Patent Application
20230383464
This application claims priority to European Patent Application No. 22175115.9, filed May 24, 2022, the contents of which is incorporated by reference in its entirety for all purposes.
The invention relates to a smoothing device comprising at least one nip formed between two rolls, at least one of the rolls being a deflection-compensated and zone-controlled roll for regulating a thickness profile of a paper or board web, according to the preamble of claim 1.
From WO 2004/001132 A2 machines are known which enable the production and treatment including application of a treatment substance on at least one side of the material web in a so-called online or offline operation. For many applications, a material quality with higher smoothness and possibly gloss is desired. To achieve such a quality, the machine has a material web smoothing device along the material web path at least upstream of the application device. The upstream use of a material web smoothing device before the application of the treatment medium is particularly expedient and effective insofar as the non-contact application process, in particular coating process, largely preserves the contour of the material web surface. This means that a rough contour of the material web is not, or only comparatively little, evened out and thus smoothed by the treatment substance. The smoother the web was before the application of the treatment medium, the higher the productivity. Therefore, it has been found that it is better to smooth first and then apply the treatment substance than to smooth a rough contour after coating. With regard to the application of the treatment substance, a so-called curtain coating process is preferably used.
The non-contact starch application processes, for example curtain sizer and spray sizer, are known from EP 3 617 403 A1 and AT 519598 A2, WO2020020626A1. In these processes, the application quantity is metered via a quantity control on the application nozzles themselves. Blade devices are not required here.
From EP 0 374 292 A1 devices for smoothing and calibrating paper are known, which compact and smooth the paper. Thickness variations of the paper are compensated, the paper is calibrated. The result is a compacted paper with a leveled surface. For this, uneven density, reduced thickness, stiffness and opacity must be accepted. In order to calibrate a paper, i.e. to give a paper an identical thickness at all locations, the pressure given by the own weight of the rolls of a calender is normally too low. The calender is therefore subjected to additional pressure. In order not to limit the flexibility of the system to a few basis weight-pressure pairs, one of the rolls is preferably designed as a zone-controlled deflection compensating roll, which replaces a rigid crown.
Paper manufacturers today are often faced with the challenge of increasingly using recovered paper fibers (reclaimed fibers) in the production of paper, especially packaging papers and board, as well as special papers. For example, testliner in the EU is mainly made from recovered paper and fillers. Due to a high proportion of secondary fibers, the strength properties of packaging papers decrease. The use of starch in paper production can compensate for the loss of strength. Starch is usually either added directly into the pulp suspension or sprayed onto the wet web in the wire section. Furthermore, the starch can be applied to the paper web immediately after the pre-dryer group using contact or non-contact application methods.
Testliner and folding boxboard can be multi-coated. After pigment-coating in the film press, a middle coating layer and a top coating layer are usually applied to the paper web with a bladecoater. The subsequent coating process with a bladecoater requires a smooth surface of the base paper (macro surface roughness Bendtsen<500 ml/min, micro surface roughness PPS<3 μm) and a very uniform paper thickness (2sigma deviation<1%-1.5%). The paper thickness must show only small deviations across the width of the paper web. For this reason, pre-smoothing with a smoothing unit or with the aid of a hard nip calender is usually carried out here as well, for pre-smoothing before coating and for calibrating the paper thickness. For one-side smoothing of testliner and folding boxboard, only one nip is used. Preferably, a calender consists of two rolls, in particular a heated steel or chilled cast iron roll and an unheated profiling roll. In most cases, cross-profile thickness control is achieved with zone-controlled elements.
Size presses and film presses are known as contact application methods, as described for example in DE 3417487 A1 and DE 4131131 A1. The starch or sizing agent is applied to an applicator roll by means of a metering applicator and metered by means of a doctor blade. The paper or board web is coated on both sides in a press nip between the applicator rolls. The temperature of the starch is usually between 50° C. and 80° C. The typical solids content of the starch is between 8% and 15% with an application weight of 0.5 to 6 g/m 2 per side. In some cases, it is also possible with a film press of this design to apply the coating color consisting of starch and pigment particles with a solids content of up to 40% to the paper or board web. The line loads between the applicator rolls are usually between 40 and 70 kN/m. Film presses can be equipped with all types of rolls customary in the industry, as described for example in DE 10 2018 100 924 A1. It can be advantageous if at least one of the rolls is a shoe roll or a deflection compensation roll, so that the pressing power acting in the nip can be adjusted to set the conditions in the treatment press nip depending on the properties of the product produced, such as sheet thickness, grammage or quality of the fiber material used, and to optimize the starch transfer to the web.
It is known from EP 2 603 636 B1 that the paper web can be smoothed with the aid of a calender. In this process, a treatment medium is sprayed directly onto the paper web while it is lying on a calender roll. However, these methods have the disadvantage that defects in the application, such as those that occur when using spray nozzles (overlap defects, blockages, etc.), become directly visible in the paper.
A major disadvantage of the known arrangements is the high energy consumption of the configuration. Here, compromises have to be made in mechanical engineering and in the production process, which is often at the expense of paper quality.
It is therefore an object of the invention to provide a smoothing device with which a uniform paper thickness and uniform cross profiles of the strength properties (CMT (flat crush resistance), SCT (short-span compression strength), RCT (bursting strength)) are achieved after a surface application of coating media, in particular starch products, by using a cost-efficient technology.
This object is solved by the features of claim 1.
Herewith a device for smoothing is provided which combines the process of pre-calendering/profiling and coating in one unit, whereby the application of the medium or substance takes place indirectly and without contact, in which the medium is applied to at least one roll of the at least one nip. The passage of the application medium “into” the web is optimized according to the invention, whereas the prior art attempts to optimize the passage “onto” the web.
The measure for ensuring the strength properties of the paper or board web, in particular the SCT value, also remains according to the invention the application of a treatment substance, in particular starch solutions, to the surface of the paper/board. However, this is done neither by a size press nor by a film press but by the device for smoothing according to the invention. The smoothing device according to the invention combines the technological measures of surfacing the web and penetrating the treatment medium, in particular starch, into the web in one unit.
Due to the pressure in the at least one nip of the device for smoothing, the surfacing of the web and the penetration of the application medium into the web are technologically carried out together and with a resulting synergistic benefit. Surprisingly, it has been found that both treatment measures affecting the paper strength can be combined in a mutually beneficial way, by the combined line load/hydraulic pressure on the paper or board web when passing through the at least one nip.
A major disadvantage of the state of the art, requiring two independent units, is eliminated. The installation space does not pose a problem for supplementary parts.
Consequently, if paper manufactures need/want to put to their existing lines additional coating units and calenders, the solution according to the invention provides high flexibility. Additional space is not required. According to the state of the art, an installation of additional calenders and coating units would require an extension of the paper machine, so that an installation in the existing mill hall would often no longer be possible. Here, compromises would have to be made in the machine construction and production process, often at the expense of paper quality.
Machines for the production of “flexible packaging” can be regarded as an example of such a disadvantageous case according to the state of the art. Due to the increased conversion to recovered paper fibers for cost reasons, a smoothing of the paper web before coating has become necessary. This space has not been provided for in machine configurations so far, which is why operators of these machines face major problems according to the prior art.
It is to the credit of the inventors to have recognized that uniform strength properties can be achieved across the width of the paper web, since the local pressing power required to profile the paper web and the locally applied amount of treatment substance or medium, in particular the amount of starch, can be matched. The required pressing power is determined by the local paper thickness. The quantity of treatment substance or medium, in particular starch, supplied is influenced by the predetermined pressing power.
The penetration of the coating medium, in particular the starch, and the resulting strength increase of the paper is directly dependent on the local pressing power. Thus, in places where a greater paper thickness was previously present, significantly improved paper strength properties result (CMT, SCT, RCT). The increase in paper strength properties results here from a higher proportion of starch (due to increased pressing pressure) and due to a higher density of the paper. This would result in an undesirable strength deviation of the paper in the CD direction, which is overcome according to the invention by the fact that the at least one coating unit has at least one delivery nozzle with a device for cross-profile adjustment of a volume flow which can be adjusted zone by zone over the web width.
A key factor is that the quantity of starch applied and the pressing power in a zone are matched to each other in such a way that the same paper strength properties result in all zones. This means that smaller quantities of starch must be applied in zones with a higher pressing power than in zones with a lower pressing power.
According to the invention, the implementation of this finding leads not only to much more uniform paper properties and thus to an improvement in quality, but also to savings in starch and thus to cost savings for the customer. This invention thus has a very high economic benefit.
According to the invention, a technologically and economically efficient technology is presented for increasing the strength properties of the paper or board by increasing the starch penetration. In terms of the invention, this objective is achieved by using a non-contact application process (preferably coating process with slot die) that allows a very high solid starch content to be applied (up to a maximum of 35%).
It is also possible to exert a specific influence on the cross-profile, for example by using the starch application and the pressing power additively (higher pressure combined with higher starch quantity) in order to achieve better strength properties in selected areas of the paper web (for example in the edge area). Of course, the strength properties could also be worsened locally if this would be advantageous for the further course of paper production. An example here is the control of the application quantity with dilution water. Under certain circumstances, it may make sense to add more water in the edge area to counteract drying of the paper edge, even if this reduces the starch content so that the strength properties deteriorate.
With the device for smoothing according to the invention, the line load in the at least one nip can consequently be profiled via, for example, scanners in such a way that a desired cross-thickness profile is produced. The volume flow of the application medium, in particular starch, can be adjusted with regard to the local, zone-controlled volume flow at the nozzle in order to control the application quantity of medium/starch depending on the strength properties of the web.
Furthermore, the functional dependence of the paper strength on the parameters—properties of the starting materials and basis weights of the fibers used—can be taken into account as a variable influencing the distribution and quantity of the volume flow of application medium. Since the consistency of the pulp suspension at the outlet of the headbox defines the basis weight produced, a measured consistency value can be measured in each zone of the headbox by means of sensors. The measured consistency transverse and longitudinal profile can be used as a controlled variable in a control loop to determine the local volume flow in CD direction. The headbox plays a key role in ensuring paper quality. Important paper properties are significantly influenced by the headbox. In the context of automatic process control, the headbox is gaining increasing influence as a corrective element for specific deviations that require downstream process steps, e.g. calendering and coating. The device for smoothing according to the invention makes use of this headbox as a correction element.
For the technical implementation of the invention, the use of a non-contact applicator with slot nozzles can be provided in conjunction with rolls of a device for smoothing serving as applicator rolls, wherein at least one applicator roll is designed as a zone-controlled profiling roll. These rolls are known, for example, under the name MHV roll or NIPCO roll. The zone widths are usually between 50 and 300 mm. The zones can be controlled individually and enable exact profile correction.
The coating systems used are also divided into zones. Such coating systems are known from DE 10 2009 036 853 B3 and DE 10 2009 048 820 A1, among others. These zones allow the application of different treatment medium/strength quantities in the machine width. This can be achieved, for example, by admixing water into the individual zones, or by feeding different amounts of starch suspension per zone. Another simple method of applying different quantities of starch across the width of the machines can be to set different slot widths at the nozzle outlet by means of a slot adjustment.
This process is preferably possible at rather moderate temperatures (as is usual for starch penetration) due to the high moisture content of the fiber web in the calendering nip. While the usual moisture content in conventional calendering is in an average moisture range of 4%-8%, in the technique according to the invention it is, depending on the area of application, for example in the range between 20% to 60%.
The increase in moisture content means for the possibility of profiling that with a conventional calender with a moisture content of 8%, the temperature of the fibers must be approx. 80° C. in order to deform them. At 4% moisture already 135° C. Since the temperature must reach a depth of approx. 15 μm to 20 μm in the short time of the nip passage in order to achieve sufficient deformation, the actual temperature of the roll surface must be approx. 20-30° C. higher than the glass transition temperature.
In this respect, the technique according to the invention may differ substantially from a conventional calendering process. One major difference is that an already heated treatment medium can be introduced into the fiber web, which in turn raises the temperature of the fibers to deeper layers. It is therefore not necessary to heat the rolls significantly higher than the glass transition temperature of the cellulose fibers.
Since the moisture content of the fiber web can range from 20% to 60%, the usual temperatures at which starch is applied (between 70° C. and 100° C.) are quite sufficient to profile the fiber web.
For example, in order to enable a high penetration of the starch into the paper web despite, for example, a high solids content, the starch can be pressed into the paper under high pressure and high temperatures according to the invention.
One or both rollers of the device for smoothing can be tempered. The rolls may further be provided with hard coatings (ceramic, chrome or the like), but may also be provided with hard covers made of polymer. One or both rollers can also be designed as soft rollers. The hardness of the surfaces is preferably less than 30 Pusey & Jones.
Further advantages and embodiments of the invention can be found in the following description and dependent claims.
The invention is explained in more detail below with reference to the embodiments shown in the accompanying figures.
As shown in
For this purpose, the roll 3 has a roll shell 7 rotating about an axis of rotation, on the inner circumference of which supporting elements 5 exert a force. The support elements 5 form individual hydrostatic pressure elements. A plurality of support elements 5 are distributed over the roll width and are divided into zones (pressure zone (1) to pressure zone (n)) so that pressure profiling is possible over the machine width. By means of the roll 3, also referred to as profiling roll with MHV elements, it is consequently possible to set a local line load distribution selectable in the at least one nip 6 over the roll width/machine width.
The at least one nip 6 is thus provided to smooth the thickness transverse profile of the material web 9 by means of an adjustable line load profile when the material web 9 passes through the nip 6. On the other hand, and simultaneously with the exertion of a pressing pressure in the nip 6, a treatment substance or treatment/application medium 2 is placed into the material web 9. For this purpose, at least one coater 1 comprising a dispensing nozzle 8 is provided for applying application medium 2, in particular starch.
The at least one applying device 1 is arranged to provide for the application of the treatment medium 2 to at least one of the two rolls 3, 4 of the nip 6. The mass flow of treatment medium 2 is applied to the surface of at least one of the two rollers 3, 4 rotating in opposite directions, for example roller 3 (
As shown in
As shown in
As
As
In a known manner, the device according to the invention for smoothing and applying a treatment medium 2 may preferably comprise preparation pans/blades 23, collecting tanks/guide plates 26, scrapers 27 and spray water or steam nozzles 28 for cleaning the (application) rollers 3, 4.
As
As
The break-down of the mass flow of treatment medium 2 into partial flows 22 in relation to a number of sections 19 is carried out in coordination with the pressure profiling in the nip 6 by the profiling roll 3. The pressure zones (1) to (n) formed can be selected to be equal to, smaller than or larger than the zones of the sections 19 of application medium 2 in the CD direction. The respective amount of treatment medium 2 in a partial stream 22 can be adjusted as well as the respective line load in the pressure zones (1) to (n) of the profiling roll 3. Together with a hydrostatic pressure element 5 of the profiling roll 3 defining one of the pressure zones (1) to (n), an associated section 19 can form a continuous zone.
If a starch is to be applied on both sides, the nip 6 can be formed by two rolls 3, 4, both of which can be designed as heated rolls. In all other respects, the roll 4 is preferably designed like the roll 3 described above.
The line loads in the press nip is preferably between 20 kN/m and 200 kN/m, preferably between 80 kN/m and 120 kN/m. The technique according to the invention can be used for starch application (sizing) and for pigmentation (pigmenting).
The starch is preferably applied with a solids content between 6% and 35%, particularly preferably between 15% and 35%, and a viscosity between 20 mPa s and 200 mPa s. The application weight is preferably in the range between 0.5 g/m2 and 6 g/m2 per side. A film of the thickness in the range of 10 ml/m2 to 40 ml/m2 can be applied to the smoothing roller with the additional function of an applicator roller.
In the case of pigmentation, the solids content can be between 15% and 40% and the viscosity between 50 mPa s and 400 mPa s. The coating weight is preferably in the range between 2 g/m2 and 5 g/m2 per side.
In
The outlet widths of the (application) nozzle 8 do not necessarily have to have the same width as the effective zones of the pressure elements 5.
Even with smaller outlet widths, it can make sense to arrange the pressure zones (1) to (n) and the outlet positions in an overlapping arrangement, as shown in
Of course, it is also conceivable to reduce the outlet widths even further if necessary. The lip adjustment as a form of execution is suitable here. The adjustment makes it possible to very finely adjust the distribution of the application medium over the machine width. Further possible variations are shown in
The following applies equally to all of the above examples of embodiments:
One or more scanners (not shown) can be assigned to the system, which record quality characteristics such as paper thickness, coating weight or paper moisture in CD direction and process them for control purposes and make them available for cross-profile control.
The zone width of the profile setting on the slot die 8 is, for example, 50 to 300 mm, preferably 150 to 280 mm.
The zone width of the hydrostatic elements 5 for profile correction is, for example, 50 to 300 mm, preferably 150 to 280 mm.
On at least one of the slot nozzles 8, the device for zone-wise slot width adjustment or a device for zone-wise adjustable mass flow control is provided.
The hardness of the covers of the smoothing rolls 3, 4 is 0 to 30 P&J, preferably 0 to 5 P&J. The diameters of the smoothing rolls 3, 4 are preferably in the range 1000 to 1800 mm, particularly preferably 1200 to 1600 mm. The discharge slot 17 of the discharge nozzle 8 is preferably 100-500 μm.
The pressure in each zone of the treatment nip 6 and the quantitative starch application are matched to achieve a desired cross profile of paper thickness and strength properties (CMT, SCT, RCT).
The zone of the applying system and the associated, downstream press zone of the profiling roll are linked with each other in terms of control technology, so that automatic balancing of the press pressure and starch application quantity is possible. The overall system can be integrated into the process control system of the paper machine. Scanner data from the machine can be used to control the zones individually so that the desired paper strengths (CMT, SCT, RCT) can be set in machine width. The macro surface roughness of the paper after treatment nip 6 is <800 ml/min at Bendtsen, preferably <500 ml/min.
The coating weight of the starch is in the range 0.5-6 g/m2 per side. The line loads in the nip is between 20 kN/m and 200 kN/m, preferably between 80 kN/m and 120 kN/m. The surface temperature of at least one of the smoothing rolls 3, 4 is 60° to 160° C. The paper or board web speed is preferably between 250 m/min and 2000 m/min, more preferably between 600 m/min and 1800 m/min. The dry content of the material web 9 is preferably more than 90% and is preferably between 92% and 98%. The starch can be applied with a solids content between 6% and 35%, preferably between 15% and 35%, and a viscosity between 20 mPa s and 200 mPa s. The temperatures of the starch are preferably between 50° C. and 110° C., particularly preferably between 70° C. and 100° C.
For further designing of the device for smoothing according to the invention, the following is provided:
The at least one deflection-adjustable and zone-controlled roll 3, which is used to influence the line pressure distribution in the at least one nip 6, can be adjusted in the transverse direction of the web for calibrating the material web 9.
The at least one deflection-adjustable and zone-controlled roll 3, which is used to influence the line load distribution in the at least one nip 6, can be adjusted to control a thickness of the material web 9 in the transverse direction of the web.
The at least one deflection-adjustable and zone-controlled roll 3 is a hydrostatically supported roll with zone-wise control of elements 5 along the width of the roll, wherein each (pressure) zone can be designed to be controlled in groups or by individual elements.
As
Alternatively or additionally, a volume flow profile can be set at the applicator by offline measurement of paper/board web samples.
The cross-profile setting of the volumetric flow/mass flow of the application medium 2, which can be set zone by zone over the web width, can be controlled as a function of an area-related mass distribution of the material web 9 in the transverse direction of the web.
As
Consistency sensors can be provided with which a consistency transverse and longitudinal profile of a pulp suspension can be measured in the resolution of an actuator division at the outlet of the headbox, which can be selected as an input variable of a control loop 35 for the quantity of application medium 2 fed in each zone. The adjustable line pressure distribution 33 and the adjustable quantity of application medium 2 can be coordinated with each other by means of a control 36 in such a way that a compacted paper with a levelled surface with selectable strength properties (CMT, SCT, RCT) 34 can be produced in the cross-web direction.
Finally, the dispensing nozzle 8 of the curtain coater, which dispenses the medium 2 under the action of gravity, may have a plurality of slit nozzles 18, each with a discharge slit 8 having an accuracy in the range±2.5 μm to ±10 μm in slit length with a slit width of 100 to 2000 μm.
The coater 1 may be designed to control the solids content of the application medium 2 in at least one layer from 6% to 55%.
The curtain coater 1 can be designed to form a multilayer curtain.
The specific volume flow from the nozzle 8 of the curtain applicator 1 can be in the range of 4 l/(min x m) to 30 i/(min x m). A film of the application medium 2, in particular starch, in the range from 5 ml/m2 to 100 ml/m2 is preferably applied to the respective smoothing roll 3, 4.
Furthermore, guide rollers (not shown) can also be provided, which can define a treatment path of the running material web 9 between curtain coater 1 and press nip 6.
A certain roll outer circumference length corresponding to the dwell time can be set between an impact of the curtain 2 and its entry into the press nip 6, thus increasing or decreasing the dwell time as indicated by the angle α. If the smoothing roll 3, 4 is heated, the dwell time can be used to influence a temperature increase of the application medium 2 before it enters the press nip 6. The treatment medium 2 can then be thermally pretreated in this way in the press nip 6 between the smoothing rolls 3, 4 and pressed into the material web 9 on one or both sides.
The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22175115.9 | May 2022 | EP | regional |
