Method and arrangement for manufacturing paper

The invention relates to a method of manufacturing various paper grades by the same paper machine, wherein the formed and at least partly dried web is treated in a coating unit (20) by spraying a liquid treating agent onto the surface of the web, whereafter the web is influenced in calendering unit (40) by means of heat and compression to at least increase the web's smoothness and, when so desired, its gloss. The invention also relates to an arrangement for manufacturing various paper grades by the same paper machine comprising a means for forming and at least partially drying the web, also comprising a coating unit (20) for treating the web, the coating unit comprising a means for spraying a liquid treating agent onto the surface of the web, whereafter there is a calendering unit (40) in the making direction of the web to at least increase the web's smoothness and, when so desired, its gloss by means of heat and compression.

SC paper grade is conventionally made of wood-containing pulp, one of its special manufacturing features being an intensive calendering by a supercalender. Previously, these supercalenders used filled rolls and chilled rolls almost exclusively, the web being directed through the nip between the rolls. To give similar properties to both sides of the web, a so-called reversible nip is used in the middle of the stack formed by the rolls, the sides of the chilled and the filled rolls being changed with respect to the web at this reversible nip. Currently, various multi-roll calenders are mostly used instead of the supercalender, their operating principle being almost the same, but instead of the filled rolls, there are stronger polymer-coated rolls provided. The number of nips treating the web has also changed; instead of the former 8 to 12 roll nips, there can be same number of roll nips or, alternatively, various calenders consisting of several sets of rolls, such as those with 3+5, 2+4, 4+5, 3+3 nips, etc. In principle, the properties of the paper grade, however, have remained the same in the respect that the new calenders can also be used to produce paper with similar properties as before.

SC paper can be divided into various subtypes SC-A+, SC-A, SC-B+, SC-B, SC-C, the distinguishing factors including gloss and smoothness, raw material, and filler content. The gloss of SC-A is in the order of 46 to 55 % and its roughness from 1.0 to 1.3 μm, while the corresponding values of SC-C are from 18 to 27 % for gloss and from 1.8 to 2.4 μm for roughness. Differences that great are not possible by a change in the calendering parameters only but, naturally, to achieve these values,

the ingredients contained in the fibrous web deviate from one another, one distinguishing ingredient being ash. As an extremely fine-grained ingredient, ash strongly influences the calendering result. In principle, with its content increasing, a glossier and smoother paper is obtained by calendering.

Similarly to the SC grade, LWC (Light Weight Coated) paper is mainly manufactured from wood-containing, i.e mechanical pulp. In addition to the sub-processes included in the manufacture of SC grades, this manufacturing process also includes coating, which is carried out before calendering. A typical coating amount of LWC paper is 10 g/m ² per side, generally totalling 16...25 g/m ² , which provides a light and smooth surface that is good in reproducing print. There is a number of various coating methods, such as blade coating, size press coating, spray coating, size press coating, curtain coating, etc. To provide better grades, these methods can also be combined; however, in the manufacture of good-quality basic LWC, both sides of the web are coated only once. Publication WO 02/103109 presents quite a versatile selection of factors related to the manufacture of the LWC grade.

The purpose of the various ways to change the surface properties of paper is to provide the properties needed by the customer of the paper mill, i.e. the printing house at the time. Typical uses for these paper grades include various newspapers, advertisements, inserts and catalogues. At the cheaper end of the scale, there is the pigmented newsprint, i.e. improved news grade, which provides the roughest print and has a low ash content, and at the more expensive end, there are the SC-A gravure paper that enables the best print quality and the LWC offset paper.

The printing methods make different demands on paper grades. In gravure printing, the ink is transferred from the cups on the surface of the printing machine's impression roller onto the surface of the paper to be printed. In this method, the smoothness of the printed surface is quite significant for providing a good print quality. When the surface is uneven, the ink transfers unevenly from the cup, air bubbles remain under the ink, and the printed figure is not repeated as it is on the surface of the impression roller. In offset, various printing inks and patterns are first established onto the surface of the impression roller by means of chemical adhesion, the ink being then ripped off therefrom after adhering to the surface of the paper under compression. As the surface has to endure the tearing off of sticky printing ink from the surface of the impression roller, this printing method requires surface strength of the printed surface, in particular. Thus, the printing ink should come off the interface between the ink and the rubber coating of the com-

pression roller. If the printing paper is inappropriate, the paper delaminates at its surface after pressing, i.e., the surface layer of the paper transfers to the surface of the impression roller, causing a failure in printing.

US 2003/0056920A1 presents a method and equipment for the on-line calendering of SC-A in a paper production line. The publication suggests that, by placing a moistening device to moisten the surface of the web from 0.6 to 1.2 s before the first nip of a multi-roll calender, paper with the desired surface properties can be manufactured. The moistening device produces fine drops with an average drop size of less than 50 or less than 20 μm. The publication states that after moistening, the moisture of the web is from 7 to 11 %, i.e., its solids content is from 89 to 93 %.

WO 02/103109 presents a method, a paper machine and a base paper for manufacturing LWC that is coated once. The publication shows the selection of suitable components for different structural groups, enabling the exploitation of certain parameters to provide a quality as good as possible for the end product under manufacture. The publication also describes the effect of the parameters used in moistening and coating on the properties of the product that is manufactured.

The object of the invention is to provide a manufacturing method and a paper machine, which can be used to produce top-quality printing papers for various purposes according to the circumstances in a way that is easy to select. On the paper market, the demand for different paper grades fluctuates intensively according to the business conditions and the like, and the object of the invention is to provide an arrangement enabling full use of the paper machine investment amid different market conditions. Another object is to extend the grade selection possibly provided by one paper machine to both cover the various SC grades from SC-A+ to SC-C and different LWC grades, such as offset and roto grades.

The object of the method is also to provide a versatile arrangement for a wide paper selection for the so-called gradient calendering of paper, wherein the moisture profile of each paper grade to be produced is individually and optimally adjustable in the direction of the thickness or the Z direction before calendering to fully exploit the calendering properties of the base paper.

The invention according to the invention is characterized in that, in the method: - the paper grade to be produced is selected, a liquid treating agent being selected according to the paper grade, influencing the web and enabling the features of the

paper grade that is produced,

- the said liquid treating agent can be selected from a group of water, water with additives, surface size and pigment-containing coating agent, and the mixtures thereof,

- a desired solids content of the web is selected before the calendering unit 40,

- a drying process 30 is selected to obtain the desired solids content, in which drying process 30, drying methods and drying powers deviating from one another can be selected between the coating unit 20 and the calendering unit 40,

- whereby the drying power can be selected from natural drying caused by a clearance to a drying power provided by a combination of several driers 30, 321 , 361 ,

- whereby the said desired solids content of the web is from 85 to 92 %, the liquid treating agent being water, and from 90 to 95 %, preferably 92 %, the liquid treating agent being the pigment-containing coating agent.

The arrangement according to the invention, in turn, is characterized in containing:

- a control unit that enables the selection of the paper grade to be produced, the liquid treating agent being selectable according to the paper grade, the liquid treating agent enabling the features of the paper grade in question, and the features being recordable in the memory of the control unit;

- a coating unit 20 comprising a means for spraying various liquid treating agents, the said liquid treating agent being selectable from a group of water, water with additives, surface size and pigment-containing coating agent, and the mixtures thereof,

- a drying unit 30, by which various drying processes can be implemented to obtain the desired solids content before the calendering unit, drying methods and drying powers deviating from one another being selectable in the said drying unit,

- the drying power being selectable from natural drying caused by a clearance to a drying power provided by a combination of several driers 30, 321 , 361 so that the said desired solids content thus obtained is from 85 to 92 %, the liquid treating agent being water, and from 90 to 95 %, preferably 92 %, the liquid treating agent being the pigment-containing coating agent.

The method and the paper machine according to the invention can be used to extend the grade selection produced by the same paper machine from the conventional, relatively small grade selection to a considerably larger one. In this way, it is easy to meet different market conditions and the requirements made by them with-

out time-consuming changes and the running-in required by the changes, which almost inevitably would cause a temporary deterioration in the utilization degree and, often, the performance of the paper machine. Using the invention, it is thus convenient to produce various paper grades which, however, are top-quality in their respective paper grades. The changes needed are already built in the paper machine; hence, they do not have to be run in in the conventional manner. In this way, a versatile arrangement for a wide paper selection for the so-called gradient calendering of paper is also provided, wherein the moisture profile of each paper grade to be produced is individually and optimally adjustable in the direction of thickness or the Z direction before calendering to fully exploit the calendering properties of the base paper. This object is implemented by selecting, in a practical way, a combination of the liquid treating agent, drying, and the dwell time, whereby, when coming to the calendering unit, the properties of the web are optimal for the end result of the calendering.

One preferred feature of the invention is characterized in that the web is treated with the liquid treating agent in one coating unit, which comprises at least a means to spray liquid treating agents with viscosities deviating from one another. The object of the invention was to manufacture quite an extensive paper grade selection which, in practice, would require the use of various liquid treating agents. Conventionally, various water spraying systems are used for the SC paper grades. To manufacture LWC paper grades, pigment-containing coating agents are used, which can be sprayed onto the surface of the web by a spray coating unit. The liquids, which include water used in the SC paper grades and the pigment-containing coating agent used in the LWC paper grades, considerably deviate from one another, regarding especially their viscosities and many other properties as well, such as surface tension, abrasiveness, and solidification and drying properties. It is preferable to arrange at least replaceable nozzles for these different types of liquid treating agents, so that a layer of liquid treating agent, which is properly covering and has even patterns, can be sprayed onto the passing web.

To reduce the energy used by the process, to decrease web breaking, and to provide as good as possible a quality of the produced paper, it is preferable that, when treating the web with the liquid treating agent, the decrease in the total solids content of the web is minimized, however, so that at least the web's gloss and smoothness will be characteristic values of the selected paper grade after the calendering unit. One way to implement this is to influence the total solids content of

the web after the coating unit by influencing the amount of liquid treating agent fed and the solids content of the liquid treating agent.

In the following, the invention is described with reference to the appended drawings, in which: Fig. 1 shows a diagram of one possible operation mode illustrating the invention;

Fig. 2 shows a diagram of another possible operation mode illustrating the invention; Fig. 3 shows a diagram of an alternative embodiment of the coating unit 20; and

Fig. 4 shows the properties of different paper grades or sub-grades, examined with respect to the gloss and roughness.

Fig. 1 shows a diagram of the operation mode according to the invention, wherein the processing of the web advances in the direction of the arrow. In the process, the web, i.e., the SC or LWC base paper to be manufactured is formed and dried to a sufficient solids content by a paper machine 10 comprising all the processing devices necessary for making the base paper. The coating unit 20 comprises all the devices necessary for spraying the liquid treating agent onto the surface of the moving web during the production. The drying unit 30 takes care of drying the web to a suitable solids content, i.e., inversely, to a suitable moisture before the calendering unit 40. In the calendering unit 40, the web is formed by means of heat and compression to increase at least the smoothness and, if so desired, the gloss of the web. A reeling unit 50 of the web reels the web on a roll, which can then be conveyed to further processing, e.g., in a slitter. Measuring units 60 control the quality properties of the web, among others, at suitable stages. One method of implementation is to preferably measure the solids content of the web before the coating unit 20, directly after the coating unit 20 and after the calendering unit 40, the measurement being preferably carried out on both surfaces of the web and through the web, whereby the solids content of the web's surface on both sides and the total solids content of the web can be distinguished. A holding point or a holding element 70 ensures a good runnability of the equipment by maintaining a suitable web tension at each point.The holding point 70 is preferably on the route of the web transfer between the coating unit 20 and the calendering unit 40 to control the tension of the web in the coating unit 20 and before the calendering unit 40. The tension of the web on different sides of the contact point between the web and the holding element can be different with production in progress, and during a

conventional production, the holding element and the web are essentially in a non- sliding contact with one another.

Fig. 1 also shows in detail the drying unit 30, showing a solution as one possible embodiment, comprising three different web transfer routes 32, 34 and 36. Thus, the dwell time of the web between the coating unit 20 and the calendering unit 40 herein can be selected from at least two alternatives 32, 34, 36, which have different web transfer routes. By selecting the dwell time, the moisture profile in the direction of the web's thickness is influenced for its part so that, between the highest and the lowest moistures in the direction of the web's thickness, a greater difference is provided by a shorter dwell time and a smaller difference is provided by a longer dwell time. Of these web transfer routes, the one shown by the reference number 34 is a short cut, on which there are no separate drying means but the web is dried to a moderately low extent, the drying being caused by a clearance. On this alternative route, the dwell time between the coating unit and the calendering unit is as short as possible, whereby the moisture contribution in the direction of the web's thickness deviates from the uniform. The solids content of the web's surface after the coating unit can be influenced by the hydrophobicity between the liquid treating agent to be fed and the web. This chemical method can easily be used to adjust the absorption of the liquid into the web. The web transfer routes 32 and 36 are alternatives containing drying solutions that deviate from one another. This drying process 30 may include power-adjustable fluidized bed driers or other through drying machines, infrared driers, clearances and drier combinations thereof; the drier combinations may include one or more of the said driers 30, 321 , 361. Cylinder driers or corresponding driers based on a contact can also be used. The lengths of the web transfer routes 32 and 36 can be different depending on the desired running parameter range. Fig. 2 shows a drying unit 30 in a corresponding place, including one web transfer route; however, the different drying methods and powers required by the different liquid treating agents are implemented by switching various driers on/off and adjusting the power of the driers when necessary. This is a preferred embodiment, if the length of the web transfer route between the coating and the calendering units remains short enough for the production of SC grades and, on the other hand, a sufficient drying power is obtained for the production of the heaviest end of the LWC grades.

The coating unit 20 in Fig. 1 is presented to be similar to that in Fig. 2, the coating unit 20 including two sets of spraying heads 22 and 24. The liquid treating agent is fed from a container T by means of a pressure production instrument 26 to be

sprayed onto the surface of the web. The pipe line includes closing devices 25, which are used to direct the liquid treating agent to a desired spraying head. Fig. 3 shows another possible embodiment containing two systems for the liquid treating agents, including two containers T and two sets of pressure production devices 26. A special advantage of this solution is that the grades can be changed quite quickly without having to wash the system in between, especially when transferring from the production of LWC to that of SC. Another advantage is that the dimensioning of the pipework and the pumps is easier to implement logically, when working closer to the optimal flowing situation of each liquid treating agent with respect to the pressures and the volume flow rates. One disadvantage, of course, is its greater initial investment.

In the calendering unit 40, the web is influenced by heat and compression at least in one nip, preferably by means of several nips; the nip can be formed by means of rolls, metal belts or belts made of an elastic material, or combinations thereof.

The method and the arrangement are controlled by means of a control based on a model or a multivariable control, preferably by means of a learning multivariable control based on a model.

A product provided by means of the method and the arrangement can comprise SC and LWC paper grades; to be more precise, the paper grades from SC-A+ to SC-C, news grades and LWC offset and LWC roto grades, by changing the spraying head 22, 24 of the coating unit and, possibly, the web transfer route 32, 34, 36 between the coating unit 20 and the calendering unit 40, and the control parameters, such as the drying process, the said changes being built in the paper machine and available by the control system.

Fig. 4 shows the typical properties of the various sub-grades of the SC paper grade manufactured by the method and the paper machine according to the invention, distributed as to the gloss and the roughness. It can be observed from the figure that there is a considerable difference in the roughness and the gloss between SC-A+ and SC-C, which enable the best print quality, and the news grades (News). The difference between these sub-grades is not unambiguous, but for the presented sub-grades, the magnitude is shown in Fig. 4.

The reference numbers in the figures:

10 Paper machine

20 Coating unit

22 Spraying head/spraying nozzles

24 Spraying head/spraying nozzles

25 Closing devices

26 Pressure production device

30 Drying unit

32 Web transfer route

321 Drying devices of the web transfer route 32

34 Web transfer route

36 Web transfer route

361 Drying devices of the web transfer route 36

40 Calendering unit

50 Reeling unit of the web

60 Measuring unit, quality measurements

70 Holding point / holding element

T Container