The present invention relates to a method according to the preamble of claim 1 for coating, in particular, cardboard with a surface sizing agent and, if desired, also with a coating mix. The method is also suited for treating paper.

The invention also relates to an arrangement for carrying out the method as well as a product produced by the method and means for carrying out the method.

Pond size presses and film transfer coaters, among others, are used for the surface sizing of board and paper. In pond size presses, the web being treated travels vertically through a nip formed by two opposed rolls. Size is applied to the nip between both rolls and the web such that it remains on the roll and forms a pool. Size is adhered to the web travelling through the pond and the metering is performed by controlling the nip between the rolls. An advantage of the method lies in the large amount applied, this being beneficial particularly in the manufacture of board. The strength of the board to a great extent correlates with the amount of size used, and accordingly, pond size presses can be used to manufacture the strongest types of board of all. Surface sizing can be used to essentially increase the compression resistance, burst strength and CMT resistance (CMT: Aflat crush resistance) of board, and in general, the aim is to maximize the strength values for a given product. When pond size presses are used, the applied amounts of size vary between 20 and 100 ml/m2, including the applied amounts on both sides of the web. The typical amounts of size applied on the surface layer of cushion board, i.e. the so called liner, and on fluting, vary between 70 and 90 ml/m2.

A disadvantage related to pond size presses is the poor runnability which is due to the swirling of the ponds and vibrations on the incoming side of the web being treated, said poor runnability restricting the coating speed. At present, 900 m/min is considered the maximum speed applicable with pond size presses. In pond application, also a lot of water is absorbed by the web partly due to the manner of application and partly because the dry matter content of size is small, merely about 10 %. As the surface sizing agent, starch mixtures are usually used, and with the above-cited dry content of the coat and the applied amount, 8 to 10 g/m2 of size will remain on the web once the size mixture is dry. The great amount of water will decrease the strength of the web and increase the drying capacity required. The extensive wetting of the web constitutes an important problem particularly in the case of heavy paperboards because particularly great drying capacity is required for removing moisture which has been absorbed to a great depth in the web. Another disadvantage with pond size presses is that the amount of size cannot be controlled such that it would be different on each side of the web, except where one side of the web is left totally untreated.

Even in film transfer coaters the web travels between two rolls but the size, coating agent or other such agent is applied by means of a separate applicator onto the surface of one of the rolls or both of them, and the treating agent is transferred onto the web surface as a film. The rolls are usually either on top of one another or, viewed from the side, arranged in an oblique position in relation to a vertical line, but there are several possible arrangements of rolls because in a film transfer coater, there is no pond which would set limits to the construction of the apparatus. Film transfer coaters provide good runnability, but in the surface sizing of paperboard, they are hampered by the restricted amount applied. The greatest possible applied amount on one size during surface sizing performed by means of a film transfer coater is approximately 20 to 25 g/m2. It is clear that when conventional film transfer coaters are used, the amounts of size required in the manufacture of strong paperboard grades cannot be attained during one sizing step. As it is not possible to have two or several surface sizing steps for cost reasons and also because of the great drying capacity required by two drying steps, paperboards requiring a large amount of surface sizing agent must be manufactured with slow pond size presses.

The present invention aims at providing a method which will allow the use of greater amounts of treatment agents than are possible with film transfer coaters, as well as greater web speeds than can be applied in the case of pond size presses. The treatment agent preferably comprises size.

The invention is based on applying treatment agent onto the one side of the web to be treated by pond application, and onto the other side by the film transfer coating method.

In more detail, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim 1.

The arrangement according to the invention, then, is characterized by what is stated in the characterizing part of claim 6.

The treated product according to the invention for its part is characterized by what is stated in the characterizing part of claim 9.

The means according to the invention, then, is characterized by what is stated in the characterizing part of claim 19.

The invention offers considerable benefits.

As there is a size pond only on one side of the web, the vibrations of the web and the splashing of the pond are less significant than in the case of a pond on both sides.

This renders a considerable increase in web speed possible.

Simultaneously, on one side of the web, the amount attainable with pond size presses, 50 ml/m2, is attained, and on the other side, the amount which can be attained by means of a film transfer coater, 25 ml/m2, is attained, resulting in a total amount of size of 75 ml/m2. Such an amount is close to that used when two-sided pond size coaters are used and is clearly greater than the maximal amount of size attainable with film transfer coaters and corresponds to the smallest amount of size applied in the manufacture of fluting and liner. Thus, by means of the invention, strong sized cardboard can be manufactured, whereby a greater speed of manufacture is possible than with previous techniques. Pond splashing and vibrations on the incoming side of the web, these being typical of pond size presses, are reduced, and for this reason the apparatus of the invention offers improved runnability in comparison to said presses. Less drying is required than in the case of pond size presses, wherefore the use of the apparatus according to the invention leads to a shorter coater length and reduced energy consumption. The coater arrangement is very compact because both sides of the web are treated simultaneously. The arrangement also makes the manufacture of many kinds of product grades possible due to its great adaptability.

By means of the invention it is possible to manufacture surface-sized paper or board, one side of which is coated with a pigment coat or another suitable coat immediately after sizing. In this manner it is possible to produce e.g. strong cardboard grades which are printable on one of their sides. The method is excellently suited for the manufacture of printable linings of cushion board and printable packing board. Because a different amount of coating mix is applied to the different sides of the web, it is possible to compensate or to enhance properties of the web surface if the quality of the different surfaces of the base paper or board varies when it leaves the paper/board machine. Such materials are exemplified by, e.g., materials based on recycled fibres. As materials based on recycled fibres often require an extensive amount of size due to their impaired strength, the invention increases the applicability of such materials because it can be used to achieve a large amount of applied coat/size without jeopardizing runnability.

In the following, the invention is described in more detail by means'of the annexed drawings.

Fig. 1 is a schematic view of an arrangement according to the invention.

Fig. 2 is a schematic representation of the arrangement of Fig. 1 in a different position of use.

Fig. 3 is a schematic representation of another embodiment of the invention.

The sizing agents used in surface sizing usually comprise starch mixtures, but in the following, the term size is employed to refer to all agents used to increase the strength of a web.

The arrangement shown in Fig. 1 comprises two applicator rolls 1, 2 arranged next to one another at the same height, namely a film transfer roll 2 and a pond applicator roll 1.

An applicator device 4 is provided for the film transfer roll 2, which can be used to apply the coat film 5 onto the surface of the roll. The applicator device 4 may comprise any known applicator device which is used in film transfer coaters, such as a short-dwell coater equipped with a rod or blade doctor. The web 3 is brought into the nip between the applicator rolls 1, 2 from the side of the film transfer roll 2 via a guide roll 9 such that the web 3 touches the film transfer roll 2 before the nip between the applicator rolls 1, 2. Thereby a basin is formed between the web 3 and the pond applicator roll 1. Above said basin, a size feeding device 6 is provided, by means of which it is possible to feed a size pond 7 into the basin. As a size pond 7 must be formed between the applicator rolls 1, 2, the web has to travel a relatively long distance along the surface of the film transfer roll 2, which will improve the transfer of size or coating mix from the film transfer roll 2 onto the web 3 surface. By varying the position of the guide roll 9 the contact time between the film transfer roll 2 and the web 3 can be controlled, but at the same time the volume of the pond 7 is altered, and if the web 3 enters the nip between the applicator rolls 1, 2 in too straight a position, the film transfer roll 2 no longer supports the web 3 on the side opposite to the pond, whereby web vibrations may occur on the incoming side of the web 3. The coated web 3 is taken to drying or other further treatment via the guide roll 8.

The apparatus of Fig. 2 is a mirror image of the apparatus shown in Fig. 1. In this apparatus the film transfer roll 2 and the pond applicator roll 1 have changed places, wherefore the web must travel to the film transfer roll 2 folded via the guide roll 9 on the incoming side. As such, the apparatuses of Figs. 1 and 2 may be different apparatuses intended for the manufacture of a certain end product. However, it is more advantageous to be able to manufacture a large selection of products treated in different ways by means of one and the same machine. Thus, in the most preferred embodiment, an applicator device 4 is provided for both applicator rolls 1, 2 for applying film onto the roll surface, and one or two movable size feeding devices 6 are provided which can be used to feed size into the basin formed by the rolls and the web independent of on which side of the web said basin is located. In such an apparatus, at least two guide rolls 9 must be provided on the incoming side, via which the web can be made to travel, or one movable guide roll 9 by means of which the travel of the web is altered to lead to the desired applicator roll.

The main incoming directions of the web are via one of the rolls or perpendicularly into the nip between the rolls 1, 2 from above the rolls. In addition to these main incoming directions it is possible to choose incoming angles between said main directions.

The above-described device can be used to treat the web in a very many-sided manner. For a film transfer coater, the amount applied per area is 0 to 25 ml/m2 and for a pond size press it is 0 to 50 ml/m2, and accordingly, the total range of variation is 0 to 75 ml/m2. However, the preferred field of application of the invention is the manufacture of grades where the total coat weight is greater than that achieved with two-sided film transfer coaters, i.e. 40 to 50 ml/m2. It should be observed that film transfer coaters do not always achieve a coat weight of more than 20 ml/m2 on one side of the web, wherefore the upper limit of the above-cited range is difficult to reach in two-sided film transfer coating. When the method of the invention is used in surface sizing, products can be manufactured having a size weight in a dry product of 4 to 7.5 g/m2 if the size has a dry matter content of 10 %. Such a product is of considerable strength but requires less energy and a shorter drying apparatus to dry.

If both applicator rolls are provided with a possibility to use both the pond application and the film transfer method, the same coating apparatus can be used for one-sided or two-sided film transferring, one-sided or two-sided pond application and naturally application where one side of the web is treated with a film and the other side is treated by applying size or a similar treating agent thereon by pond application. As a film transfer coater can be used to apply coating mix onto the web instead of size, it is also possible to choose a combination where one of the sides of the web is treated with a size mixture in order to obtain increased strength and the other side is treated with a coating mix in order to achieve improved printability.

Fig. 3 illustrates an embodiment of the invention where one of the sides of the web is treated twice. The apparatus corresponds to the apparatus of Fig. 1 in other respects except that here the web 3 is arranged to travel round the pond applicator roll 1 and does not depart via the guide roll. A short-dwell applicator 10 is arranged to operate against the web which is supported by the pond applicator roll 1, the short-dwell applicator being used to apply a second layer of treating agent onto the once treated surface of the web 3. By means of this apparatus size is applied to one side of the web using the pond applicator roll 1 and to the other side using the film applicator roll 2. The side of the web 3 which has been treated by means of the film transfer roll 2 is then treated with a short-dwell coater 10. This arrangement is particularly well suited for the manufacture of strong printable boards because the base board web can be treated with a large amount of size and a coating layer is applied directly onto the undried size layer. The adhesive layer applied by means of the film transfer coater has time to set and the water contained therein is sufficiently absorbed by the web when the web travels round the pond applicator roll for the short-dwell coater not to remove all of the size off the web surface by doctoring. The side of the web which is treated by film transfer coating can also be coated twice by the wet-on-wet coating method, if a coating mix is used instead of size during the film transfer step.

The invention also covers means for manufacturing a paper or board product, said means being fitted in connection with a paper or board machine as a part of an integrated or a separate apparatus to cooperate with at least one apparatus which treats the web. The means can be arranged as part of an on- or off-line treating unit after a dryer or between dryers, e.g., groups of drying cylinders. The drying apparatus may also comprise a device which guides and simultaneously dries the web without contacting it. In particular, the means is arranged in an apparatus which contains a device or devices from the following group: a drying cylinder or cylinder group arranged prior to the means, a drying cylinder or cylinder group arranged prior to the means and a drying cylinder or cylinder group arranged after the means, a non-contact dryer and guide device which at least partly dries the web and is arranged in connection with the means or another web-treating or - producing device arranged to cooperate with the means. In this embodiment the means comprises guide means for bringing the web to a coating arrangement comprising two applicator rolls and for applying coating mix onto both sides of the web, means for guiding the web into the nip between the two applicator rolls, and a film transfer apparatus for applying a treating agent layer onto the web surface by the film transfer method in the nip, and means for simultaneous application of treating agent on the other side of the web by the pond application method.

The above-described means is particularly well suited for an apparatus having a web speed of 300 to 2500 m/min, advantageously 400 to 1000 m/min for a board web and 900 to 2200 m/min for a paper web.

In addition to what is stated above the invention also has other embodiments. The method of the invention is best suited for the manufacture of boards, such as liner board, fluting, packing board and drawing board, but it can also be used for treating paper products requiring surface sizing in a corresponding manner. The invention is particularly well suited for products where the properties desired on one side differ from those desirable on the other. Correspondingly, the treating method of the invention is extremely well suited for base material webs manufactured partly or totally from recycled fibres, advantageously for base webs containing more than 50 % of recycled fibres or being entirely made from recycled fibres. Instead of a guide roll, other known means for guiding and supporting the web can be used, such as belts or guides based on the use of air. The size pond is most easily implemented in solutions where the rolls are arranged next to each other at the same height, but in some cases it is feasible to shift the rolls from this position, whereby, however, it is necessary to make sure that a coating pond of sufficient depth can be formed against the web by means of at least one of the rolls.