The invention further relates to an arrangement for implementing the method.

In the manufacture of fine grade paper, the liner of corrugated board and fluting, treating the web with size is an essential part of the manufacture, essentially determining the tensile properties of the end product. Generally, size is added to the surfaces of the web, and with the multi-layer technology becoming common in the web manufacture, it is also added between the layers of the web into its middle. In the case of fine grade paper, the purpose of sizing is to improve the printing properties of the paper surface by increasing its surface strength, whereby the paper is better in resisting stresses during printing, and will not lint very much when used in a copying machine, for example. Furthermore, as the capillary spread of the ink in the paper is decreased, the print is improved. Surface sizing carried out before coating evens out any irregularities on the surface of the web, resulting in a smoother spreading of the coating material onto the web. In the case of cardboards, the most important objective of surface sizing is mostly to increase the strength. Accordingly, when strength is required, as great an amount of size as possible is often used for board grades to obtain the highest possible strength.

Surface sizing is traditionally carried out by a pond size press, wherein the web to be treated travels vertically between a nip formed by two opposite rolls. Surface size is applied to the nip between the rolls so that it forms a pond on top of the nip. When the web travels through the pond, size sticks to it and the dosing of the amount of size is mainly carried out by adjusting the amount of dry matter of the size. One of the advantages of the method is its high application rate, which is of benefit in the manufacture of board in particular, where the purpose is to provide the board with as good strength properties as possible. The strength of the board is very much dependent on the amount of size used.

Pond size presses can be used to obtain the highest strengths for thick board grades.

Typically, the application rates for the pond size presses are within a range of 30 to 100 g/m2, when adding up the application rates on both sides of the web. Typical application rates of size on the surface board of the corrugated board, i. e. , on the liner and on fluting are within a range of 60 to 90 g/m2.

One disadvantage of the pond size presses is their poor runnability caused by turbulences in the pools and vibrations on the inlet side of the web that is treated, limiting the coating speed. At the moment, the greatest running speed of the pond size presses is about 1000 m/min. In pond size application, a lot of water is also absorbed by the web because of the way of application. On a typical dry content of 8% and rate of application, the amount of size remaining on the web after the size has dried is about 4 to 8 g/m2. The large amount of water weakens the web and increases the need for drying power. Strong wetting of the web

is a big problem for light cardboard in particular, because the humidity absorbed deep inside the web weakens the strength properties of the web causing web breaks after the size press.

Surface sizing can also be carried out by a film transfer coater, wherein the web to be treated also travels through a nip between two rolls. The size is applied with a special applicator on the surfaces of one or both rolls, from where it transfers onto the surface of the web being treated in the nip between the rolls. The thickness and the profile of the size layer on the surface of the roll can be adjusted by means of a rotating coating rod or coating knife. The runnability of the film transfer coaters is good, but one of their weaknesses is the limited application rate. The highest application rate of the film transfer coaters in surface sizing is about 30 to 60 g/m2.

Currently, webs are made from fibre materials of ever decreasing quality, which is why larger amounts of size than earlier must often be used to achieve the desired web properties. In that case, when using paper and board grades of low grammage, the web becomes too wet in sizing, which in turn weakens the runnability of the coater and may cause track breaks or otherwise damage the web. The above problems begin to occur, when the amount of size applied on the web is so great that the moisture content of the web becomes over 40%.

The purpose of the invention is to provide a novel method that can be used to apply larger amounts of size on the web.

The invention is based on the fact that surface size is applied on the web, which is to be treated, in two separate stages. The web is dried between the first and the second application stages.

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

Furthermore, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim 7.

The invention provides significant benefits.

Because of the two separate steps of application of size and the intermediate drying between them, the total amount of wet size applied on the web can be considerably larger than in an application carried out in one single stage. In the solution according to the invention, the total amount of wet size applied on the web can be, in practice, as much as 80 to 120 g/m2 in the manufacture of liner and fluting, for example. The second applicator of the mixed size and the intermediate drying unit do not essentially increase the length of the paper or board production line, which is why the invention can also be applied to existing production lines. The runnability of the web improves, and it is often possible to use higher running speeds, because the amount of wet size applied on the web at a time is smaller compared with a case, in which the size is applied on the web in one single stage.

The size application carried out in two stages increases the flexibility of surface sizing ; for example, the first applicator can be used to surface-size one or both surfaces of the web and the second applicator can be used to apply size on one surface of the web and a pigment coating on the other, or different types of sizes can be used in the various stages of application.

In the solution according to the invention, the moisture content of the surface-sized web can be higher compared with surface sizing carried out by one unit, because the humidity of the web does not become too high after the size press, as only part of the total amount of size is applied at a time.

In the following, the invention is described in detail with reference to the appended drawings.

Fig. 1 is a schematic view of the initial part of a paper or board machine.

Fig. 2 is a schematic view of the end part of the paper or board machine of Fig. 1.

The manufacture of a paper or board web 4 begins from a headbox 1. The web 4 is formed in the headbox 1 by spraying fibrous pulp containing a great deal of water between two wires. In addition to water and fibres, the pulp can also contain many other ingredients, such as mineral fillers. The web is taken from the headbox 1 to a press section 2, where water is removed from the pulp and the solid web 4 begins to form. After the press section 2, the web is taken to a dryer section 3, where the web 4 is dried by heating. The direction of travel of web 4 is marked by arrows in the drawings.

After the dryer section 3, the web 4 is taken to a first surface sizing device 5, where size is applied on the web 4. The moisture content of the web 4 coming to the first surface sizing device 5 is not higher than 15%, typically 5 to 15%. Size can only be applied on the first surface of web 4 or, alternatively, on both the first and the second surfaces of the web 4. In order for the size to penetrate the web 4 as deep as possible, the dry content and the viscosity of the size applied on the web 4 by the first surface sizing device 5 are low. In that case, also the amount of dry size remaining in the web is low. A so-called bleed through is preferably carried out on the web 4 by the first surface sizing device 5, whereby the size penetrates throughout the thickness of the web 4 or essentially throughout the thickness of the web 4. In the first stage of application 5, the penetration of size into web 4 is at least on one surface higher than in the second stage of surface sizing.

The dry content of the size applied by the first surface sizing device 5 is typically less than 10% and its viscosity less than 40 mPas. In the first stage of surface sizing 5, the amount of wet size applied on the web 4 is typically 30 to 60 g/m2. The surface sizing device 5 is preferably a pond size press or a film transfer coater.

In surface sizing carried out by the pond size press, the web to be treated travels through a nip between two rolls. The treatment can either be carried out on both surfaces or only on one surface of the web. Generally, the application devices used are injection pipes of size,

which are not described in this connection. In the first stage of surface sizing, the applicator of size is preferably a pond size press.

In surface sizing carried out by the film transfer coater, the web that is treated travels through a nip between two rolls. Size is spread with a special applicator on the surface of one or both rolls, from where it transfers onto the surface (s) of the web that is treated in the nip between the rolls. The thickness and the profile of the glue line on the surface of the roll can be adjusted by means of a rotating coating rod or doctor blade.

After the first step of surface sizing 5, the web 4 is dried by preferably heating it by means of a set of dryer cylinders 6, which comprises dryer cylinders 7 that can be heated. The dryer cylinders 7 support the web 4 during drying. The web 4 is dried to the desired moisture content.

After intermediate drying 6, the web 4 is taken to the second surface sizing device 9, wherein size is applied on the web 4 once again. Size can be applied on the first surface of the web 4 only or, alternatively, on both surfaces of the web. In the second step of surface sizing 9, the dry content of size that is to be applied is preferably higher than that of the size applied in the first step of surface sizing 5. Typically, the dry content of size in the second step of surface sizing 9 is 9 to 20%, and that of the wet size that is spread onto the web is 30 to 60 g/m2. In the second step of surface sizing, the film transfer coater is preferably used as the applicator 9 of size.

After the second step of surface sizing 9, the web 4 is dried by cylinder driers 10. Finally, the paper or board machine comprises a winder 11 and other necessary roll handling devices.

The first step of application 5 of surface size, the intermediate drying 6 and the second step of application 6 of surface size take preferably place directly one after the other. In that case, the web 4 is not treated in any other way between the above treating steps.

Correspondingly, the first applicator 5, the intermediate dryer 6 and the second applicator 9 are placed directly one after the other in the direction of travel of the web 4 so that no other web treatment devices are located between them. The web treatment device does not refer to the guiding or application devices of the web.

In connection with this invention, surface sizing mainly refers to a treatment that improves the strength of the paper and/or board. Generally, the treating substance is starch that is dissolved in water. The purpose of the surface size is to absorb into the surface that is treated, whereby it binds the fibres of the web so as to form a stronger layer. No adhesive layer should be formed on the surface of the web that is treated, and the surface sizing does not refer to malcing laminated products by gluing layers together.

The purpose of the surface size is not to form a separate layer on the surface that is treated but the size should be absorbed into the web to form a uniform layer together with the web's fibres. Accordingly, the dry content of the size is generally low. On the other hand,

any increase in the dry content is limited by the fact that the starch becomes more difficult to handle and its fluidity weakens when the dry content increases.

In the solution according to the invention, in the first step of surface sizing 5, one or both surfaces can be surface-sized and in the second step of surface sizing 9, instead of merely surface sizing, either one or both surfaces of the web can be pigment coated or one surface can be surface-sized and the other one can be pigment coated. This type of a method is especially advantageous for liner or for paper or board that is pigment coated on one surface only. In the first step 5, for example, surface sizing of a maximum penetration can be applied on one surface of the web and in the second step 9, pigment coating can further be applied on this surface and surface sizing can be applied on the other surface of the web.

Alternatively, both surfaces of the web can be surface-sized by the first device 5, and one surface of the web can be pigment coated by the second device 9 and increase its strength properties by surface-sizing the other surface of the web.

The equipment according to the method can also be used to pigment coat both sides of paper or board, whereby the first device 5 is used to surface-size one or both surfaces of the web; and the second device 9 is used to pigment coat both surfaces of the web. In that case, the equipment can be used to make pigment coated paper and board grades without the need to make large additional investments. In this embodiment, it is preferable to use an intermediate calender in connection with pigment coating, for example, after surface sizing carried out by the first device 5 ; and a certain amount of non-contacting drying before cylinder drying and after the pigment coating carried out by the second device 9.