BACKGROUND ART

Paper or board, for example fine paper or containerboard such as liner or fluting is in certain point of the production process desired to treat with a web strength increasing process medium such as a liquid size. Processing the web with a size prescribes substantially the strength properties of the end product. Different starches are used mostly as size to which starches many kinds of additives can be added. This invention is not restricted to any specific size mixture and by the size should be understood all mixtures which are at least partly absorbing in the web to be treated and are increasing the strength of the web. In sizing of the containerboard and the fluting good penetration is sought in order to achieve CMT and delamination strengths (Huygen, Scott-Bond). In film sizing or spray application, no good strengths dependent on penetration have been gained with soft press nips, when the basis weight was high. In application apparatuses, rolls with soft surfaces are notoriously used in the press nip in connection with the application. Rolls with soft surfaces are used among others in order to form an even size film. The length of the press nip is also increased with soft press surfaces in order to increase impact duration of compression pressure and penetration of the sizing. Many difficulties are, however, associated with the application of size mixture with high dry solids content and soft press surfaces when the dry solids content is increasing. When the water content of the size mixture is decreased in order to form the high dry solids content, the size mixture is changing tacky and sticking to the soft surface of the press member. It is difficult to maintain the soft press surface clean. The soft press nip is rubbing the web and releasing material from the web. It is difficult to clean material which is sticked to the web from a soft press surface, for instance a soft surface of a nip roll. The material which is sticked to the soft press surface will end up to the web which is sized in the press nip and will worsen the quality of the end product.

Removing water which is proceeding to the web in connection with the sizing is limiting efficiency of paper machines and board machines. Because only a limited amount water can be removed from the moving web with one drying apparatus, the amount of successive drying apparatuses has to be increased when the speed of the web is increasing. The increasing amount of the drying apparatuses is increasing the length and the price of the machine. The viscosity of the size is limiting the dry solids content of the size in spray application and film sizing. It is known that by means of pure spray application one has not been able to even a size mixture with high viscosity on the surface of the web but the size mixture was remaining in drops. One has been trying to keep the viscosity low so that an overhigh viscosity does not cause irregularity of sprays or the size film to be applied remaining striped. With highly modified and expensive starches, 20% dry solids contents are gained before the viscosity is increasing above 40 mPas. Such a ready converted starch costs typically about 70% more than non-modified native starch. The viscosity of the size which is converted enzymatically from native starch and cooked can be modified to a certain limit by dosing more enzymes or by lengthening the impact duration. Adding enzymes is expensive and lifetime of modified starch is short. The starch begins to disintegrate to sugar after a certain point, in addition to which it is difficult to change the impact duration when the preparation of the size is measured for a certain amount.

Operating with a low dry solids content of the size causes a high drying demand, enlarges the after drying section and drops potential speed of the line. Operating with a high dry solids content and thus a high viscosity with a pond sizer in pond sizing can cause a heavy web braking force and collapse the runnability.

The publication Fl 2001 1455 discloses a method for producing sized paper or board. The dry solids content of the size-water mixture being applied is at least 15% and the size-water mixture is applied to one side of the web by an amount not greater than 5 g/m ² indicated as amount of aqueous size mixture.

The patent publication Fl 121084 B discloses a method and an apparatus for processing a paper or board web. A processing mixture is spread onto the surface of the web, the web is lead to pass from a press nip and wet processing mixture is pressed to the web in the press nip.

SUMMARY

According to a first aspect of the invention there is provided a method for producing paper or board, the method comprising spraying size mixture from spray nozzles direct onto at least one surface of a web and pressing the web, after spraying the size mixture, by leading the web through a press nip which is formed by two rotating press members which are loaded against each other, the method comprising spraying the size mixture with dry solids content above 15% to a liquid film onto the surface of the web and pressing the web in a hard press nip which is formed between press members having hard surfaces. Preferably the size mixture is sprayed with dry solids content above 15%, more preferably above 40%.

Preferably the size mixture is sprayed with viscosity above 100 mPas. Preferably the viscosity of the size mixture can be above 100 mPas when the dry solids content of the size mixture is above 40%.

Preferably a turbulent size mixture flow is formed in the spray nozzles by means of a high spraying pressure. Preferably the size mixture is arranged in a spraying pressure of 20 to 100 bar in the spray nozzles. According to some embodiments the size mixture is arranged in a spraying pressure of 20 to 50 bar in the spray nozzles. According to some embodiments the size mixture is arranged in a spraying pressure of 20 to 35 bar in the spray nozzles.

Preferably the web is pressed with a roll or a belt comprising at least in the surface metal, steel, polymer, epoxy, rubber or a combination thereof. Preferably the web is pressed with a deflection compensated roll. Preferably recycled fiber is used in the manufacture of the web and the size mixture is sprayed at least on the side of the web which comprises recycled fiber.

Preferably the web is pressed in the press nip with a line load of 30 to 80 kN/m. Preferably the web is pressed in the press nip with a line load of 80 to 200 kN/m.

Preferably the application amount of the size mixture is adjusted by distribution of the spray nozzles, by nozzle size and/or by dry solids content of the size mixture. Preferably the application amount of the size mixture is fine-adjusted by changing rate of spraying pressure.

Preferably starch, PVA, silicone, coating pasta, latex or pigment-surface size- mixture is applied as the size mixture.

According to a second aspect of the invention there is provided an apparatus for producing paper or board, which apparatus comprises spray nozzles for spraying size mixture direct onto at least one surface of a web and a press nip for pressing the web after spraying the size mixture, which press nip is formed by two rotatable press members which are loadable against each other. In the apparatus the size mixture with dry solids content above 15% is adapted to be sprayed to a liquid film onto the surface of the web and pressed after that in a hard press nip which is formed between press members having hard surfaces.

Preferably the apparatus is configured to spray the size mixture with dry solids content above 15%, more preferably above 40%. Preferably the apparatus is configured to spray the size mixture with viscosity above 100 mPas.

Preferably the spray nozzles are configured to spray a turbulent size mixture flow by means of a high spraying pressure. Preferably the apparatus is equipped with a high pressure pump which is configured to produce a spraying pressure of 20 to 100 bar. According to some embodiments the high pressure pump is configured to produce a spraying pressure of 20 to 50 bar. According to some embodiments the high pressure pump is configured to produce a spraying pressure of 20 to 35 bar.

Preferably a roll or belt which comprises at least on the surface metal, steel, polymer, epoxy, rubber or a combination thereof is arranged as the press member having the hard surface. Preferably a deflection compensated roll is arranged as the press member.

Preferably the apparatus comprises loading means for loading the press nip with a line load up to 200 kN/m.

The size mixture may be applied on the web with higher dry solids contents than it was previous thought to be possible. Operating with higher viscosity allows also the use of inexpensive starches without a precise viscosity limit.

The size mixture with high dry solids content can be put in a turbulent state with the high spraying pressure. The turbulent state may be generated by increasing the flowing speed of the size mixture by means of the high spraying pressure. The high spraying pressure helps in weakening and splitting bindings in the size mixture with high dry solids content wherein the size mixture gets better applicable. The increase of the flowing speed can be influenced also by a smaller nozzle size.

A higher contact pressure can be achieved with the hard press nip than with a soft press nip. The strength of the web in the sizing treatment with the hard nip and with the high dry solids content can be increased when compared to the strength of the web in the sizing treatment with a soft nip with a high dry solids content which is one half smaller. The press members can be maintained in the hard press nip clean without stickings during running. A proper penetration of the surface size can be achieved with the spray application and a particularly hard nip. A proper penetration of the surface size can particularly be achieved with heavy qualities, for instance with qualities above 90 g/m ² . The penetration of the size can be improved with one-sided application without superfluous increase of total size amount. Two-sided sizing is also possible.

The sizing can be implemented on a size press, a calander, a wire section, a belt press, a drying section or another paper production apparatus. The types of paper may be, among others, boards, liner, fluting, fine paper, news. The liquid mixture to be applied may be starch, PVA, silicone, coating pasta, latex, pigment-surface size-mixture or a corresponding processing medium to be applied typically on the surface of the fiber web. Preferably the liquid mixture to be applied is increasing the strength of the web.

Different embodiments of the present invention will be illustrated or have been illustrated only in connection with some aspects of the invention. A skilled person appreciates that any embodiment of an aspect of the invention may apply to the same aspect of the invention and other aspects alone or in combination with other embodiments as well. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described, by way of example, with reference to the accompanying drawing, in which: Fig. 1 shows a spray application apparatus in which two press members with hard surfaces are forming a press nip;

Fig. 2 shows schematically the difference between a hard and a soft press nip and the Huygen strength of a surface sized board when a size mixture with high dry solids content is sprayed on a base board of 1 10 g/m ² ; and

Fig. 3 shows schematically the difference between a hard and a soft press nip and the CMT30 strength of a surface sized board when a size mixture with high dry solids content is sprayed on a base board of 1 10 g/m ² .

DETAILED DESCRIPTION

It should be appreciated that the illustrated figure is not entirely in scale, and that it mainly serves the purpose of illustrating embodiments of the invention.

Fig. 1 shows a spray application apparatus 10 which comprises two press members with hard surfaces to be loaded against each other, a first roll 1 with a hard surface and a second roll 2 with a hard surface. The rolls 1 , 2 with hard surfaces are forming a press nip 3 between each other. A paper or board web 6 is lead to the hard press nip after a size mixture with high dry solids content is sprayed on the web. The spray application apparatus 10 comprises first spray nozzles 4 and/or second spray nozzles 5 before the press nip. The web can be size treated one-sided or two-sided with the size mixture by means of the first spray nozzles 4 and/or the second spray nozzles 5.

The size is applied on the web before the press nip by the spray application apparatus which preferably comprises spray nozzles. The web is lead to the press nip which has preferably a heavy contact pressure. The size can be forced to the web with the heavy contact pressure. When the size film is particularly large the size can be brought to penetrate inside the web. The high pressure in the press nip also flattens the web and brings the surfaces of the web closer to each other wherein the distance proceeded by the size is increasing. The other side of the web can also be surface sized. The sizing can take place on the dirtier bottom side of the web, particularly on a recycled fiber machine and in connection with a web containing recycled fiber. The size can be applied on the web also mainly one sided wherein a large liquid film can be formed on the surface of the web. In order to raise the contact pressure level of the press nip 1 the hard press nip 3 is equipped with press members having hard surfaces for forming a sufficient high contact pressure and for gaining a sufficient penetration of the size mixture with high dry solids content to the web 6. Preferably the press nip 3 is a combination of two steel rolls or a combination of one steel roll and a hard polymer roll . Also rubber, polyurethane, epoxy, steel, a steel belt, a combination thereof or another material which is typically used in a hard nip of a paper production machine can be arranged as the hard material on the surface of the press member. A press roll may be coated in order to prevent sticking of the size, because a film forming ability is not necessary required. The rolls may also be doctored or equipped with a cleaning apparatus. Cleaning of the hard surface will succeed considerably better than cleaning of a soft surface and the size mixture will not stick so easy to the hard surface.

The penetration of the size to the web can be intensified by running the web after the application through the hard roll nip. That has not been possible in film sizing because the rolls were scratched when a grooved rod was used or because there was no film forming ability. Also a line load for instance 80 to 200 kN/m or even more if desired which is higher than typically, can be used in the press nip 3.

It is also possible to use a belt or belts as a press member. The press member may be a hard belt. Preferably the belt is lead through a roll nip. The press roll 2; 3 may also be a deflection compensated roll in order to enable the high line load in the press nip. By means of the deflection compensated roll a large and even contact pressure can be formed in the press nip and the penetration of the size mixture to the web can be improved. By using the deflection compensated roll also roll diameters can be reduced substantially when compared to tube rolls.

The size is applied onto the web 6 preferably by means of conventional spray nozzles 4, 5 using a spraying pressure which is higher than conventionally. With the pressure a viscous liquid can be made to flow faster from the nozzles wherein the liquid flow is changing turbulent. Forming of a regular spray requires a turbulent flow in the hole of the nozzle. Preferably the pressure is about 20 to 35 bar, although lower or also higher pressures can be used. After the application the web is run to the nip where the size drops are evening out and the size is penetrating in the web. The size mixture can be pumped first through a pressure sieve with a low pressure and then the pressure is increased by means of a separate high pressure pump to a desired level. Because the pressure cannot any longer be used significantly for adjusting the amount of the application, the amount of the application is adjusted with a selected distribution of the spray nozzles 4, 5 in a spray tube, the size of the nozzle and the dry solids content of the size. The pressure can be used for fine- adjusting in the spray application.

Significant differences in increase of strength of size treated web 6 can be illustrated with Figs. 2 and 3. Remarkable differences are generated between size treatments made with hard and soft press nips when size mixtures are sprayed on the web which size mixtures have different dry solids contents and thereafter the web is compressed in the press nip with different loads. The Huygen strength of a surface sized 1 10 g/m ² base board is shown in Fig. 2 where the percentage increase of the Huygen strength is shown on the vertical axis and the amount of starch measured from the web as grams per square meter on the horizontal axis. The CMT30 strength of a surface sized 1 10 g/m ² base board is shown in Fig. 3 where the percentage increase of the CMT30 strength is shown on the vertical axis and the amount of starch measured from the web as grams per square meter on the horizontal axis.

A test arrangement in which differences of a hard and a soft nip were compared to strengths of liner/fluting ended up to results shown in Figs. 2 and 3. The increase of the strength was tested in the test arrangement with a dry solids content 40% of the size. Two modified corn starches were used as the size mixtures in the following way:

- 40% dry solids content - viscosity 1 18 mPas

- 20% dry solids content - viscosity 24 mPas.

Additionally a low viscosity oxidized potato starch with a low dry solids content 25% was used as a size mixture in the strength comparison. The size mixtures with high dry solids contents were applied on both sides of the web with high pressure pumps of a spray coating apparatus using conventional sprays nozzles of a spray application beam. The amount of starch which is left on both sides of the web is measured in the results shown in Figs. 2 and 3, so the amount of starch which is dosed on one side is one half of the values shown in the graphs. In the running with high viscosity the amount of the size was limited by the yield of the high pressure pump. In the test arrangement, press rolls with hard surfaces of a hard calender nip were used as the press members 1 , 2 with hard surfaces. In the test arrangement, the first press roll 1 was a polymer roll with a hard surface (diameter 375 mm, hardness 95 ShD) and the second press roll 2 was a steel roll (diameter 500 mm). As line loads for the hard press nip 3 were used line loads 30 and 60 kN/m for a high 40% dry solids content size mixture and a series of line loads 10, 30, 60 and 80 kN/m for a 20% dry solids content size mixture.

In the test arrangement, press rolls with soft surfaces of a spray press were used as the press members with soft surfaces, the diameter of which was 1425 mm and hardness 20 P&J. As the line load for the soft press nip was used 50 kN/m.

It can be seen from the graphs of the Figs. 2 and 3 that the same strength improvement for the size treated web can be gained with smaller size amounts when the web is compressed in the hard nip. With the size mixture of the high dry solids content 40% and the high viscosity a good strength improvement can be gained cost effectively, because less water is required to be dried from the web. Thus drying energy can be saved, investments of drying apparatuses can be reduced and, if desired, production capacity can be increased. The foregoing description provides non-limiting examples of some embodiments of the invention. It is clear to a person skilled in the art that the invention is not restricted to details presented, but that the invention can be implemented in other equivalent means. Some of the features of the above-disclosed embodiments may be used to advantage without the use of other features.

As such, the foregoing description shall be considered as merely illustrative of the principles of the invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended claims.