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Method and apparatus for treating a fibre web

Method and apparatus for treating a fibre web description/claims

The present invention relates to a method according to the preamble of Claim 1 for processing a fibre web, which method includes the spreading of a size mixture, a coating, or a corresponding processing mixture on a board web in particular. The invention can also be applied to paper webs and other similar fibre webs.

The invention also relates to an arrangement intended to apply the method.

The application relates to sizing, pigmentation, and coating and the processing of a web by calendering. Hereinafter the invention is described using as an example sizing in particular, which is the most preferred application of the invention. The invention can, of course, also be applied to the other purposes referred to above.

The term size usually refers to a water-starch mixture, which is spread on the surface of board or paper and is pressed inside the web, for example, in a nip between rolls. Size is used to increase the strength of the web. Size also improves the surface properties and printability of paper. In addition to starch, the size mixture can also include many additives while in some cases latex can also be used as the binder in the size. In principle, the present invention can be applied to all size mixtures and to different kinds of fibre web. The strength of the product is one of the most important properties of some board grades, such as the liner and fluting used in the manufacture of corrugated board. Because the use of size can, among other things, considerably increase the bending and surface strengths, the aim is to use large amounts of size in these board grades. At present, a greater application amount are achieved using pond-application devices, but their greatest running speed is too low for modern requirements. Therefore in modern board machines film-transfer application used to spread the size. In film-transfer application the size is spread first of all on the surface of a film-transfer roll, by means of which it is transferred to the web. In board machines, simultaneous two-sided application is often used, in which case the web travels through a nip formed by two film-transfer rolls. Each roll has its own application beam. Using these devices, it is possible to achieve a reasonably large application amount on the surface of one web, expressed as wet film of 30-35 g/m2.

The manner of using a coating mixture differs from sizing in that the aim is to make the solid particles of the coating mixture remain on the surface of the web.

Film-transfer coaters have, however, limitations, which reduce their efficiency and economy. Thanks to the nip pressure obtained in the nip of the film-transfer rolls, good size penetration into the fibre structure of the web is obtained, but the length of the nip is, however, quite short, so that the actual absorption of the size into the web only takes place after the nip. Because in practice the drying of the size must be started immediately after the nip, so that the web is made touch dry and can be controlled, the total absorption time remains short. Even though even large amounts of processing agent can be spread on the web using a film-transfer spreader or spreaders, a pond can arise between the surface of the web and the surface of the film-transfer roll. The creation of the pond causes the detriment of splashing at high web speeds, but a pool can arise easily even at low speeds. The creation of this so-called mini-pond limits the greatest application amount, because the pond causes splashing, web vibration, and when it rapidly rotates even the penetration of air through the nip. In a film-transfer nip, the web becomes wet with the great amount of size, which leads to runnability problems. When it becomes wet, the web weakens so that it can tear more easily. In addition, the wetting and drying change the dimensions of the web and the changes in dimension must be taken into account by adjusting the web tension and possibly by using spreader rolls. The application chambers of film-transfer coaters are designed quite precisely, to ensure an even spread of size. However, material continually detaches from the surface of a fibre web, which can travel either directly on the surface of the film-transfer roll, or along with the machine circulation into the application chamber. This causes disturbances, particularly when it gets under the doctor rod and can even block the application chamber. Further, the doctor rods and their cradles bounding the application chamber, as well as the roll surfaces wear rapidly, which leads to short service intervals. This causes direct maintenance costs and a reduction in production due to service breaks. These problems are emphasized in machines using recycled fibre, because there are always foreign substances in the fibre that cause wear, the most important of which being sand and particularly abrasive pigment-coating particles. Because recycled fibre is an important raw material in the manufacture of board, these problems become emphasized especially in machines used in the manufacture of board. During application, air and impurities can easily enter the machine circulation of the coater, and must be removed by efficient air removers and filter devices. These devices are expensive and the necessary machine circulation becomes long and its capacity large.

The problems and limitations appears in film-transfer application have different emphases in the manufacture of different products. It is obvious that the danger of a web break will not be as great in board grades as it will be with thinner papers, possibly made from recycled fibre. On the other hand, when making printing paper or board, there is less dirtying and wear of the devices.

The invention is intended to create a method and apparatus by which it is possible to is spread processing agent on a moving fibre web an amount that can be varied in wide range, the agent being advantageously surface size or pigment coating and to avoid the limitations present in application by conventional film transfer- or pond press sizer.

The invention is intended to create a method and arrangement, with the aid of which separate drying after application and the following pressing in a nip can be avoided.

The invention is based on the processing-agent mixture being spread on at least one surface of the web using a spray-application device, after which the web is led into a nip between two moving surfaces at a distance from the spreading point. Further, according to one embodiment of the invention the web is led after application to a nip, in which at least one surface that is in contact with the web is heated.

More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The arrangement according to the invention, in turn, characterized by what is stated in the characterizing portion of Claim 17.

Considerable advantages are gained with the aid of the invention.

The most important advantage of the invention is the variable amount of application over a very large area. In particular, the large application amount makes the method according to the invention more advantageous than previous methods. Thanks to the long delay time, even a large amount of size mixture has time to set on the surface of the web and inside the web and not to be pressed out in the nip of the rolls. In principle, with the aid of the method it is possible to use as large an amount of size are desired, at least within the scope of the limitations of existing known sized products. If necessary, a large application amount as permits the manufacture of new types of products. The amount of size and the profile can be adjusted with the aid of the feed amount of the nozzles or by changing the nozzle size. The delay is adjusted simply by altering the distance between the application unit and the press rolls. The final adjustment of the penetration of the size is made by altering the nip pressure of the press rolls' nip.

Preferably application is performed simultaneously on both sides of the web, in which case only one application station and the dryers it requires will be needed. There is no problem in spreading different amount of size mixture on each side of the web or using different processing-agent mixtures on each side of the web. The method also permits double application directly onto the wet surface. This is carried out by the roll nip being formed in the normal film-transfer-unit manner, in such a way that the press rolls have conventional application beams, so that a second layer of size is transferred to the web in the roll nip. The processing-agent mixture can be spread hot on the surface of the web, in which case it will be more fluid than usual and dry and set more quickly on the surface of the web. The evaporation of water is also promoted by the fact that the web can brought hot to the application device directly form the dryer.

The calender nips can also be used as the roll nip, for example, in such a way that one of the rolls is a soft roll and the other a heated roll. Correspondingly, one or both of the rolls can be replaced with belts, or the nip can be formed from a shoe calender. Thanks to the heated roll or belt and the long, preferably adjustable delay, drying after pressing can be eliminated, if the drying effect transferred in the nip to the web is sufficient. In that case, it is possible with the aid of the invention to substitute application, nip pressing of the application, calendering, and drying with only application and a hot calender nip. In particular, the expensive floating or infrared dryers are then eliminated and the purchase cost of the system can be reduced and it can be fitted into a small space, which is advantageous particularly when rebuilding a manufacturing line. This solution is best suited to pigment application, because in the case of a size with a starch content the adhesion of the size to the hot roll or belt surfaces can prevent the use of the method. In connection with sizing, particular attention must be paid to the selection of the materials of the hot surfaces, so that adhesion will be avoided.

In addition to the above advantages, the weaknesses of sizing with a film-transfer press are avoided, these being the insufficient penetration of the size to the web, i.e. limitations in the strength of the product, getting a large amount of size through the film-transfer nip, i.e. the avoidance of too great a pond forming in the roll nip, and the rapid wear of the application rods and rod cradles and roll surfaces, particularly in recycled-fibre machines. Because there are no application chambers, blockage of the chambers with material detaching from the web is avoided. Material detaching from the web does not enter the machine circulation, so that the processing-agent mixture need not be filtered in order to remove impurities. A return circulation is usually not even needed, because all of the application mixture can be directed to the web. If excess processing agent must be collected from the rolls or the spray chamber, it can be led in a simple manner back to the feed tank. The amounts are then small and the necessary transfer devices cheap. In addition to these, a pond presses have the weakness of the speed limit imposed by splashing from the pond, which limits the possibility to use them at the web speeds demanded nowadays.

In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

FIG. 1 shows a schematic view of one embodiment of the invention.

FIG. 2 shows a schematic view of a second embodiment of the invention.

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