METHOD AND APPARATUS FOR COATING A MOVING PAPER OR CARDBOARD WEB

Abstract: The invention relates to a method of coating a moving web of paper or paperboard. In the method, the web to be coated is passed to a coater station, wherein a coat layer is applied to at least one surface of the web by means of high-pressure spraying nozzles, whereby the pattern width covered by a single nozzle is essentially narrower than the cross-machine width of the web being coated. The nozzles are adapted into an enclosure and the excess coat mist formed in the process is removed by means of suction tubes, and advantageously, with the help of a falling film of coating mix flowing down the internal wall(s) of the enclosure.

Description: Method and apparatus for coating a moving paper or cardboard web The present invention aims to provide a method according to the preamble of claim 1 for coating a moving web of paper or paperboard in a manner permitting applicator or levelling apparatuses to operate noncontactingly on the web. The invention also aims to provide an apparatus suited for implementing said method, that is, an applicator apparatus in which the coat is applied to the web in a noncontacting fashion directly as a coat layer of desired thickness. To improve the printability of paper, the paper may be coated with a coating formulation containing mineral pigment and binder components. Over the years, application and levelling of the coat have been carried out using a variety of apparatuses. Higher web speeds and increased demands on process efficiency and paper quality in combination constitute the stimulus driving the devel¬ opment of applicator equipment. Initially, paper coating with a pigment-containing formu¬ lation was performed using coaters of the gate roll type, in which the coating mix was first metered with the help of furnish rolls to a set of transfer rolls, and there¬ from further to the moving web of paper. However, the function of such a coater is impaired at web speeds exceeding 400 m/min. The nips of the rolls start to throw out splashes of the coating mix, and the coating process lacks the stability required to achieve an acceptable coat quality. Furthermore, well-behaved control of coat weight is difficult to achieve when using the above- described technique. Particularly for surface sizing, sizing presses have been used in which the downward running web is passed through a coat mix pond sealed by the rolls. Herein, a problem arises from the strong increase of moisture content in the web and difficult controllability of the correct amount of applied size. In the kiss-coating technique, the coating mix is metered directly in a nip from the casting roll to the surface of the paper web. In the early days, and in paperboard coat¬ ing even today, excess coat is doctored away with the help of an air knife. At web speeds above 500 m/min, however, the impact force of the air flow from the slot orifice of the air knife is insufficient for effective doctoring of the coat layer applied to the web surface. An essential increase in coating speed was facilitated by the adoption of the doctor blade levelling technique for controlling the final coat weight. In the first genera- tion of blade coaters, the web was arranged to run from above downward and the coating mix was pumped into a pond formed in the recess between the backing roll and the blade. In fact, the same technique is still being used in two-sided coating. The actual break-through of the blade coating technique occurred along with the adoption of the transfer coating method. Herein, the coat is applied directly to the web surface in the nip between a transfer roll and a backing roll. Excess coat is removed by means of a doctor blade extending over the entire web width. This kind of coating technique makes it possible to increase the web speed to about 1300 m/min. At web speeds above this, splashing of the coat at the nip and the air film which is entrained in the nip along with the moving web, thereby causing skip marks on the coated web, make the use of this method extremely complicated if not impossible. The higher the web speed, the fewer degrees of freedom will be available in the selection of coat mix components. Herein, the coating mix formulations must be selected under the con¬ straints of web runnability, sometimes even compromising the quality of the end product. Due to the poor runnability of transfer coaters, a short- dwell doctor blade coater was developed to provide an alternative technique for applying light coats to thin- caliper paper grades. In this type of coater, the web is guided past a slot orifice box which is formed by a short-dwell application chamber and the doctor blade and is adapted to operate against a backing roll. This method has been extremely popular in the art and facilitated effective on-machine coating. Also in this method, the maximum practicable web speed has turned out to be the limiting factor for further development. At web speeds above 1300 m/min, striping will appear at coat weights higher than 9 g/m 2 due to turbulence in the applicator flow chamber. In addition, an essential impairment of the cross-machine coat profile occurs with higher coat weights. Improvements in the design of film transfer-type coaters typically used in surface sizing of paper have extended the use of these coaters to the application of pigment coats, too. Herein, the coating mix is first metered by means of an apparatus similar to a short-dwell coater onto a transfer roll, wherefrom the coat film is further transferred in a nip of two rolls to the surface of the paper web. This novel technique was initially introduced to surface sizing and later also to the application of a pigment coat at unconventionally high web speeds. How¬ ever, problems are encountered in the form of coat mist and splashing occurring at the splitting point of the coat film when the web exits the film transfer nip. When applied at high web speeds, coats heavier than 10 g/m 2 suffer from an orange-peel texture and other low-quality surface properties incapable of fulfilling all the speci¬ fications that may be set on a finished end product. The coat splashing and web skipping problems occurring on the application roll have generally been overcome by means of the nozzle application technique, which gives a wider latitude in the direction of higher web speeds. Additionally, better capabilities of applying heavy coat weights have been attained through more effective water drainage offered by the longer dwell time. Moreover, the coat forms close to the base sheet surface a layer of higher solids content that provides support to the doctor blade, whereby blade stability is improved and cross- machine profiles of improved evenness are attained. When the nozzle-application step based on a doctor blade and a subsequent levelling step based on a scraper ele¬ ment are performed against the same backing element, a runnability complication in the form creases and/or bags in the web generally occurs. This problem can be elimi¬ nated by implementing the application and levelling steps against separate backing elements. Due to the resultant increase of dwell time and paper moisture content, some difficulties will be encountered in the runnability of lightweight and high-moisture-absorbance paper grades. The striping problem of short-dwell coaters has been alleviated with the dam blade construction known from the film transfer method of coating. However, all the above- described application methods are hampered by the mecha¬ nical contact and load imposed on the web by the coater. Particularly in blade coaters, paper production will easily be disrupted by defects in the base sheet. The paper mills have a strong drive to improve the efficiency of coater lines. Obviously, valuable production time will be lost due to web breaks. In conventional application techniques, the time to regain an acceptable quality after a web break takes an unduly long time. For wet-on-wet coating, a blade coater is not necessarily the best possible alternative. In this coating method, to the same side of the web are applied at least two coat layers so that onto the first coat, while still moist, is directly applied the next coat layer without intermediate drying. Particularly in the application of a precoat, web defects like striping and unevenness are extremely detri¬ mental. Therefore, a blade coater requires continuous control to keep the coat weight at its set value. Hence, a facility for measuring the precoat weight is mandatory in order to maintain controlled coat application. Such a coat weight measurement system operating between the successive application steps of coat layers is expensive and sometimes even impossible to arrange. Therefore, stable operation is required from wet-on-wet coaters so that the application and levelling of subsequent coat layers can be carried out without spoiling the already applied, still moist coat layers. Attempts have been made to improve runnability in paper machines and coating stations with the help of supported web threading. Herein, an extremely smooth surface is required from the support wires or belts used in coaters. Furthermore, even the smallest irregularities of backing surfaces will cause coat marking not only particularly in blade coaters, but also in transfer coaters. At higher web speeds, the rate of successfully performed flying splicing on the unwinder of off-machine coaters falls significantly. Splicing apparatuses required herein become expensive, and nevertheless problems will occur in exact timing of splicing. Therefore, future development of coaters must aim to provide an on-machine coater em- bodiment in which such problems associated with splicing and roll change cannot disturb the finishing treatment. A blade performing the doctoring of the coat applied to the web tends to accumulate aggregations of dirt under the blade edge that cause striping of the coat. Due to such coating defects, large amounts of finished paper turn into scrap. The rheological properties of the coating mix may cause web runnability problems due to the extremely strong fields of high shear rate acting on the coat mix in the blade tip region. Accordingly, the selection of possible coating mix formulations is often curtailed by the rheol- ogical constraints associated with the blade geometry. In order to overcome the above-described drawbacks, paper coating should preferably be carried out using a noncon¬ tacting method. Through the use of a noncontacting method for coating the web, defects of the base sheet are pre¬ vented from disturbing the finishing treatment. Comple¬ mented with a web threading syεtem which is fully sup¬ ported by wires and belts, it is possible to achieve a break-free, even a fully automated coating process. Herein, paper web defects can be identified by means of defect detectors and removed during intermediate winding in order to prevent them from interfering with further processing. Development of equipment for higher web speeds is no more hampered by load imposed on the web. The opacifying power of the applied coat becomes so good that the air knives, which today are the major factor limiting the maximum speed of paperboard coaters, can be replaced by the novel technique. Thus, the efficiency of coating lines and production throughput of coaters can be elevated to a remarkably high level. Noncontacting coating methods are mentioned in, e.g., patent applications PCT/US91/03830, FI 925404 and FI 933323. In the coaters discussed therein, the coating mix is fed into the nozzle via a separate duct, and atom- ization of coating mix is performed with the help of com¬ pressed air passed to the nozzle. However, tests have shown that insufficient atomization results from the use of a nozzle based on blast-diffusion by compressed air. Moreover, such a strong airflow causes excessive evapora- tive drying of the coating mix droplets before they can impinge on the sheet surface. Droplets of excessive size in the coat mist make the finished surface pitted and unsmoothly coated, which is manifested in the coat profile as craters and mounds. Patent applications FI 911390, US 248,177 and PCT/FI89/00177 discuss applicator apparatuses in which the coating mix aerosol is formed in a separate chamber or apparatus using a gas-liquid nozzle or ultrasonic diffusor nozzle. The coat aerosol is passed into an applicator nozzle, wherein the aerosol is directed by means of separate gas injection to impinge on the sheet surface. The portion of the coating mix aerosol not adhering to the web is returned by suction back into the coating mix circulation. In such an apparatus, the coating mix droplets undergo evaporation before reaching the sheet surface, whereby their adherence to the sheet is impaired. Subsequently, when the paper is used in a printing shop, a large amount of dirt will build on the printing machine rolls and the coat will release dust in the trimming and folding equipment. In the apparatus described in the patent application PCT/FI93/00453, the coat is applied using the above- described methods and then levelled using a doctor unit. This method represents a kind of direct application with the exception of its conventional doctor blade technique, whose shortcomings were described above. Noncontacting coater equipment are well-known and fre- quently used apparatuses in the art of painting and coat¬ ing systems technology. High-pressure spraying equipment with suitable nozzles are commercially available for painting. However, the use of high-pressure spraying for applying coating mix to a moving web of paper or paper- board in the fashion described in detail later is a novel application of the noncontacting application technique. In order to make it possible to spray a coating mix or material onto a surface to be coated, the fluid material must be dispersed into small droplets. This step is called atomization. The basic idea of atomization covers a variety of different uses ranging from painting to varied combustion installations, engines and apparatuses for mass and heat transfer such as gas scrubbers and evaporation towers. As a general term, atomization refers to conversion of fluid material into droplet form (that is, particles of round or similar form) . The type of the spray is categorized according to the cross-sectional shape of the spray jet. Normally, a hollow or solid coni- cal or fanned spray is used. Spray coverage is defined as the width of the spray pattern at a certain distance from the nozzle tip. The spray angle is the opening angle of the spray cone emitted by the nozzle. Atomization nozzles fall into four different classes: 1) High-pressure nozzles (pressure atomizers) 2) Atomizers based on rotary centrifugal atomization (rotary atomizers) 3) Air-assist and air-blast nozzles (twin-fluid atomizers) 4) Other methods. High-pressure atomizers are characterized in that therein atomization occurs driven alone by the internal pressure of the fluid being atomized. No atomizing air is used. In practical tests, airless atomizing nozzles have been found superior to air-blast nozzles. In pilot-scale tests of the present invention, the spray¬ ing technique was first adapted to the application step of the coating mix. Levelling of the applied coat was performed using conventional doctor blade techniques. However, this combination did not offer any benefit over prior-art nozzle application methods. Following shortcomings were found in this method: - for the nozzle types used in the test, the viscosity of the coating mix was too high to permit sufficient atomi¬ zation of the coating mix so that a smooth coat could be applied; - coating mix droplets did not gain sufficient kinetic energy to adhere and spread sufficiently on the sheet surface; and - pressure levels used in the fluid atomizing nozzles were insufficient for the atomization of the coating mix. Coating mix used in the atomization application method must have a sufficiently high kinetic energy to drive the coat droplets formed at the nozzle home against the sheet surface so as to flatten and adhere the droplets to the web surface. At higher web speeds, the droplets must also be capable of penetrating the barrier formed by the air film travelling along with the moving sheet surface. These requirements cannot be fulfilled by means of an air-blast atomizing nozzle. This is because the blasting air flow causes strong evaporation of the coat droplets, whereby the deposition and spreading of the coating mix droplets on the sheet surface is worsened. Hence, the achievable coat quality remains unsatisfactory. It is an object of the present invention to provide a noncontacting method of coat application free from the shortcomings of the above-described techniques. The goal of the invention is achieved by means of per¬ forming the coat spraying step onto the sheet surface with the help of high-pressure nozzles. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 10. The invention offers significant benefits. The present method of entirely noncontacting coat appli¬ cation, which is free from any need for coat doctoring, is capable of significantly improving the runnability of coating equipment. The method applies no strong forces loading the web, whereby coating may be carried out against a web running over a backing roll, belt or even unsupported. High-pressure airless spraying nozzles give an extremely smooth surface, which has a coat profile similar to that obtained by means of an air knife, however, with a smoother profile, in some cases even smoother than that of a doctored coat. Obviously, the smoothness of the coated web is affected by the base sheet profile, and therefore, the base sheet to be coated is advantageously run through a precalendering step prior to the application of the sprayed coat. In the method, the coat settles as a uniform layer of constant thickness on the base sheet surface, whereby a high opacifying power of the coat layer is attained. Hence, the method is particularly suited for coating only semibleached paper¬ board grades. The control of coat weight and profile is easy by way of altering the number of nozzles and coat pumping rate to each individual nozzle. On the basis of tests performed, it appears that the impact of the coat spray on the sheet does not cause strong migration of water from the coating mix into the base sheet. The method is extremely well suited for wet-on-wet coating, because the coat sprays emitted by the nozzles do not agitate the previously applied layer and the load imposed on the moist web is low. The applicator apparatus according to the invention has a simple and compact construction requiring minimal space permitting relatively free integration of the applicator as a unit of the coating line and, if so desired, even the installation of the coater unit inside the paper machine. Owing to the cost-efficient structure of the apparatus, multilayer coating at lower cost than in the prior art becomes possible, whereby the overall coat thickness can be increased, and by applying different coat layers, the paper quality can be controlled in a more cost-advantageous fashion than in the prior art and differently coated paper grades can be made more flexibly in a single coating line. In the following, the invention will be examined in greater detail by making reference to the appended drawings in which Figure 1 shows a first coating line configuration imple¬ mented using an applicator apparatus according to the invention; Figure 2 shows a second coating line configuration imple¬ mented using an applicator apparatus according to the invention; Figure 3 shows an applicator apparatus according to the invention; Figure 4 shows another applicator apparatus according to the invention; Figure 5 shows a third applicator apparatus according to the invention; Figure 6 shows a fourth applicator apparatus according to the invention; Figure 7 shows a fifth applicator apparatus according to the invention; Figure 8 shows a sixth applicator apparatus according to the invention; Figure 9 shows a seventh applicator apparatus according to the invention; Figure 10 shows a linear nozzle array suitable for use in applicator apparatus according to the invention; Figure 11 shows an eighth applicator apparatus according to the invention; Figure 12 shows a ninth applicator apparatus according to the invention; Figure 13 shows a coating mix circulation system; and Figure 14 shows another coating mix circulation system. According to the invention, the coat is applied to the web by means of high-pressure airless spraying nozzles. The fluid is atomized in the nozzle heads by passing the pressurized liquid through a small-orifice nozzle. Thence, the core component of the spray-coater apparatus is the coat-atomizing nozzle. Test results indicate that high-pressure spraying nozzles of the airless type are generally to be preferred. The fluid may be pressurized in the range of 1 - 1000 bar. However, typical pressures vary in the range 100 - 300 bar. It has been found that pressures under 100 bar can under no conditions atomize the coating mix into droplets of sufficiently small size. Typically, the spray-coater apparatus includes a nozzle assembly incorporating nozzles designed to emit fan- shaped sprays. The main axes of the fanned spray patterns of the nozzles are rotated by approx. 7 - 15° with respect to the cross-machine main axis of the nozzle set, whereby a relatively smooth coat profile results. The nozzle assembly is also characterized by an adjustment facility of the internozzle distance and the distance of the entire nozzle assembly from the base sheet. The most uncomplicated design of the nozzle adjustment is such that offers a simultaneous adjustment for all the nozzles of the system and provides as identical conditions as possible for all the nozzles. A separate adjustment for each nozzle gives certain latitude for the coat profile control over the cross-machine width of the nozzle spray pattern. Additionally, individual control of the nozzles could be used to some extent for compensation of orifice wear in the nozzles. On the basis of tests performed, it has been found that the effective practicable spray pattern width achievable by means of a single nozzle is about 10 to 30 cm. This means that from 10 to 3 nozzles, respectively, are re¬ quired per linear meter of web width. As it is plausible that a uniform coat quality cannot be attained by means of a single linear array extending over the entire web width, the spray-coater apparatus must be constructed using a plurality of linear nozzle arrays. Formation of coat mist is one of the problems of the spray-coating method needing an effective solution. Elimination of coat mist formation can be categorized into four tasks: 1) the conditions of coating mix spraying are made such that the deposition of sprayed particles on the web occurs as unobstructed as possible, which in practice means removal of the air film travelling along with the surface of the moving web; 2) such nozzle designs are selected that produce droplets of as uniform size as possible, whereby the number of droplets of small size and kinetic energy is minimized; 3) the adhesion of the coat droplets to the web is maxi¬ mized by all means, whereby such operating parameters as the electrostatic charging of the droplets, coating mix formulations and appropriate impact force of the fluid droplets on the web must be evaluated; and 4) suitable mechanical mist collector systems are used. The spraying-nozzle unit must be located so that it can be sealed sufficiently tightly against a suitable backing surface. Such surfaces are offered at least by a web- supporting roll, belt, felt or wire. In this context, the term sealing refers to air-tight sealing of the peri¬ pheral areas of the applicator unit and of the edge areas of the web as well as controlled travel of the web at the ingoing and outgoing ports of the spray-coater. Such sealing is extremely crucial for proper collection of excess coat mist. Spray-coating requires efficient removal of the air film travelling along with the web. The air film forms a barrier to the deposition of the sprayed particles on the web. As the removal of the air film also helps to reduce the formation of coat mist, the air film should be re¬ moved as effectively as possible and as close as possible to the ingoing port of the spray-coater unit. The removal of the air film can be accomplished by means of an ar¬ rangement operated in a doctor blade fashion, or alter¬ natively, by adapting an air knife to blow against the web travel direction. By contrast, removal of the air film from the web surface inside the spray-coater unit may become a complicated task, because the coat mist tends to deposit on any surface inside the spray-coater unit. Doctoring-away of the air film is an important step to be carried out just before the ingoing side of the spray- coating assembly. Such a doctoring of the air film can be implemented by means of, e.g., counterblowing based on air injection from an air knife reverse to the web travel direction. Also various doctor blade arrangements are suitable for the removal of the air film. The optimum location for such an air-layer-doctoring accessory is in the immediate vicinity of the spray-coater ingoing side. While the accessory elements may also be located inside the enclosure of the spray-coater unit, such a placement necessitates additional clean-keeping arrangements. The coating mix must be furnished into the coating mix machine tank of the coater separately for each coating run with a specific formulation suited for spray-coating. The replenishment of fresh coating mix into the machine tank can be arranged to occur continuously or batch-wise. An essential requirement herein is that the coating mix must have a homogeneous composition with suitable physi¬ cal properties. The constituents of the desirable coating mix formulation are determined separately for each base sheet type and grade. The viscosity and solids content of the coating mix are adjusted compatible with the spray- coating method. Generally, coating mix formulations optimized for spray-coating have a low solids content and viscosity as compared to coating mixes used in doctor blade coater. In the use of a spray-coater unit, at least three differ¬ ent operating modes may be categorized: 1) run mode, 2) wash mode, and 3) nozzle replacement mode, all of which can be arranged to occur without causing interrup- tion in the actual function of the coater unit. The spray-coater unit must be provided with a sufficient¬ ly rigid body, which can be sealed reasonably tightly against the backing surface supporting the web, whereby the coater unit body is arranged to include fixtures for mounting the spray-coating nozzles or nozzle arrays. The entire unit must also be attached by its body to an external support. The coater unit body shall be designed so that the different operating modes related to run, wash or replacement can be easily carried out. The attachment of nozzles to the coater unit body can be implemented in a plurality of different fashions. A basic arrangement is the assembly of the nozzles into linear nozzle arrays extending over the cross-machine width of the web, or alternatively, attaching each nozzle separ¬ ately to the body of the spray-coater unit. The linear array arrangement offers such benefits as easier robotic handling in automated removal of an entire linear array with the nozzles from the spray-coater unit for servicing and other operations. Furthermore, a linear array of nozzles is easier to provide with a common coating mix infeed channel having a single inlet port. Depending on the operating principle of the coat mist collection system, the interior of the spray-coater unit can be provided with jet flow deflectors to improve the aerodynamics of the coater unit to perform successful application of the coat to the paper web, and on the other hand, to collect excess coat mist away from the coater unit interior with maximum efficiency. The internal aerodynamic flow patterns of the coater unit can be controlled at least by means of the following elements: deflectors, steam tubes, air injection, water- moistening and sweating on surfaces (condensation) . The feed of coating mix to the nozzles must be arranged compatible with the nozzle technology used. High-pressure airless nozzles are more demanding than low-pressure spray nozzles with the regard to the feed system. An essential task, however, is presented by the need for some degree of independent control of the nozzles or nozzle arrays. In practice, this requires the infeed line to be equipped with a sufficient number of control valves. It is necessary to provide means for cutting off the coating mix feed to selected nozzles or nozzle arrays for the duration of servicing or replacement of the nozzles/nozzle arrays without causing disturbance to the operation of the other nozzles, linear nozzle arrays or spray-coater units. In the design of the coater unit, the control result can be affected by following factors: nozzle type, including ultrasonic and electrostatic techniques; application technique, together with control of nozzle distances and spray angles; and control of coat mist formation. In its simplest embodiment, the spray-coater unit comprises a linear nozzle array placed at a suitable distance from the web and having a desired spray geometry of the spraying nozzles. Using this kind of an applicator apparatus, the coating mix spray is impinged on the web so as to apply an even coat layer extending over the entire cross-machine width of the web. When the spray- coater unit is used for coat profile control, the spraying nozzle assembly need not necessarily apply a coat layer for the full width of the web, but rather, the coat profile control can be achieved by local application as desired. Besides the nozzles, a full-function spray- coater unit must include a coat mist collection system capable of recovering and/or separating the coat mist formed as an excess of the coating process. Different implementations of coat mist collection systems are described later in the text. For achieving a high quality for the paper web coated on the spray-coater unit, the most important step is the spraying of the coat onto the paper web surface. Herein, the nozzle technology used forms the principal design factor which alone determines the coat quality achievable by the present method. In various tests, high-pressure airless nozzles (operated at pressures above 100 bar) have been found to provide optimal performance in spray- coating. Single nozzles of this type can be assembled into a linear array of nozzles extending over the entire cross-machine width of the web. A facility must be provided for the collection of the coat mist formed in the spray-coating process and its separation from the air. Successful collection of coat mist requires that the spraying nozzles are located in an enclosed space separated from the environment. The volume of the sealed space can be varied widely. In its smallest form, a separate closed space may be designed about each nozzle. In its largest form, e.g. , an entire coater unit can be contemplated to be enclosed by a kind of hood. Also the enclosure of the entire coating line under a hood would be a feasible arrangement. Plausibly, the optimal size of the coat mist collection system is such that encloses a number of linear nozzle arrays. In the following, the term spray-coater unit is used to denote an applicator apparatus comprising at least the nozzles/nozzle arrays and a coat mist collec¬ tion system with its operating means. The connection of the coater unit to the other parts of the coating line is not crucial, and thence, the location of the coater unit along the coating line can be varied. In the simplest arrangement of coat mist collection, only vacuum suction is used for the removal of the coat mist hovering in the spray-coater unit. A problem in the design of this system is how to find the proper rate of air removal and select the optimal suction points so that the coating process itself will not be affected. As the function of this type of coat mist vacuum suction system is gravity-independent, the designer can freely align it in any physical position. In this technique, the actual separation of the coat mist is performed outside the spray-coater unit. The placement and size of the vacuum ducts can be varied, and internal air flow pattern of the coater unit may be optimized by selecting a suitable alignment, size and suction rate of the vacuum ducts. In another embodiment of the coat mist collection system, a falling flow of coat or other liquid down the internal walls of the spray-coater unit enclosure is used for trapping the coat mist. Here, various contraptions can be employed to guide the coat mist hovering in the coater unit to make contact with the downward falling film of coat or liquid, whereby the coat mist aerosol particles are adhered to the falling film. This arrangement requires continuous pumping to both establish and remove the falling film of coat or liquid. The separation of the coat mist occurs here already inside the coater unit, instead of taking place outside the unit as is the case in the vacuum suction technique described above. However, a spray-coater unit equipped with the falling-film coat mist separation system cannot be located freely in different positions, because gravity-assistance is required to establish the falling film. Thus, the spray- coater body design can be varied to implement the most flexible arrangement of the coat mist collection system. The two basic variants comprise the horizontally operating suction-based coat mist collection system and the vertically aligned falling-film system. The two basic techniques described above can be combined. This offers the maximal efficiency of coat mist removal. Additionally, the suction flow pattern can be arranged to force the random flow of hovering particles into an efficient contact with the falling film of coat/liquid. In the following, two examples are given of coating line arrangements which use spray-coater units according to the invention for coat application. The spray-coater sys¬ tems themselves are described in a co-pending application based on the FI Pat. Appl. No. 954,745. Shown in Fig. 1 is a simple off-machine coating line adapted for single-layer, two-sided coating of paper web. The first unit of the line is an unwinder 1, after which the web is taken to a precalender 2 comprised of, e.g., a nip of two soft rolls and one hard roll. Next after the precalender 2 is a spray-coater unit 3 in which a desired coat layer is applied to the first side of the web. The actual coater unit comprises a belt-backed coater in which the coat is applied in two steps to the web sup¬ ported by the belt. Such a coater unit is capable of applying a heavy coat in a single pass. Subsequent to the coating step, the web is threaded to an infra-red dryer 4, followed by drying on an airborne dryer 5 and finally on a cylinder dryer 6. Immediately after drying, the dried web is passed to a second spray-coater unit 7, followed by another sequence of the above-described equipment comprising drying on an infra-red dryer 8, airborne dryer 9 and cylinder dryer 10. Subsequent to drying, the paper web is recalendered on a machine calen¬ der 11 comprising four nips and rewound into rolls on a winder. The coating line of Fig. 2 differs from the above-described system in that the winder is adapted immediately after the second coater and dryer section. The coating line is complemented with different calenders such as a soft-nip calender 13 and a supercalender 14. One of the benefits of the coating lines shown in Figs. 1 and 2 is their simple structure which, nevertheless, with the help of pre- and postcalendering is capable of pro¬ viding a very smooth coat combined with the extremely good opacifying power which is characteristic of spray- coating. Additionally, the equipment of Fig. 2 can be readily modified for making paper grades of different finishes by varying the coating mix formulation and the calendering. In Figs. 3 and 4 are shown two embodiments of the spray- coater apparatus. The apparatus of Fig. 3 comprises a backing roll 15, a guide roll 18 passing the web to the backing roll and four applicator units 16 each including three parallel linear nozzle arrays 17. The linear nozzle arrays may be replaced by a nozzle assembly having the nozzles in a layout different from that of linear arrays so arranged that the coverage of the nozzles is equiva¬ lent to that of at least 1 or 2 linear nozzle arrays. Thus, this coating method is capable of applying the coat in four steps performed in a single coater unit. The nozzle arrays 17 are located so staggered that the spray jets of one nozzle array 17 are always aligned at the internozzle spaces of the preceding nozzle array. The nozzle arrays 17 are enclosed in an enclosure 25 which borders the application area against the web. The coater shown in Fig. 4 has three applicator units. As is evident from the diagrams, the applicator units have an extremely simple construction permitting them to be installed in a very small space, whereby a single backing roll 15, for instance, can be provided with 1 to 4 adjacent applicator units 16, or even more by making the roll diameter larger. Due to their compact structure and small space requirement, the applicator units can be placed almost anywhere along a coating line, even inside a paper machine, whereby this type of coater makes it possible to implement coating lines of most varied configuration. Already an applicator unit equipped with three nozzle arrays 17 can provide a relatively smooth coat, and when desired, the number applicator units can be increased to make smoother coats and higher overall coat thicknesses. Fig. 5 shows a belt-backed coater unit. This unit in- eludes two belt-guiding rolls 19, over which a support belt 20 running parallel with the web is passed. The applicator units 16 are arranged to rest tightly sealed against the support belt 20, and the web is arranged to pass in front of the applicator units backed by the sup- port belt 20. A scraper 21 is adapted to work in coopera¬ tion with the other backing roll 19 for keeping the belt 20 clean. With the help of such a belt-supported applica¬ tor device, a substantially great number of the applica¬ tor units 16 can be adapted into a single coater unit if so desired. The most significant benefit of the apparatus shown in Fig. 5, which has the applicator units placed in a succession over the linear support belt 20, is that the web can be passed to the coater unit directly without changing its direction. If the drying of the web is implemented using noncontacting dryers, such as infra-red or airborne dryers, this type of a coater unit makes it possible to configure the entire coating line so that the web to be coated passes straight through the entire length of the coater installation. Because the spraying nozzles must be changed at certain intervals due to wear and dirt accumulation, the coater unit is provided with a robotic nozzle changer 22 for automatic replacement of the nozzles 23. In Fig. 6 is shown a belt-supported applicator unit having the web adapted to pass backed by a support belt 20 over a belt-guiding roll 19. To both sides of the endless belt are placed two applicator units 16 and the robotic nozzle changers 22 of the nozzles 23. Such a coater unit is extremely flexible in use. For instance, it is possible to have one of the applicator units 16 continuously set off-duty due to nozzle change or clean¬ ing, or alternatively, a schedule can be drawn so that at any one of the applicator units on one side of the sup¬ port belt can withdrawn off-duty due to servicing. Also herein, the belt 20 is cleaned by means of a scraper 21. In Fig. 7 is shown an applicator unit 16 of the above- described type including a reverse-blowing air-knife assembly 24 for removal of the air film travelling along the web. The reverse-blowing air-knife 24 comprises an air tube placed at the incoming edge of the applicator unit enclosure 25 and having a slot orifice 34 for blowing an air jet reverse to the travel direction of the web. Further, the applicator unit incorporates a coat mist collection system comprising vacuum ducts 26 for removal of hovering coating mix aerosol from the appli¬ cator unit enclosure 25. Close to the outermost nozzle arrays 17, at a distance from the inner walls of the enclosure 25, are adapted air-flow deflectors 27 serving to form, with the help of the flow induced by the suction of the vacuum ducts 26, such an air flow passing along the inner walls of the enclosure 25 that can remove the coat mist escaped from the main spray emitted by the nozzles 23 away from the enclosure 25 without disturbing the spray pattern of the nozzles. In the apparatus shown in Fig. 8, the removal of the air film from the web surface is arranged by means of a mechanical scraper 28, while the vacuum ducts 26 for coat mist removal are adapted between the nozzle arrays. This arrangement is suitable for use at lower web speeds, wherein the amount of air film travelling along the web and the effect of the excess coat mist formed from the spray have a lesser effect. In Fig. 9 is shown another method of coat mist collec¬ tion. Therein, to the sides of the applicator unit en¬ closure, close to the ingoing and outgoing points of the web, are placed coating mix infeed tubes 29 having slot orifice openings 30 for feeding coating mix onto the in¬ ternal side walls of the enclosure 25. The suction ducts 26 for removal of collected coat mist are located at the lower corners of the side walls. In this arrangement, the liquid film falling down the side walls captures the coating mix aerosol particles hovering in the enclosure 25 and conveys the collected coat mist directly into the suction tubes 26. In Fig. 10 is shown a detachable linear nozzle array which can be employed instead of individually detachable nozzles 23. The nozzle array comprises a manifold tube 31, along which the coating mix is fed to nozzles at¬ tached to the manifold tube 31, and fixtures 34 for con- necting the nozzle array to the applicator unit 16. Thus, it will be easy to change the entire nozzle array as an integral entity. In Fig. 11 is shown an applicator unit suitable for mounting in a vertical position. Such an applicator can be used, e.g., in belt-backed coater equipment having the belt arranged running vertically. In the enclosure of this applicator unit, the side wall lower edge facing the web is equipped with a coating mix infeed tube 29 serving to the feed coating mix along the side wall. The suction tube 26 for the removal of the collected coating mix is located at the lower corner edge of the same side wall. The coating mix aerosol hovering in the enclosure is directed to impinge onto the coating mix film falling down the interior side wall of the enclosure with the help of blow tubes 33 from which in the area remaining between the nozzle arrays 17 air or steam is injected toward the falling film of coating mix that captures the aerosol particles of hovering coating mist. In Fig. 12 is shown an applicator unit which is equipped with a reverse-blowing air knife assembly 24 adapted therein for the purpose of removing the air film travel¬ ling on the web surface and additionally has coating mix infeed tubes 29 adapted to the interior side walls of the enclosure 25 for forming a falling film of coating mix that captures the hovering coat mist. Additionally, the enclosure houses tubes 33 adapted between the nozzle arrays 17 for injection of air or steam and further includes air flow deflectors 27 serving to guide the flow of the hovering coat mist toward the interior side walls of the enclosure 25. In Fig. 13 is shown an embodiment of the coating mix circulation. Therein, the coating mix is pumped from a coating mix machine tank 35 by means of a high-pressure pump 36 via pressure accumulators 37 serving to equalize the feed pressure of the coating mix, and from the press¬ ure accumulators 37, the coating mix is taken to the linear nozzle arrays 17. A separate low-pressure pump 39 is used to feed the coating mix from the machine tank 35 to the coating mix feed tubes 29, and the excess coat mist together with downward-falling flow of coating mix collected from the interior walls is removed from the applicator unit enclosure 25 by means of a pump 38. The removed coating mix with abundant air entrained is taken to a strainer 40, wherein aggregates are filtered away from the coat, which is then returned to the coating mix machine tank. In Fig. 14 is shown another coating mix circulation arrangement having an additional lamellar or cyclone separator via which the coat mist with the large volume of entrained air can be passed. In Fig. 15 is shown the most preferred arrangement of coating mix circulation having all of the coating mix, which is removed from the applicator unit, advantageously taken to a lamellar or cyclone separator for separation of entrained air. Typically, the coat returned from the spray-coater contains so much entrained air that effi¬ cient air separation is a mandatory step before the coating mix can be recycled back to the coating mix machine tank. In the circulation system illustrated herein, the separator is provided with an additional infeed from the coating mix machine tank 35 to aid the separation of air from the coating mix. The circulation system of Fig. 15 additionally includes a washing line comprising a water tank 43 with a pump 44 and valves 45 for feeding water to the nozzles 17 and the pressure accumulator 37. The method according to the invention has been applied in coating tests with results discussed below. Coating of a full-width web in the tests was generally successful, even to an unexpectedly good degree. Three adjacent spraying zones did not provide a sufficient capacity for attaining high web speeds. The coating capacity was approx. 10 g/m 2 at 220 m/min web speed and approx. 5 g/m 2 at 470 m/min. The solids content of the coating mix was 40 %. This test did not aim at deter¬ mining the maximum performance values of the method. Spray-coating is hampered by strong dusting of the spraying point environment by coating mix particles. The atomized spray of small coating mix droplets can spread everywhere along with air streams unless collected away in a controlled manner. Additionally, the air film travelling with the moving web surface tends to drag along the dust. In the test runs, a blade made from a polymer sheet was used for doctoring the air film away. The kinetic energy imparted to the sprayed droplets must be sufficiently high, particularly at high web speeds, in order to prevent the moving air film from entraining the coating mix spray even before the spray can impinge on the web surface. In the test run, the capacity of the nozzles per unit time was measured. When the amount of coating mix adhering to the web is known, also the portion lost in the environment can be calculated. Adjustment of suction fan capacity was found to affect the applied coat weight to a significant degree. The stronger the suction, the less coat could be deposited on the web surface. The capacity of the nozzles was measured for two differ¬ ent types of nozzles. Nozzle code FF-610 indicates a nozzle with 60° spray angle and 0.010" (0.254 mm) nozzle orifice diameter. The other nozzle tested was with the same spray angle but with 0.012" (0.305 mm) orifice. The actual tests were performed on the FF-610 nozzle at 160 bar pressure, whereby the nozzle output was 7.5 g/s of wet coating mix. The coating efficiencies (portion of coating mix adhered to the web from the overall amount of sprayed coating mix) at different web speeds are calcul¬ ated in Table 1. Table 1. Test point Av. coat weight Web speed Coating over 1 m unit [m/min] efficiency width of web [%] [g/m 2 ] 218 10.0 220 87 221 5.0 470 93 223 7.5 280 83 229 5.0 449 89 As can be seen, the coating efficiency varied in the range 83 - 93 %. On the average, the loss of sprayed coating mix was 12 %. The webs were measured for cross-machine profiles of base weight, ash and caliper. To speed the measurement, all five profiles were printed sequentially into the same profile plot. The measurement results shown that the fan-shaped spray pattern of the individual nozzles remains very clearly detectable and the coat weight profile is peaked. Profile deviation from nominal coat weight can be as much as about 6 g/m 2 per side. A peak is seen in the coat weight profile at the intersection of the fan edges. Examination of the coat profiles gives a peak-to-peak deviation of 40 - 60 % from the overall coat weight. An interesting observation is, however, that the profile errors are not particularly visible on the finished product, which tells of the good opacifying power of the coat. The edge areas of the sprays can be blended smoother by making the spray angle of the nozzles wider, and the greater number of spraying zones required at higher web speeds will finally reduce the fan intersection errors to an insignificant level. Low web speeds necessitate the use of lower-output nozzles to prevent the errors of a single application zone from becoming excessively pronounced. When arranged into a three-row array, the nozzles tested herein are sufficient for applying a coat weight of 10 g/m 2 at 220 m/min web speed. To apply the same coat weight at a web speed of 440 m/min, the spray-coater would require a 6-row nozzle assembly, for a web speed of 880 m/min a 12-row assembly and so forth. Then, the profile error caused by a single nozzle will be reduced respectively. While the coat profiles of a paper passed through a Sy Sizer size press are peak-free, a certain amount of skew toward the drive side can be seen. A pronounced valley occurs in the coat weight profile very close to the drive-side edge. Prior to the tests, the greatest doubts were expressed with regard to the surface strength of the sprayed coat. Intuitively, the coat mist was expected to settle in the same fashion as snowflakes on the sheet surface. However, no differences could be found in the coat surface strength in contrast to paper passed through a SymSizer size press. Also the rolls of the supercalender and the printing machine remained free from buildup of coat dirt. Additionally, such a high coat surface strength indicated that the coating mix does not undergo phase separation when exiting the nozzle. The coated paper was supercalendered to test runnability of spray-coated paper on a full-scale supercalender and to compare its behavior with that of supercalendered paper passed through a SymSizer size press. The spray- coated paper grades were found uncomplicated to run on a calender. The calender rolls remained free from buildup of coat dirt. The spray-coated paper grades were readily printable. On the basis of samples returned from the printing shop, the following observation could be made: spray-coating is a viable application method for coating a web; - buildup of coat dirt on the rolls of printing machines using spray-coated paper remains insignificant; a pronounced difference is seen between the surfaces of transfer-coated and spray-coated paper grades that becomes more accentuated at higher coat weights; - spray-coating gives a smoother visual appearance, but not as good a printed surface gloss and density as that offered by a transfer-coated paper; orange-peel texture is more pronounced on a transfer coated sheet; - supercalendering of the base sheet clearly improves the surface quality of spray-coated paper. Overall results of web coating by spraying techniques widely surpassed the expectations laid on the method. Paper surface strength in calendering and printing is imperative prerequisite for further development of the method. At least on the basis of tests performed, suffi¬ cient strength of coat surface seems to be attainable. As compared visually to a comparative sample passed through a SymSizer size press, the paper surface and printing quality seemed smooth, even promising. Under visual examination, the print gloss and density of spray- coated paper did not reach the quality level of the comparative sample. The paper surface appears well-opacified and no sign of "cracker bread" effect (that is, splashing of coat as large droplets on the sheet surface) was present. Obviously, due to the fully conformant deposition of the coat layer applied by the spraying technique, the method has some special characteristics and thus sets certain requirements for the coating process. Accordingly, the base sheet should have a maximally smooth surface. The operating life of nozzles could not be evaluated within the time span of tests performed. Experiences from similar nozzles used in painting technology indicate that the nozzle life will be rather limited, because abrasive wear of the nozzle causes progressive narrowing of the spray angle and widening of the nozzle orifice, whereby both the surface quality and coat profile will suffer. Therefore, the service life of nozzles in the spraying of coating mixes need to be assessed in detail.

Claims: Claims : 1. A method of coating a moving web of paper or paper¬ board, in which method the web to be coated is passed to a coater station, wherein a coat layer is applied to at least one surface of the web, c h a r a c t e r i z e d in that - the coat is applied by spraying the coating mix on the web surface by means of high- pressure spraying nozzles extending over the cross-machine width of the web, whereby the pattern width covered by a single nozzle is essentially narrower than the cross-machine width of the web being coated. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the coat is sprayed using at least three nozzle arrays (17) aligned in the cross-machine direction to the moving web. 3. A method as defined in claim 1 or 2 , c h a r a c ¬ t e r i z e d in that the cross-machine coat profile of the web is controlled by closing and opening the of individual spraying nozzles (23) . 4. A method as defined in claim 1, 2 or 3, c h a r ¬ a c t e r i z e d in that the amount of coat applied is controlled by closing and opening the individual nozzle rows (17) . 5. A method as defined in any of foregoing claims, c h a r a c t e r i z e d in that the spraying pressure at the nozzles is 1 - 200 MPa, advantageously 1 - 20 MPa. 6. A method as defined in any of foregoing claims, c h a r a c t e r i z e d in that the distance of the spraying nozzles from the web is 10 - 500 mm, advan¬ tageously 80 - 150 mm. 7. A method as defined in any of foregoing claims, c h a r a c t e r i z e d in that the distance between adjacent spraying nozzles (23) is 5 - 500 mm, advan¬ tageously 30 - 200 mm. 8. A method as defined in any of foregoing claims, c h a r a c t e r i z e d in that the web is supported on the opposite side with regard to the sprayed side by means of a support element, advantageously by a belt. 9. A method as defined in claim 1, c h a r a c t e r - i z e d in that the coat is sprayed on the web using at least one nozzle assembly. 10. An applicator apparatus for coating a moving web of paper or paperboard, said arrangement including - an enclosure (25) open toward the web to be coated, - means adapted into the enclosure (25) for applying coating mix to the web to be coated, and - supporting means (15, 20) for supporting the web from the opposite side with regard to the open side of the enclosure (25) , c h a r a c t e r i z e d in that - said means for applying the coating mix are high-pressure spraying nozzles (23) adapted into at least one row (17) extending over the cross-machine width of the web and having the pattern width on the web covered by a single spray being essentially narrower than the width of the web being coated. 11. An apparatus as defined in claim 10, c h a r a c ¬ t e r i z e d by comprising at least three linear nozzle arrays (17) in which the nozzles (23) of one array are always at least partially staggered with respect to the nozzles of the adjacent array. 12. An apparatus as defined in claim 10 or 11, c h a r a c t e r i z e d in that said supporting means is a backing roll (15) and, operating against this backing roll (15) , are adapted at least three applicator units (16) comprising an enclosure (25) and high-pressure spraying nozzles (17) . 13. An apparatus as defined in claim 10 or 11, c h a r a c t e r i z e d in that said supporting means is a belt (20) and operating against this support belt are adapted at least two applicator units (16) comprising an enclosure (25) and high-pressure spraying nozzles (17) . 14. An apparatus as defined in any of foregoing claims 10 - 13, c h a r a c t e r i z e d by a reverse-blowing air-injection assembly (24) adapted to the ingoing edge of the enclosure (25) , said assembly comprising an air tube having a slot-orifice opening (34) aligned reverse to the travel direction of the web to be coated. 15. An apparatus as defined in any of foregoing claims 10 - 13, c h a r a c t e r i z e d by an air-knife (28) adapted to the ingoing edge of the enclosure (25) . 16. An apparatus as defined in any of foregoing claims 10 - 15, c h a r a c t e r i z e d by at least one coating mix infeed tube (29) adapted to feed a falling film of coating mix onto the wall of the enclosure (25) of the applicator unit (16) .

Inventors: KINNUNEN JORMA, KIRVESKARI ERKKI, KOSKINEN JUKKA, LUOMI SEPPO, NISSINEN VILHO, KINNUNEN, JORMA, KIRVESKARI, ERKKI, KOSKINEN, JUKKA, LUOMI, SEPPO, NISSINEN, VILHO
Applicants: KINNUNEN JORMA, KIRVESKARI ERKKI, KOSKINEN JUKKA, LUOMI SEPPO, NISSINEN VILHO, VALMET CORP, KINNUNEN, JORMA, KIRVESKARI, ERKKI, KOSKINEN, JUKKA, LUOMI, SEPPO, NISSINEN, VILHO, VALMET CORPORATION
Application year: 1996
Main classification: D21H23
IPC codes: B05B1, B05B12, B05B13, B05B15, B05C11, B05D1, B05D7, D21H23