a useful roll on the other side of the application unit or the coating amount control unit. The counter rollers can operate on the friction of the web or they can be rotated, for example, by a motor. Coating units and coating amount control units and counter rolls are devices known per se and extend to or slightly beyond the width of the web.

8 95946

In the coating station alternative shown in Fig. 2, the web 10 is fed to a web guide roll 5, which operates in question. in the coating station as the first counter roll. The coating paste C is passed through the nozzle chamber 3 to the application nozzle 2 and then to the application blade 1, which forms an application nip with the roll 5.

These members 3, 2 and 1 of the applicator unit A are placed on a special applicator beam 15 supported on axle pins (not shown) on a frame of a paper machine or the like, the arrangement of which is explained below in connection with Fig. 4b.

Part of the paste C moves to the surface of the web 10 and most of it returns to the return tray 4 of the application unit. The paste C forms a layer 18 on the surface of the web 10.

In the application unit A, the pre-coated web continues to run on a counter roll 8, against which a scraper blade 6 resting on a special beam 9 is placed. This blade 6 acts as a level scraper to adjust the amount of the final coating 18 '. By means of the scraper blade 6, the excess paste C flows into the return tray 7 of this scraper unit 9.

Figure 3 shows an application of the coating method according to the invention to roll application, wherein the application unit A comprises only rollers 5 and 11. When explaining the different parts of the device and their operation, the same reference numerals as in Figure 2 have been used where applicable.

The coating paste C is applied in the coating position according to Fig. 3 to the surface of the web 10 by an application roller 11 which rotates in the pasta tray 12 at an adjustable speed so that a suitable amount of coating 18 can be applied to the surface of the web 10.

In addition to the rotational speed of the roll 11, the amount of the coating 18 is controlled by the size of the gap between the rollers 5 and 11.

After the coating unit A formed by the rollers 5 and 11, the pre-coated web 10 is conveyed in the same way as shown in Fig. 2 to a roll 8 against which a beam 9 acts.

II

9 95946 supported scraper blade 6, by means of which the final adjustment of the amount of coating takes place so that the desired amount of coating 18 'remains on the surface of the web 10. By means of the scraper 6, the excess C 'of the coating paste formed on the surface of the pre-coated web flows back into the pasta tray 7, which is supported on the beam 9. The arrows indicate the directions of rotation of the rollers.

In the embodiments of the invention according to Figures 4a, 4b and 5, a combined application and adjustment beam 13 is arranged in connection with the roll 8, the structure of which allows the web 10 coming to the roll 8 via the roll 5 to be treated in different ways. Not only an adjustment scraper 17 but also a conventional short-stay (SDTA) coating unit 20 is arranged on the beam 13 so that these can optionally be used depending on the requirements of the process. In order to feed any paste used for SDTA coating, a so-called an extruder applicator consisting of a paste supply chamber 16 and a nozzle 20 and a steel 17.

* 'According to Fig. 4a, the reference numbering of which corresponds mainly to the references of Fig. 2, the paste feed nozzle 16 in the beam 13 is closed by a closure 16a, so that only the scraper device 17 belonging to this beam is used. In this case, the embodiment of the invention shown in Figure 4a is functionally identical to that shown in Figure 2. The scraper device 17 corresponds to the scraper 6 in Fig. 2 and the beam 13 corresponds to the beam 9 in Fig. 2.

Figure 4b shows a structure in which the beam 13 acts as the only coating unit against the roll 8 of the coating method according to the invention. It must therefore operate on the principle of SDTA. In this case, against the roll 5 shown in Figures 2 and 4a: the functional applicator beam 15 is not used at all but is resting on its shaft pins turned to its lower position and thus out of the coating system. In this case, the roll 5 thus functions as a conventional web guide roll.

Figure 5 shows an arrangement for double coating a web made possible by the invention. The apparatus comprises application units A and B. Unit A is the same as shown in Fig. 2. The web 10 thus having its surface coating 18 'is passed on to application unit B, which is formed by a counter roll 8 and an SDTA unit 20 acting against it. by means of which it is possible to provide an additional coating 19 on the surface of the web 10 and on the coating 18 'already present therein. Thus, an additional coating is obtained on the surface of the web by means of the application unit B. The operation corresponds to that shown in Figure 4b.

Figure 6 shows a view of a further embodiment of the invention, in which the web coating and / or coating amount control devices are each opposed to their own counter roll 5, 8 and 22. Opposite the first web roll 5 is a roll applicator device already shown in Figure 3. The web 10 is passed to the second to a coating unit 15 opposite the counter roll 8 and operating in accordance with the nozzle application method with the preforming blade shown in Figs. 2 and 4b. In this case, the web 10 is finally fed to a scraper 6, where the final adjustment of the amount of coating takes place. C '' shows the excess paste II coated on the roll 11 and the excess paste coated by the coating unit 15.

C '' 'describes the material removed by the scraper 6. 18 '' 'describes the coating left on the paper web. It will, of course, be appreciated that coating compositions and grades may be varied. The amount of coating can be up to more than 20 g / m2. This so-called wet on wet coating is a particularly advantageous solution for the implementation according to the invention. It wets the paper twice, which doubles the benefits of using multiple counter rolls.

Figure 7 shows a coating station comprising a first counter roll 5 opposed by an application unit 11, 12. After application there are two scraping steps 1, 6, each of which has its own counter roll 8, 22. When the weight of the paper coating in the application unit is n 50 g / m 2, after the first scraper 1 the amount of coating is about 10 g / m 2 and after the second scraper 6 about 5 g / m 2.

Figure 8 shows schematically the amount of coating as a function of driving speed. Alternative combinations according to the invention 11 95946 also provide edge areas, such as coating amounts of up to more than 20 g / m 2 and on the other hand 5 g / m 2 or less, as shown by the slashes / lines. The paper path speed can be adjusted as needed. Combinations with which a low amount of coating can be obtained are, for example, SDTA, extrusion or coating and at least preferably two formulations against different counter-rolls. An amount of coating of 20 g / m 2 is possible if at least two successive coatings SDTA, extrusion and / or coating are used and at least after the last coating is scraped against the last counter roll.

In addition, it can be stated that the counter rolls 5, 8, 22 can be of the same size or different sizes, the distance between the centers of the adjacent rolls being less than or equal to twice, preferably 1.1 to 1.5, the sum of the radii of the adjacent rolls. The angular velocities of the counter rolls are adjustable so that the angular velocity of the latter on the circumference can be equal to or greater than that of the former. Pleats caused by wetting can thus be eliminated. The diameters of the counter rolls can vary but are usually about 600 to 1000 mm. The scraper blade can consist of, for example, two steels, one of which removes more coating than the other. A separate counter roll can be placed against each coating station or scraper. Different products can be manufactured on one and the same production line. According to Scheme 8, the driving speeds can vary from less than 800 m / min to more than 1600 m / min, depending on the amount of coating, so that the paper quality and driving properties are acceptable.

In the invention, the coating station is most preferably in the case of three counter-rolls a maximum of 4-5 times the total length of the counter-roll diameters and in the case of two counter-rolls 2.5-4 times the total length of the counter-roll diameters.

The invention has been described above only with reference to some of its preferred embodiments, but it is clear that the invention can also be implemented in other possible ways within the limits set forth in the claims without changing the scope of the invention. It is possible to use different coating methods or devices or coating pastes or combinations thereof, coating station combinations from two or three application steps. Delays can be better taken into account. Significant improvement has been made to the prior art. 1

II

·

Claims (16)

1. The coating method for paper, paperboard or the like, in which the paper web (10) is coated with one or more layers of coating mixture, the paper web (10) is scraped after the last coating, and the web (10) is supported in the previous steps by the means (5) from the opposite side of the web, characterized in that the web (10) is conveyed to at least two support rollers (5, 8, 22), the web (10) being coated; against either one or more support rollers (5, 8, 22) having at least one coating agent (9, 12, 13, 15) in use, and after each coating, the overflow coating mixture is scraped from the surface of the coated paper, wherein the control means (1, 6, 17) for the coating amount that follows the coating agent (9, 12, 13, 15) is positioned against the same support roll (5, 8, 22), correspondingly so that the coated paper is at least scraped against the same support roll after the first coating roll. (5, 8, 22) used by the coating agent (9, 12, 13, 15) or in the case of three or more support rolls (5, 8, 22) is coated against at least one support roll (5, 8, 22) and scraped after at least the last coating. Method according to Claim 1, characterized in that for each coating nip (1, 6, 17) which rises, a separate support roll (5, 8, 22) is placed, which coating nip consists of the coating means feeding means (9, 12, 13). 15) and / or the control unit (1, 6, 17) for the coating amount. Method according to claim 2, characterized in that the support rollers (5, 8, 22) can be rotated between each other at different speeds advantageously, in that the latter is rotated at a greater peripheral speed than the previous one. Method according to claims 1-3, characterized in that the overflow coating mixture is scrubbed from the surface of the coated paper against the third or subsequent support roll (22). Method according to claims 1-4, characterized in that there is more than one coating device (9, 12, 13, 15) and a leveling means (1, 6, "against one and the same support roll (5, 8, 22)" ). Method according to claims 1-5, characterized in that the same and / or different coating method and / or composition of the coating mixture is used against each support roll (5, 8, 22). • ♦ - Method according to claims 1-6, characterized in that the web (10) is first coated against the first support roll (5), after which the overcoat coating agent is stripped from the coated web (10) to the same support roll (5), coated with the second coating. the device (13) against the second support roll (8) and the overflow coating mixture are scraped against the same support roll (8). 18 95946 Method according to claims 1-7, characterized in that the web (10) is coated according to the short interval method. Method according to claims 1-8, characterized in that the paper web (10) is fastened. against At least three or more support rolls (5, 8, 22), of which are coated and scraped against kt-minimumone two support rolls (5, 8, 22), and the coated web (10) is scraped against at least the last roll (22). Method according to claims 1-9, characterized in that the web (10) is pre-coated with the first coating layer according to the short interval method against the first support roll (5), and the second coating layer is applied to the web according to the short interval method against the second support roll (8). 11. Apparatus for coating paper, paperboard or the like, which apparatus has means (9, 12, 13, 15) for coating the web with one or more layers of coating mixture, means (1, 6, 17) for making it coated web (10) after eating at least the last coating, and means (5) simultaneously supporting web (10) from the opposite side, characterized in that for support of web (10), there are at least two support rollers (5, 8, 22), wherein either one or more support rollers (5, 8, 22) for coating the web (10) are provided with a coating means (9, 12, 13, 15) and a control unit (1, 6, 17) for the coating amount, which is against the same support roll (5, 8, 22) as the coating means (9, 12, 13, 15), correspondingly, so that after the at least the first coating, the control unit (1, 6, 17) is positioned against the same support roll (5, 8, 22) used by the coating agent (9, 12, 13, 15), or against three or more support rollers (5, 8, 22) are in addition to the coating the breeding means *: at least two control units (1, 6, 17). Apparatus according to claim 11, characterized in that the support rollers (5, 8, 22) are of the same size or of different sizes. Apparatus according to claim 11, characterized in that at least one applicator unit (9, 12, 13, 15) and / or a control unit (1, 6, 17) is adapted to each support roll (5, 8/22). ) for the coating amount. 14. Apparatus according to claim 11, characterized in that the distance between the pivotal supporting rollers (5, 8, 22) is smaller or equal in size to the sum of the radii of the successive rollers (5, 8, 22). . Apparatus according to claim 14, characterized in that the distance between the pk successive support rollers (5, 8, 22.) is less than or equal to 1.1 - 1.5 times the sum of the pk successive rolls (5, 8 , 22) radii. 16. Method of coating paper with a coating mixture of high-speed paper web (10), in which method one side of the movable paper web (10) is coated with a coating slurry in a defined coating sector with at least one coating agent (9). , 12, 13, 15), from the surface of the coated web, the excess coating of the coating smear is scrubbed with at least one scrubbing agent (1, 6, 17) and the scraping is made as at least the last coating step slurry, said web is supported by at least two support rolls. (5, 8, 22) in the delimited coating sector by placing at least one coating agent (9, 12, 13, 15) and at least one scrubber (1, 6, 17) against a separate support roll (5, 8, 22), characterized in that the web (10) is transported against at least two support rolls (5, 8, 22), the web (10) being coated against either one or more support rolls (5, 8, 22) with at least one in use durable coating ( 9, 12, 13, 15) and after each coating, the excess coating mixture is created from it; the surface of the coated paper wherein the control means (1, 6, 17), which follows on the coating roller (9, 12, 13, 15) are placed against the same support roll (5, 8, 22) so that at least after the first coating, the coated paper is scraped against the same support roll (5, 8, 22) used by the coating agent (9, 12, 13, 15), or in the case of three or more support rolls (5, 8 22), is coated against a minimum of one support roll (5). , 8, 22) and is scraped after at least the last coating. t ii