WO2006125856A1

WO2006125856A1 - Method of and equipment for manufacturing a fibrous web formed at high consistency

Info

Publication number: WO2006125856A1
Application number: PCT/FI2006/000165
Authority: WO
Inventors: Pasi Ahonen; Markku Karlsson; Pauli Kytönen; Kari RÄISÄNEN; Tomi Ulmanen; Reijo PIETIKÄINEN; Erkki Ilmoniemi
Original assignee: Metso Paper, Inc.
Priority date: 2005-05-25
Filing date: 2006-05-26
Publication date: 2006-11-30
Also published as: FI119251B; CN101171391A; ATE425304T1; CN101171391B; FI20050555A0; EP1899530A1; DE602006005665D1; FI20050555A; EP1899530B1

Abstract

A method of and equipment for manufacturing a fibrous web, comprising the formation of at least one layer of fibrous web from stock with a solids content of over 1.5%, water being removed from the said fibrous web by first pressing the fibrous web to a solids content of about 45 to 65%, then evaporating the fibrous web to a solids content of about 75 to 95%, after which the fibrous web is treated by means of a pressing contact to increase its smoothness, the fibrous web being treated in the manufacturing process before the reeling stage by at least one pressure and heat treatment, wherein the forming pressure exerted on the fibrous web is not higher than 20MPa.

Description

Method of and equipment for manufacturing a fibrous web formed at high consistency

The invention relates to a method of manufacturing a fibrous web, wherein at least one layer of the fibrous web is formed from stock with a solids content of over 1.5%, water being removed from the said fibrous web by first pressing the fibrous web to a solids content of about 45 to 65%, then evaporating the fibrous web to a solids content of about 75 to 95%, after which the fibrous web is treated by means of a compressive contact to increase its smoothness.

The invention also relates to equipment for manufacturing the fibrous web, wherein at least one layer of the fibrous web can be formed from stock with a solids content of over 1.5%, the equipment including a former and a press section for pressing the fibrous web to a solids content of about 45 to 65%, after which there is a drying section for evaporating the fibrous web to a solids content of about 75 to 95%, and after that there is equipment for treating the fibrous web by means of a compressive contact to increase its smoothness.

In the wet web forming technique currently used, the fibrous web is converted into a web from stock formed by water and fibrous pulp, generally, at a solids content of about 0.5 to 1 %. However, a web formation known as high consistency (HC) web formation takes place at a solids content of about 1 to 5%, mostly at a solids content of 2 to 4%. In principle, the stock to be formed can either contain virgin fibre or recycled pulp or both.

For prior art HC web formation, reference is made to publication WO 2004/022844 A1 , wherein high-consistency stock is conveyed symmetrically onto two wires, af- ter which the two layers are pressed against each other in a prepressing nip for providing a single fibrous web. By doing so, a space-saving machine solution and quite symmetrical surfaces and structure of the fibrous web are provided.

Publication WO 97/45589 is also known in the prior art, disclosing a method of providing a fibrous web from high-consistency stock by means of a Fourdrinier- type former. The publication mentions an advantage of the method being, among others, that in this way, a smooth outer surface is provided, meeting the requirements made of high-quality printing. Generally, the object of the HC web formation is to reduce, already at the beginning, the amount of water removed from the fibrous web. The major part of the structure of a paper and board machine is designed for dewatering either by pressing or evaporation. If the amount of water in the fibrous stock could already be reduced at the beginning, not compromising the printing properties of the fibrous web, however, it would be possible to considerably decrease the energy needed for making the fibrous web and to reduce the investment costs and the need for space of the equipment.

However, in the present high-consistency formation technique, an even distribution and a suitable orientation of fibres are more difficult than in the conventional web formation. With the solids content of the stock formed by water and the fibrous pulp increasing, the flow and the web formation process of the stock are more difficult, easily causing an uneven formation of the fibrous web, wherein the fibres are accumulated in clusters or flocks. On the other hand, it has been observed that in the HC web formation, the fibres mostly tend to settle in the direction of the web's thickness (the Z direction). Generally, the Z direction refers to the direction of thickness of the fibrous web, the X direction refers to the machine direction of the fibrous web and the Y direction refers to the direction transversal to the machine direction of the fibrous web.

The HC formation can be applied to single-layer products, wherein the entire web is formed from HC stock. It can also be applied to multi-layer products, as presented in the said references, whereby at least one layer is HC-formed. It is particularly preferable to use the web formed from HC stock in cardboards so that a layer inside the surface layers is made by HC formation. In that case, a coarser reject or recycled pulp can be included in the inner layers and the surfaces can preferably be manufactured from chemical pulp.

The HC formation can further be applied to coated cardboard and paper products, whereby any possible defects caused by the HC formation in the structure of the fibrous web and the surface quality are covered by a coating layer.

In HC formation tests, it has been observed that a fibrous web formed at high consistency is bulkier than normal and the fibres settle less parallel or in the in-plane direction than in a web that is formed from conventional stock with a solids content of 0.5 to 1.0%. The number of fibres settling in the Z direction of the fibrous web in particular is higher. With the number of fibres of the fibrous web in the Z direction increasing, the bulk of the formed fibrous web increases. This increase can be from 2 to 5% for mechanical pulp and as much as 5 to 20% for chemical pulp containing CTMP. Furthermore, when the number of fibres settling in the Z direction increases, the strength, especially the Scott-Bond strength of the fibrous web also increases.

Prior art publication WO 03/064761 A1 is also known, describing the formation of a fibrous web by a belt calender or the like.

The object of the invention is to provide a method and equipment for achieving a smooth and, if needed, glossy surface, which enables a good print quality for the web that is formed from the HC stock. Another object of the invention is to main- tain the good properties of the fibrous web provided by the HC formation, such as bulkiness and good strength properties in the Z direction. A further object is to enable the use of stock with a solids content higher than before to form the web and to reduce any disadvantages related to the smoothness and possibly caused by the high solids content. A special object is to provide a solution that helps reduce the energy consumed by the paper machine line per surface area of the fibrous web produced, while also producing fibrous web grades with a quality comparable to the conventional fibrous web grades that are made from stock with a low solids content. Regarding multi-layer fibrous webs, one object is to influence the structure of the fibrous web in the Z direction so that a denser and smoother layer is provided on the surfaces, while the middle layer(s) remain bulkier than the surface layers, whereby excellent rigidity properties per grammage are obtained.

In the case of uncoated paper and cardboard grades, the object of the invention is to make an advantageous surface of the fibrous web in order to achieve the required properties of the printing surface. Essential requirements of the printing sur- face include a good surface smoothness (such as Bendtsen and PPS) and surface contour and a fairly low variation in density and pore size (an even adsorption of the printing ink). A special object is to maintain the bulkiness of the layer made by the HC formation and yet treat the outermost surface layer of the web intensively enough to achieve the required properties.

Regarding coated paper and cardboard grades, an important object of the invention is to make an advantageous fibrous web surface for the coating process. A special object is to provide a small topographic variation in medium and high (> about 1mm) scales, whereby in the coating process, it is easier to apply a smooth layer of coating agent. The smooth layer of coating agent dries more evenly, whereby the absorption properties of the coating layer and the print quality are good and even (a low tendency to mottle).

The method according to the invention is characterized in that, in the manufacturing process, the fibrous web is treated before the reeling stage by at least one pressure and heat treatment, wherein the forming pressure exerted on the fibrous web is not higher than 20 MPa. The equipment according to the invention, in turn, is characterized in that, in the manufacturing process, there is at least one set of pressure and heat treatment devices to treat the fibrous web before reeling, the devices being capable of treating the fibrous web at a forming pressure of less than 20 MPa. The solution according to the invention can be used to convert HC stock into a fibrous web with a high-quality surface, the web being bulky and strong in the Z direction compared with corresponding wet formed fibrous webs.

According to an additional feature of the invention, the formation of the fibrous web is carried out by a pressure and heat treatment, wherein the factors causing the formation, such as the forming pressure, the time of action of the forming pressure, and the temperature are adjustable within a certain distance or zone. Correspondingly, the pressure and heat treatment equipment comprise means for the above-mentioned adjustments. In the pressure and heat treatment of a specially preferred embodiment, the fibrous web is treated in the machine direction of the fibrous web at least once to improve the smoothness of the fibrous web at three stages so that, at the first stage, the fibrous web is preheated, at the second stage, the actual pressing is carried out, smoothening the surface of the fibrous web and, at the third stage, the fibrous web is subjected to a stabilizing post-treatment. Correspondingly, the equipment has at least one set of pressure and heat treatment devices in the machine direction of the fibrous web to improve the smoothness of the fibrous web's surface, the pressure and heat treatment devices being adapted to three stages so that the first stage is arranged to preheat the fibrous web, the second stage is arranged to actually smoothen the fibrous web's surface by pressing, and the third stage is arranged as a stabilizing post-treatment of the fibrous web. The pressure and heat treatment zone of the pressure and heat treatment devices is thus adapted to be three-stage so that, at each stage, the pressure effect (the value and/or the time of action of the pressure) is separately adjustable. In another embodiment, the pressure and heat treatment zone consists of one part only, wherein almost the same pressure influences the fibrous web. The required qualities of the printing surface are attained by forming the final surface in the pressure and heat treatment either in connection with drying or immediately there- after. In the pressure and heat treatment zone, at least one surface of the fibrous web is in contact with a surface that is heated to 70-250⁰C.

Compared with conventional calendering, the most significant advantage of the above-described pressure and heat treatment is its considerably longer time of action, whereby, because of the intensive heat supply, the surface parts of the fibrous web are quickly heated as early as at the first stage of the pressure and heat treatment. Heating evaporates water and softens and plasticizes the fibres in the surface layer. The pressure charge applied at the second stage of the pressure and heat treatment effectively forms the plasticized surface layer; instead, the set of fibres deeper in the structure are mainly formed elastically, i.e., the structure is mostly restored. This elastic behaviour is possible because of the level of the prevailing moderate pressure charge, and partly also because of the increased inner steam pressure of the fibrous web, resisting the compression. At the third stage of the pressure and heat treatment, the pressure charge is reduced in a con- trolled way so that the inner steam pressure of the fibrous web cannot discharge too quickly.

The gradually adjusted pressure and heat treatment process described above enables a heat supply adequate for the formation of the fibrous web in relation to the compression pressure, which for its part contributes to the structure of the fibrous web in the Z direction. Because of the intensive heat supply and the light compression pressure used in the formation, a denser layer is provided on the surfaces of the fibrous web, while the middle section of the fibrous web remains bulkier. For the overall economy, the method and the equipment accomplish the object of manufacturing a fibrous web, the amount of energy required for its manufactur- ing process per unit area being lower than that of a corresponding fibrous web manufactured of conventional stock. These measures can be implemented at a suitable stage in the manufacturing process of the fibrous web; for a board web, in particular, before the coating stage, and yet before reeling up.

According to a preferred embodiment of the invention, the said compression pres- sure is preferably less than 12.5MPa, more preferably less than 10MPa, more preferably less than 7.5MPa, most preferably less than 5MPa. By reducing the compression pressure used for the formation in this way, the bulk of the fibrous web, in particular, is better retained.

The equipment and the method according to the invention are suitable for manu- facturing packing boards, such as folding boxboard and liquid packaging board and kraft liner, as well as certain printing paper grades. The fibrous webs, which comprise the target of the application, can either be coated or uncoated. The fibrous web can be a coated or uncoated paper web with a grammage of 40 to 150 g/m². The fibrous web can also be a coated or uncoated board web with a gram- mage of over 100 g/m².

The invention is described in detail with reference to the appended drawings, in which:

Fig. 1 shows a rough draft of the equipment for forming a fibrous web,

Fig. 2 shows a preferred forming profile of the pressure and temperature as a function of time,

Fig. 3 shows another preferred forming profile.

Fig. 1 shows one possible embodiment of the pressure and heat treatment equipment for forming the fibrous web W. This embodiment comprises a belt calender 1 , which has a metal-built belt 2 extending around guide rolls 3, at least one of the guide rolls being adapted to be movable to adjust the tenseness or the tension of the belt 2. The forming belt 2 travels past a roll 5, constituting a pressure and heat treatment zone N between the belt 2 and the roll 5. The fibrous web W to be formed travels through the pressure and heat treatment zone N, at the same time being subjected to formation under a pressure momentum and the temperature, which act as a function of time. The belt 2 can be heated or cooled by means of a heating/cooling device 6. The line of dots and dashes 9 in Fig. 1 shows the pressure that is formed, when a nip roll 4 is installed inside a loop made by the forming belt 2, the roll 4 working as a press element and pressing the belt 2 against the roll 5, providing a greater pressure in the area of the nip of the pressure and heat treatment zone N. On the other hand, the dashed line 8 shows the distribution of the pressure that is formed, when only the tension of the belt 2 is working, i.e. the roll 4 is not making a nip with the belt or the roll 4 is fully off the said position. The roll 5, as well as the roll 4, can be variable crown rolls or non-variable crown roils, and they are selected from a group containing rolls with flexible surfaces, such as rolls with polymer, rubber or elastomer surfaces, shoe rolls, thermo-rolls, metal rolls, filled rolls, and composite rolls. Instead of the roll 4, the press element can be a press element, which can be profiled or which forms a fixed profile and which may contain several sequential zones in the X or Y directions. The press element can have a continuous or non-continuous surface. The press element 4 can be arranged to be fixed or movable and it influences the length of the pressure and heat treatment zone or the tension of the belt 4.

Fig. 2 shows a possible compression pressure curve for the fibrous web as a function of time. In a particularly preferred embodiment, the pressure and heat treat- ment zone consists of three portions, the pressure effect of which (the value or the time of action of the pressure) is separately adjustable, as shown by the zones 21 , 22, and 23 in Fig. 2. The figure shows how the fibrous web is pressed at three stages to improve the smoothness of the fibrous web's surface so that, at the first stage 21 , the fibrous web is preheated, at the second stage 22, the actual pressing that smoothens the fibrous web's surface is carried out and, at the third stage 23, the post-treatment that stabilized the fibrous web is carried out. According to the invention, the fibrous web is treated by at least one of these forming pressure and heat treatment sequences, but several treatments can also be carried out sequentially. In another embodiment, the pressure zone consists of one portion only, wherein almost the same pressure is acting (the stages 21 , 22 and 23 are combined). As needed, any other treating operation can be carried out between these formations, such as lining, coating, steaming, moistening, drying or the like.

The curve in Fig. 2 also shows by means of a dashed line another possible location for the position of the second stage 22 compared with the first stage 21 and the third stage 23. For example, at the first stage, the fibrous web can be influenced by a pressure of 0.05 to 0.5 MPa, the dwell time being from 20 to 100 ms. In this way, the fibrous web can be heated during the moderately long dwell time to a temperature suitable for the formation, whereby the fibrous web can be formed plastically. The advantage of such a long dwell time is that a reasonable temperature difference can be used between the heating belt 2 or the roll 5, 4 and the fibrous web W. In this way, a local overheating of the fibrous web possibly caused by too great a temperature difference is avoided. At the second stage 22, the fibrous web is influenced by a pressure of 0.2 to 20 MPa, for example, the dwell time being 0.5 to 10 ms. Using a reasonably low forming pressure in this way, the good properties of the fibrous web related to the bulkiness can be maintained, but still have a sufficient effect on the smoothness and the gloss of the fibrous web. At the third stage 23, the fibrous web is influenced by a pressure of 0.05 to 0.5 MPa, for example, the dwell time being from 20 to 100 ms. The third phase equalizes the state of the fibrous web so that part of the elastic deformation of the fibrous web caused by the second stage 22 changes into a plastic deformation. Similarly, the inner steam pressure differences of the fibrous web and individ- ual fibres can gradually become even during such a "release" without the pressure discharging too quickly and causing a weakening in the fibrous web's structure. When operating within the above-mentioned values, the end result is that a fibrous web according to the objects of the invention can be formed from the fibrous web formed by the high-consistency stock, enabling a good print quality.

Fig. 3 shows another alternative forming pressure/temperature curve of the fibrous web as a function of time. Compared with Fig. 2, the differences here mainly comprise "rounded" transitions from one stage to another, whereby the parameters influencing the fibrous web do not change as quickly. Similarly, the transient has been slightly cut, whereby the dwell time at the maximum pressure used is slightly longer than in the situation according to Fig. 2. The fine adjustment of such changes in the shapes of the curve is mostly dependent on the special properties of the fibrous web; thus, the fine adjustment must always be adapted case- specifically. Fig. 3 also shows an example of the heating of the fibrous web W compared with the forming pressure used. This is also highly proportional to the properties of the fibrous web and the material used, the fibre grade, the filling agents, the amount thereof, the porosity of the fibrous web and, in particular, to the heat conductivity produced as a result of the above. It is preferable for the method that, before the pressure and heat treatment, at the evaporation stage of the dry- ing of the fibrous web, the fibrous web is heated by means of a smooth contact surface, such as a drying cylinder or the cylinder of a Yankee dryer so that the total contact time is at least 1000 ms. The pressure and heat treatment equipment comprise a metal belt conveyor, long nip calender, conventional belt calender, and Condebelt dryer or Yankee dryer.

Claims

1. A method of manufacturing a fibrous web, wherein at least one layer of the fibrous web is formed from a stock with a solids content of over 1.5%, water being removed from the said fibrous web by first pressing the fibrous web to a solids content of about 45 to 65%, then evaporating the fibrous web to a solids content of about 75 to 95%, after which the fibrous web is treated by a pressing contact to increase its smoothness, characterized in that, in the manufacturing process, the fibrous web is treated before the reeling stage by at least one pressure and heat treatment, wherein the forming pressure exerted on the fibrous web is not higher than 20 MPa.

2. A method according to claim 1 , characterized in that, in the pressure and heat treatment, the forming pressure, the time of action of the forming pressure and the temperature are adjustable.

3. A method according to claim 1 , characterized in that, in the pressure and heat treatment, the fibrous web is formed at least once in the travel direction of the fibrous web to increase the smoothness of the fibrous web's surface at three stages so that, at the first stage, the fibrous web is preheated, at the second stage, the actual pressing is carried out, smoothing the surface of the fibrous web and, at the third stage, the post-treatment stabilizing the fibrous web is carried out.

4. A method according to claim 1 , characterized in that the said forming pressure is preferably less than 12.5 MPa, more preferably less than 10 MPa, more preferably less than 7.5 MPa, and most preferably less than 5 MPa.

5. A method according to claims 1 and 3, characterized in that, at the said first stage, the fibrous web is influenced by a pressure of 0.05 to 0.5 MPa, the dwell time being 20 to 100 ms.

6. A method according to claims 1 and 3, characterized in that, at the said second stage, the fibrous web is influenced by a pressure of 0.2 to 20 MPa, the dwell time being 0.5 to 10 ms.

7. A method according to claims 1 and 3, characterized in that, at the said third stage, the fibrous web is influenced by a pressure of 0.05 to 0.5 MPa, the dwell time being 20 to 100 ms.

8. A method according to claims 1 and 3, characterized in that, at the pressure and heat treatment stage, at least of the surfaces of the fibrous web is in contact with a surface that is heated to 70 to 250 ⁰C.

9. A method according to claim 1 , characterized in that, at the evaporation stage of the drying, the fibrous web is heated by a smooth contact surface, such as a drying cylinder or the cylinder of a Yankee drier so that the total contact time is at least 1000 ms.

10. A method according to claim 1 , characterized in that the fibrous web is a coated or uncoated paper web with a grammage of 40 to 150 g/m².

11. A method according to claim 1 , characterized in that the fibrous web is an uncoated board web with a grammage of over 100 g/m².

12. A method for manufacturing a fibrous web, a board web in particular, characterized in that at least one pressure and heat treatment according to claim 1 is carried out before the coating stage.

13. Equipment for manufacturing a fibrous web, wherein at least one layer of the fibrous web can be formed from a stock with a solids content of over 1.5%, the equipment containing a former and a press section for pressing the fibrous web to a solids content of about 45 to 65%, after which there is a drying section for evaporating the fibrous web to a solids content of about 75 to 95%, and after those, there are equipment for treating the fibrous web by means of a pressing contact to increase its smoothness, characterized in that, in the manufacturing process, there is at least one set of pressure and heat treatment devices to treat the fibrous web before reeling, the devices being capable of treating the fibrous web at a forming pressure of less than 20MPa.

14. Equipment according to claim 13, characterized in that the pressure and heat treatment devices comprise a means for adjusting the forming pressure, the time of action of the forming pressure and the temperature.

15. Equipment according to claim 13, characterized in that, in the travel direction of the fibrous web, there is at least one set of pressure and heat treatment devices for increasing the smoothness of the fibrous web's surface, the pressure and heat treatment devices being adapted to three stages so that the first stage is arranged to preheat the fibrous web, the second stage is arranged to actually smoothen the fibrous web's surface by pressing, and the third stage is arranged to provide a stabilizing post-treatment for the fibrous web.

16. Equipment according to claim 13, characterized in that the said forming pressure is preferably less than 12.5 MPa, more preferably less than 10 MPa, more preferably less than 7.5 MPa, and most preferably less than 5 MPa.

17. Equipment according to claims 13 and 15, characterized in that, at the said first stage, the fibrous web is influenced by a forming pressure of 0.05 to 0.5 MPa a length, which length corresponds to a dwell time of 20 to 100 ms.

18. Equipment according to claims 13 and 15, characterized in that, at the said second stage, the fibrous web is influenced by a forming pressure of 0.2 to 20

MPa a length, which length corresponds to a dwell time of 0.5 to 10 ms.

19. Equipment according to claims 13 and 15, characterized in that, at the said third stage, the fibrous web is influenced by a forming pressure of 0.05 to 0.5 MPa a length, which length corresponds to a dwell time of 20 to 100 ms.

20. Equipment according to claim 13, characterized in that, in the drying section, there is a smooth contact surface, such as a drying cylinder or the cylinder of a Yankee drier.

21. Equipment according to claim 12, characterized in that the pressure and heat treatment devices comprise a metal belt calender, long nip calender, ordinary belt calender, Condebelt dryer or Yankee cylinder dryer.