The objective of the invention is a method of and apparatus for coating paper, cardboard or a corresponding material.

Paper and board coating aims at improving certain properties of these products, especially their printability and surface smoothness. As coating materials are used water suspensions of suitable mineral pigments, such as e.g. kaolin, talk, carbonates, chalk etc. as so called coating slips, which contain further i.a. binding agents, dependent on the use of the end product. Paper generally requires coating on both sides, which can be applied either simultaneously or as consecutive processes. Surface sizing can also be used in coating, e.g. in copy paper making, when only size is used as a coating material.

Cardboard usually requires only a one-side surface treatment. The application is generally done by leading the web to be treated through the coating station, the main structure features of which are presented below. The coating process comprises two main phases, namely the actual feeding of the coating slip, i.e. the application to the web surface in excess, and thereafter the doctoring and leveling of the resulting coating slip layer into a homogenous and even coating.

There are different types of coating methods and apparatus, the applicability of which for different paper and board qualities depends on process variables and the desired quali¬ ty properties of the end product. Such process variables are e.g. the amount of coating and the speed of the web during the coating process. The quality properties are also affected by the slip types and the amount of coating layers, which in turn put demands on the coating method and apparatus.

As is known in the art, the application of the coating to the web surface can take place either as an application roll

application, in which the slip is distributed to the paper web by rotating a roll dipped into the slip, as an extrusion or jet application, in which the coating composition is delivered to the web surface by an extrusion jet, a short dwell time application, in which the coating composition is lead to an application chamber just behind the doctoring blade, having as one wall the paper web supported by the backing roll, whereby the travelling web draws the coating to its surface, which is being immediately doctored, or by using a special application doctor blade arrangement. The leveling of the coating is made by using a trailing blade, which controls the final quantity and/or thickness of the coating. Between the initial coating phase and the actual doctoring, is further also introduced a so called leveling blade to steer the flow of the coating.

All presently known coating methods are based on the concept that the application of the coating as well as the control of its quantity by a trailing blade or some other doctor blade take place against one and the same backing roll. This re¬ stricts considerably the possibilities to choose the optimum size, shape and location of the application and doctoring devices.

Two main high speed-methods are at present in use. By high speed is today meant application speeds exceeding 800 m/min. One of these methods is a roll-application/trailing blade- combination and the other one is a so called Short Dwell Time-application. The first mentioned is called the LDTA- method (Long Dwell Time Application) .

The main difference between these methods is the time differ¬ ence, i.e. the dwell between the application moment and the blading moment. In the Short Dwell Time-method, the dwell is very short, as the doctoring blade is placed at one edge of the slip feeding jet, so that the slip is actually fed just behind the blade. The arrangement is primarily the same as the apparatus (1, 2, 3) against the blade (5) presented in

fig. 2.

In the LDTA-method, the dwell is longer, as the application is made by using a roll or an application jet furnished with a pre-doctoring blade, whereafter the doctoring is made by a separate blade. According to the method, the coating slip is applied to the paper surface with an application jet fur¬ nished with a pre-doctoring blade and with an application roll, whereafter the doctor blade removes the excess of the slip. The long dwell is caused by the fact that the applica¬ tion device and the doctor are placed against one and the same backing roll, by which they cannot structurally be placed close to each other. It has been experienced, that in the LDTA-method, consecutive applications without intermedi- ate drying will not succeed, because the consecutive applica¬ tions tend to mix and contaminate the previous coating and the absorption of the binding agents separately into the paper. The dwell between the coating and the doctoring has to be short in order to prevent the coating material binding agents to absorb into the web.

The applicability of the methods at different speeds, indi¬ cates that the roll-application functions well only at speeds below 1000 m/min, when again the short dwell time application method functions well only at web speeds over 1000 m/min. The application roll starts creating problems at speeds exceeding 1000 m/min. The increased rotation speed of the roll, pre¬ vents the even distribution of the slip due to splatter and flow disturbances, causing an increase in coating defects. With the short dwell time application method again, a suffi¬ cient amount of coating material cannot be attached to the paper surface when the speed is below 1000 m/min.

The objective of the invention is to provide a coating method and a coating apparatus, operating within a broader range of speed and coating quantity than previously known methods and devices. Further, the different modifications of the appara¬ tus according to the invention can quickly be taken into

operation when switching from one paper quality to another, thus optimizing the application process including its meth¬ ods, slips and coating quantities, without losing in cost efficiency.

The problem with the known application devices, having against one and the same backing roll an application jet fur¬ nished with a blade and a separate doctor blade, has been the poor runability of the application device. The paper is wrinkled before the doctor blade, as a result of which the paper can break at the doctor blade, later at the dryer or reeling drum. For this there can be several reasons. E.g. the paper between two roll nips, applied with aqueous coating slip, absorbs water from the slip, thus decreasing the ten- sion of the paper between the nips.

Conventionally, the application of the paper surface with two coatings is made in consecutive phases, in which the applica¬ tion means, two SDTA-stations, are located in sequence against the same backing roll, the so called wet on wet- method, or the coating station and the doctor are initially placed against a first backing roll, and after the drying phase, a second layer of coating is applied to the dry and once coated paper (wet-on-dry) in a separate coating station against a second backing roll. Separate coating stations being in question, one coating station has only one backing roll.

The US Patent 5112653 describes a coating station, which is against one and the same backing roll, or two stations, comprising an applicator 42 and a primary and secondary trailing blade 44, 46. The Fl-patent 71801 describes the control of the amount of coating in one phase either by air¬ brush 4 or in the nip between the rolls 9 and 17. The publi- cation describes the drying of the coating. Two or several backing rolls to form the coating nips are not presented.

The Fl-patent application 915964 describes the method of and

an apparatus for coating travelling webs, having against one backing roll an applicator, whereafter the coating is doc¬ tored against this backing roll in one or several phases. The Fl-patent application 930757 describes the coating of a paper web with a coating layer, having against one backing roll an application roll and a control plate or plates for the excess flows of the coating, and one doctor blade.

The Fl-patent 71801 presents paper coating. The coating material is fed to one side of the web, whereas a vacuum is directed to the opposite side of the web, and the coating is simultaneously dryed by heating. The vacuum is said to im¬ prove the smoothness of the paper. Against the coating roll and the doctor blade there is one and the same backing roll.

The US-patent 4 963 397 presents an applicator, having a doctoring unit placed downstream. There is a smoothener in order to avoid a turbulence before the doctor blade. Against the application roll and the doctor blade there is one and the same backing roll. Wochenblatt 23/24, pp 1056-1062 Sommer et al. describes a so called Combi blade structure, with an exchangeable trailing blade against one and the same backing roll.

The method and apparatus according to the invention can better control changes caused by the visco-elastic character¬ istics of the paper during the application phase than conven¬ tional application methods. A decisive difference compared to other currently used application methods, is, that this invention provides at least two backing rolls in the same coating station, having an application unit as at least one coating nip, and alternatively an application unit or a control unit for the coating amount, i.e. favorably against each coating nip there is one backing roll. The coating nip is simplest a doctor blade, with a backing roll on one side. The application can especially favourably be carried out in embodiments with two backing rolls, so that the application is conducted against both the backing rolls, and the doctor-

ing is made against at least the latter one. There can also be more than two backing rolls, e.g. three. Thus, the appli¬ cation and doctoring is conducted against at least the first roll, the application and/or doctoring against the second roll and doctoring is conducted against at least the last roll. The application and doctoring can also be conducted against the same roll, whereafter e.g. doctoring is conducted against only one or favorably several backing rolls. This provides especially wide application possibilities for small and big coating amounts with the same apparatus, because the moistening / visco-elastic change in the paper caused by the coating, can now be controlled by a completely new control quantity (the tensile difference between the coating nips). By adjusting the angular speed of the adjacent backing rolls, an even run of the paper is achieved without wrinkles or sacks before the doctor blade or the application device. By this the slackening of the paper web due to moistening can be taken into account with different draws of the backing rolls.

The invention can also be applied to already existing appli¬ cation devices with only minor structural modifications. Variations in and combinations of different slips are more easily implemented.

The method according to the invention is characterized in what is disclosed in Claim 1, and the apparatus is character¬ ized in what is presented in Claim 11.

The invention and its main features are below presented with reference to the structural examples disclosed in the en¬ closed drawings.

Fig. 1 presents a schematic illustration, in side view, of a prior art coating station, having an application roll and a trailing blade

Fig. 2 presents a schematic illustration, in side view, of a simple embodiment of a coating sta-

tion according to the invention.

Fig. 3 presents one form of embodiment of a coating station according to the invention.

Fig. 4a and 4b present a schematic illustration, in side view, of two different embodiment solutions of a coating station according to the invention.

Fig. 5 presents one form of embodiment of the subject invention.

Fig. 6 presents an additional form of embodiment of a coating station with more than two backing rolls.

Fig. 7 presents the control of the coating amount with two doctor blades after the application.

Fig. 8 presents a diagram of the coating amount as a function of the web speed.

In the description below, the following expressions are being used for the coating station, the coating nip, the applica- tion unit, the coating amount control unit and the backing rolls. The coating station means the apparatus used for paper coating to achieve one or several coating slip applications, and one or several controls of the coating amount. The coat¬ ing station comprises one or several coating nips, but does not contain other process phases essentially related to the application (e.g. the actual drying). In the paper nip, the paper web is subjected to mechanical/hydrodynamic forces. These forces are caused by the fact that the paper web trav¬ els through the nip, having on one side a backing roll and on the other side alternatively a pressure impulse caused by the slip application (application unit, e.g. in extrusion or jet application) or a mechanical element caused by mechanical stress (coating amount control unit) . The application/distri-

bution of the coating slip to the surface of the paper web against the backing roll, takes place in the application unit. In some embodiments, a pre-control of the coating amount is connected to the application unit. The control unit of the coating amount forms the coating nip, so that one side of the paper web there is a backing roll and on the other side some mechanism used to control the coating. Generally some mechanical doctor blade is used for the control of the coating amount, but also an air-knife can be used. The back- ing roll is a practical roll on one side of the application unit or of the coating amount control unit. The backing rolls can operate by the web friction, or can be driven by e.g. a motor. The coating units and the coating amount control units and the backing rolls are as such prior art equipment, and they project across the width of the web or slightly beyond it.

In the prior art structure in Fig. 1, the web 10 is led to the roll 5, which operates as a backing roll of the coating station. Against this is provided an application roll rotat¬ ing in the slip-pool 12, which roll corresponds to the nip between the coating slip rolls 5 and 11, in which the web 10 obtains a defined coating layer 18 from the coating station operation parameters (the compression between the rolls 5 and 11, the rotation speed of roll 11, etc.) Against the roll 5 is further arranged a bar 9, to which has been attached a doctor blade 6, which removes the excess from the coating 18, so that after the coating station 5/11, the web surface contains a desired amount of coating slip as a uniform layer 18'.

In the coating station alternative presented in fig. 2, the web 10 is led to a web guide roll 5, being the first backing roll in this coating station. The coating slip C is led through the jet chamber 3 to the application jet 2 and there¬ after to the application blade 1, which forms an application nip together with the roll 5. These means 3, 2 and 1 of the application unit A are placed to rest on a special applica-

tion bar 15, being supported by its axle journals in the paper machine or corresponding frame, the purpose of which arrangement is described below in connection with fig. 4b.

Part of the coating slip C moves to the surface of web 10 and the main part of it is returned to the application unit return tray 4. The coating slip C forms on the web surface the layer 18.

In the application unit A, the precoated web continues its travel to the backing roll 8, against which has been placed a doctor blade 6, supported by a special bar 9. This blade 6 operates as a leveling doctor, controlling the final amount of the coating 18'. By means of the doctor blade 6, the excess of the coating slip C flows to the return tray 7 of this doctor unit 9.

Fig. 3 presents an embodiment of the application method according to the invention in a roll-application, by which the application unit A comprises only the rolls 5 and 11. When describing the different parts of the device and their operations, the same reference numbers have been used as in fig 2, when applicable.

The coating slip C is applied in a coating station according to fig. 3 to the surface of web 10 by the application roll 11, which rotates in the slip tray at a controlled speed, so that a suitable amount of the coating 18 is applied to the surface of web 10. The amount of the coating 18 is adjusted by the rotation speed of the roll 11, and also by the size of the gap between the rolls 5 and 11.

The precoated 18 web 10, after the coating unit A formed of the rolls 5 and 11, is transferred in the same way as pre- sented in fig. 2, to the roll 8, against which operates the doctor blade 6, supported in the bar 9, by which the final control of the coating amount is made, leaving at the web 10 surface the desired amount of coating 18'. The excess C of

the coating slip doctored from the precoated web surface by the doctor blade 6, flows back to the slip tray 7, which is supported in the bar 9. The arrows indicate the rotation directions of the rolls.

In the embodiments of the subject invention in figs. 4a, 4b and 5, is provided, in connection with the roll 8, a combined application and control bar 13, the structure of which en¬ ables a different handling of the web 10 that comes to roll 8 through the roll 5. In connection with the bar 13 is provid¬ ed, in addition to a control blade 17, also a traditional short dwell time coating unit 20, so that these can be used optionally depending on the process requirements. For the possible feeding of the coating slip used for the SDTA-coat- ing, the structure of bar 13 is provided with a so called extrusion or jet applicator, which comprises a slip feeding chamber 16 and a jet 20 as well as a blade 17.

According to fig. 4a, the reference numbering of which pri- marily corresponds to the references of fig. 2, the slip feeding jet 16 in the bar 13 has been closed with the barrier 16a, by which only the doctor blade 17, belonging to this bar, is in use. In this case, the embodiment of the invention as presented in fig. 4a, is functionally identical with the one presented in fig. 2. The doctoring device 17 corresponds to the doctor blade 6 in fig. 2, and the bar 13 corresponds to bar 9 in fig. 2.

Fig. 4b presents a structure, in which the bar 13 operates as the only coating unit against roll 8 in the application method according to the invention. It has thus to function according to the SDTA principle. In this case, the applica¬ tion bar 15, functioning against the roll 5 presented in figs. 2 and 4 a, is not in use at all, but has been turned by its axle journals in a lower position, and is thus not par¬ ticipating in the coating system. In this case, the roll 5 operates as a conventional web alignment roll.

Fig. 5 presents an arrangement enabled by the invention to double-coat the web. The apparatus comprises the application units A and B. Unit A is the same as presented in fig. 2. The web 10, thus applied with the coating 18', moves forward to the application unit B, being formed of the backing roll 8 and the SDTA-unit 20 operating against it, which enables the application of the additional coating 19 to the surface of the web 10 and the already existing coating 18'. The coating unit B thus provides for an additional coating to the web surface. The function corresponds to the one presented in fig. 4b.

Fig. 6 shows a picture of an additional embodiment of the invention, in which an own backing roll 5, 8 and 22 is pro- vided against the web coating and/or coating amount control device. Against the first backing roll 5, there is an appli¬ cation roll device, as presented already in fig. 3. The web 10 is led to the second coating unit 15, being against the backing roll 8, and which to its functional principles corre- sponds to the jet-application method equipped with a pre- doctoring blade presented in figs. 2 and 4b. The web 10 is in this case lastly led to the doctor blade 6, where the final control of the coating amount takes place. C' presents the excess of the slip applied by the roll 11, and the excess of the slip applied by the coating unit 15. C'' depicts the material removed by the doctor blade 6. 18''' is the coating remained on the paper web. The coating compositions and qualities can naturally be changed. The amount of coating can even be over 20 g/m2. This so called wet-on-wet application is an especially beneficial solution of the embodiment ac¬ cording to the invention. In this the paper is moistened twice, which gives a double benefit to the use of several backing rolls.

Fig. 7 presents a coating station containing a first backing roll 5, against which there are the application units 11, 12. The application is followed by two doctoring phases 1, 6, against each of which there is an own backing roll 8, 22.

When the weight of the coating in the application unit is approx. 50 g/m2, the weight after the first doctoring 1 is approx. 10 g/m2 and after the second doctoring 6 approx. 5 g/m2.

Fig. 8 presents a diagram of the coating amount as a function of the web speed. The alternative combinations of the inven¬ tion, enables also covering of the ranges, having coating amounts of even over 20 g/m2 and on the other hand of 5 g/m2 or less, as indicated by diagonal lines /. The speed of the paper web can be adjusted according to need. Combinations to achieve a low coating amount, are e.g. SDTA, extrusion or brushing and favorably at least two doctorings against dif¬ ferent backing rolls. A coating amount of 20 g/m2 is possi- ble, if at least two consecutive SDTA-coatings, extrusions and/or brushing are being used, and the doctoring is conduct¬ ed at least against the last backing roll after the last coating.

Further, can be established that the backing rolls 5, 8, 22 can be of the same or different size, wherein the distance between the centres of the adjacent rolls is smaller or same than twice, favorably 1,1 - 1,5, the amount of the radii of the adjacent rolls. The angular speeds of the backing rolls can be adjusted so that the peripheral angular speed of the latter one can be the same or bigger than the former one. Wrinkles caused by moistening can thus be eliminated. The diameters of the backing rolls can vary, but are usually approx. 600 - 1000 mm. The doctor blade can be formed of e.g. two blades, of which the one removes more coating than the other. Against each coating station or doctor blade can be provided an own backing roll. Different products can be manufactured on the same production line. The running speeds of the invention can, according to diagram 8, vary depending on the coating amount, from below 800 m/min to even over 1600 m/min, providing acceptable paper quality and running proper¬ ties.

The coating station of the subject invention is favorably in embodiments with three backing rolls, maximum 4-5 times the total length of the diameters of the backing roll, and in embodiments with two backing rolls, 2,5 - 4 times the total length of the backing roll diameters.

The invention has been described with reference to only some of its favorable forms of embodiment. The invention is not to be considered as so limited, but all modifications within the scope of the inventive idea defined by the claims are possi¬ ble. Different coating or application methods and coating apparatus or coating slips or their combinations can be used, as well as combinations of coating station in two or three application phases. The dwells can be considered better than before. A considerable improvement of the prior art has been achieved.