Method for rewetting a web made from paper, and plant

Abstract

A method is specified, for rewetting a web (4) made from paper, wherein the web (4) is guided through a plant (2), wherein the plant (2) comprises a processing unit (10), by way of which the web (4) is processed, wherein, downstream of the processing unit (10), the plant (2) comprises a drying unit (12), for drying the web (4), wherein, downstream of the drying unit (12), the plant (2) comprises a plurality of wetting units (18, 20), by means of which the web (4) is rewetted in multiple stages using a wetting agent, by pre-wetting of the web (4) taking place by way of a first one of the wetting units (18), and post-wetting of the web (4) taking place after a minimum dwell time (T) by way of a second one of the wetting units (20), wherein the minimum dwell time (T) is selected in such a way that the web (4) has an increased absorption speed for the wetting agent in the second wetting unit (20) on account of the pre-wetting. Furthermore, a plant (2) is specified, for processing a web (4) made from paper.

Description

[0001] Description

[0002] Method for remoistening a paper web, and system

[0003] The invention relates to a method for remoistening a paper web and a plant for processing a paper web.

[0004] Examples of systems for processing paper webs are printing machines for printing such a web, and corrugated board systems for producing a web (more precisely corrugated board web) which is composed of several individual layers of paper.

[0005] When processing a paper web, the web is regularly dried, e.g. after printing in a printing press. During drying, a significant amount of moisture (i.e. water) is removed from the paper, which is then expediently added back to the paper later as part of a remoistening process. The problem with this, however, is that the paper only has a limited ability to absorb water that is applied to the web during remoistening. In some circumstances, the previously removed moisture cannot therefore be fully returned. This then regularly leads to defects such as wrinkling, tin can effect, roping and the like. Such defects are in turn problematic when the web is wound up at the end of the system or when the web is transferred directly to a downstream system in inline operation.

[0006] US Pat. No. 5,596,930 A describes a method for moistening a moving web of printed material. A moistening agent can be applied to the web in several steps. Against this background, the object of the invention is to improve the remoistening of a paper web. To this end, a correspondingly improved method and an improved system are to be provided.

[0007] The object is achieved according to the invention by a method having the features of claim 1 and by a system having the features of claim 15. Advantageous refinements, developments, and variants are the subject of the dependent claims. The statements relating to the method also apply mutatis mutandis to the system, and vice versa. Where steps of the method are specified below, advantageous refinements for the system result from the system being designed to perform one or more of these steps.

[0008] A key concept of the present invention is, in particular, multi-stage remoistening, in which the remoistening of a paper web is divided into several stages, in each of which a moistening agent is applied to the web. This makes it possible to utilize the fact that a pre-moistened web subsequently absorbs further moisture significantly better than a non-pre-moistened web, making it possible to introduce significantly more moistening agent, specifically water, and thus moisture into the paper.

[0009] The method according to the invention serves to remoisten a paper web and is, in particular, also an operating method for a system in which remoistening takes place. Without limiting the generality, the following assumes a single-layer paper web, i.e., a paper web in the narrower sense. However, the invention is equally applicable to other, particularly multi-layer paper webs, e.g., a corrugated cardboard web or a precursor for such a web, which is then composed of several individual paper layers.

[0010] The web is guided through a system. The web is unwound, for example, by an unwinder of the system, then processed, and finally wound up again by a rewinder of the system or, alternatively, passed directly to a subsequent system. In the following, it is assumed, without limiting the generality, that the system is a printing press; however, the statements made here apply quite generally to any system that processes a paper web and, in connection with this, carries out remoistening. A configuration in which the system is a corrugator, for example, is particularly advantageous for the production of corrugated board.

[0011] The system generally comprises a processing unit with which the web is processed. Furthermore, the system has a drying unit downstream of the processing unit for drying the web (i.e. the processed web). The drying unit has in particular one or more hot air dryers and / or infrared dryers for drying. The type of processing and drying are initially irrelevant; what is more important is that moisture is fundamentally removed from the paper in the web during drying. This moisture should be replenished as completely as possible by means of remoistening as part of the process. For this purpose, the system has several moistening units downstream of the drying unit, by means of which the web (i.e. the dried web) is remoistened in several stages with a moistening agent.Accordingly, the moistening units rehydrate the web after drying by applying a moistening agent to the web. In this case, the web is remoistened with the moistening agent in several stages: a first moistening unit pre-moistens the web (also known as "initial moistening or pre-wetting"), and a second moistening unit, after a minimum dwell time, re-moistens the web (also known as "secondary wetting"). Pre-moistening and re-moistening typically occur on the same side of the web; in one practical embodiment, even on both sides.

[0012] The wetting agent contains, in particular, water, preferably predominantly (i.e., > 95% or even > 99%) or is even exclusively water. The water is suitably as soft as possible and / or demineralized. In a particularly advantageous embodiment, the wetting agent is a mixture of water and a wetting additive, preferably with a proportion of at most 1 vol.%, particularly preferably of at most 0.1 vol.%, which is usually sufficient for high-quality wetting additives to achieve a minimum surface tension. The wetting additive serves to reduce the surface tension of the water (e.g., from 72.7 mN / m (polar portion: 51 mN / m, disperse portion:

[0013] 21.7 mN / m) at 20 °C to 28.5 mN / m (polar fraction: 5.2 mN / m, disperse fraction: 23.3 mN / m), thus further increasing the absorption rate. Optionally, the humidifying agent can also be mixed with air or another inert gas (e.g., nitrogen).

[0014] In this context, “A is upstream of B” and equivalently “B is downstream of A” mean that A is ahead of B along the trajectory, i.e. the trajectory first passes through A and then B. Instead of “upstream / downstream”, the terms “before / behind” and “before / after” are also used.

[0015] For the sake of simplicity, we will refer to "the web" being treated (processed, dried, moistened, etc.). This specifically means that the web undergoes the appropriate treatment section by section, not first being fully processed, then completely dried, and only then moistened. Rather, the web is continuously (i.e., inline) conveyed through the entire system, with each web section being treated at a specific location and then directly passed on.

[0016] The minimum residence time is now selected such that the web in the second moistening unit has an increased absorption rate for the moistening agent due to the pre-moistening (by the first moistening unit). In a suitable, but not mandatory, embodiment, this is an absorption rate in the range of 0.1 s to 1 s, particularly for a quantity of 3 g / m 2 Humidifier understood (ie then 3 to

[0017] 30 g / (s*m 2)) and for other amounts a corresponding absorption rate. In other words: through pre-moistening, moistening agent is applied to the web and is absorbed into the paper during the minimum residence time, so that after the minimum residence time the web has increased hydrophilicity. This takes advantage of the knowledge that during pre-moistening the moistening agent initially forms a liquid film on the web which, due to the previous drying, is not immediately completely absorbed into the paper, but at least partially remains and in any case requires a certain amount of time to be completely absorbed by the paper. During this time the web is, so to speak, flooded with the moistening agent and further supply of moistening agent is pointless because it cannot be absorbed into the paper. The liquid film forms a corresponding barrier and limits the maximum absorption capacity of the paper.This maximum absorption capacity is also limited by the conveyance of the web through the system and the available distance, because as soon as the web is to be further processed or wound up, no excess moistening agent may remain on the web and must be skimmed off if necessary.

[0018] Accordingly, in this case the remoistening is divided into several stages, namely into at least two moistening units, which then form a first and a second remoistening stage. The pre-moistening does not yet take place with the actual target amount of moistening agent, but with a reduced amount of moistening agent. Additional moistening agent is then only added during the post-moistening with the second moistening unit. However, since the web is now pre-moistened, the moistening agent in the second moistening unit penetrates the paper much faster than without pre-moistening (i.e. the absorption rate is increased), so that in total and over the same distance more moistening agent can be introduced into the web than with just one-stage remoistening. This advantage also applies analogously to remoistening with more than two stages.

[0019] The minimum residence time serves in particular to wait for the most favorable time for re-moistening. As already indicated, during pre-moistening the moistening agent initially forms a liquid film on the web, which should have been broken down as largely as possible by the time the web reaches the second moistening unit. As long as a liquid film is present on the web, the absorption rate for further applied moistening agent is practically zero and only increases once the liquid film has been absorbed by the web. Therefore, the minimum residence time is preferably selected such that the moistening agent applied by the first moistening unit is completely absorbed into the web when the web reaches the second moistening unit. Especially in this state and at this time, an increased absorption rate results.In principle, it is also possible to simply skim off any liquid film, e.g. with a squeegee, but in this case, any moistening agent that has already been applied is discarded and remains unused.

[0020] The rate at which the humectant is absorbed into the paper depends heavily on the initial moisture content (i.e., initial moisture content) of the paper. Dried-out paper, as it is after the drying unit, absorbs water more slowly than pre-moistened paper. The reason for this is that the hydrophilicity is increased by the adsorbed water molecules on the capillary walls of the paper. This reduces the contact angle and thus increases the absorption rate. This relationship is also summarized in the so-called Lucas-Washburn equation (or simply Washburn equation). Accordingly, a lower contact angle results in a greater penetration depth per unit time, and thus an increased absorption rate. Equivalent to an increased absorption rate are, in particular, an increased penetration depth, increased hydrophilicity, a reduced contact angle, and a reduced surface tension.

[0021] A particularly problematic condition for the absorption rate of the web is what is known as keratinization of the paper. The paper contains paper fibers which absorb moisture and release it again during drying. Keratinization is understood to mean any fiber change (i.e. actual change in the paper fibers) that occurs during drying and re-moistening processes. Examples of such fiber changes are irreversible pore closure (reduction of the specific surface area) due to the formation of hydrogen bonds between neighboring cell walls or the fusion of microfibrils. Keratinization is particularly irreversible. Keratinization regularly occurs during drying in the drying unit. Starting with keratinized paper, a renewed supply of moisture is particularly difficult, i.e. the absorption rate is particularly low.The process presented here does not attempt to introduce as much moisture as possible within a given time and over a given distance using just a single humidification unit, but rather, preferably, to initially achieve an increased absorption rate in a first stage by means of pre-humidification. Only once the absorption rate has been significantly increased, ideally upon reaching a suitable threshold value for the absorption rate (e.g., from the range already mentioned above), is further moisture introduced into the paper in a second stage by means of post-humidification. This time, however, is significantly more efficient than with keratinized paper, so that overall, more moisture is introduced into the paper over the same time and distance.Due to the multi-stage remoistening and the resulting higher moisture content of the paper, keratinization of the paper in the system is advantageously reduced or even completely avoided, especially when drying after remoistening or drying between the two stages of remoistening.

[0022] As a result, significantly more moistening agent, especially water, can be advantageously introduced into the web, thereby achieving a particularly high moisture content. Similarly, a particularly large loss of moisture during drying can be advantageously completely compensated for or, if necessary, even overcompensated for. This reduces or completely prevents defects such as roping, wrinkling and the tin can effect during conveying and processing of the web in the system. Of particular importance here is the wrinkling-free nature of the web, which is ensured by the multi-stage remoistening, particularly both within the system and at the end of the system. In addition, the multi-stage remoistening has a positive effect on web shrinkage in the system, in particular by significantly reducing the effect of web shrinkage caused by drying through the subsequent remoistening.In addition, multi-stage remoistening makes it possible to achieve a difference in the moisture cross-section of less than 2%, and in particular a maximum of 1.3%, i.e. the difference in moisture at any two points along the width of the web is less than 2% or a maximum of 1.3%. From a difference of 2%, what is known as warp occurs during further processing of the web in a corrugator, i.e. the corrugated board web distorts. Finally, multi-stage remoistening also enables high machine dynamics for the system, particularly with regard to changes in web speed and job changes, since shrinkage of the web is compensated for at the appropriate point as required. This can be demonstrated in particular by means of suitable sensors for measuring the width of the web in the system and is also noticeable at the end of further processing in a corrugator as a precisely cut blank.

[0023] The minimum residence time is preferably in the range from 0.5 s to 30 s, particularly preferably in the range from 1 s to 30 s, although the upper limit of 30 s is essentially unnecessary, i.e. the minimum residence time is preferably at least 0.5 s or at least 1 s. Tests have shown that such a minimum residence time is sufficient to achieve the described effect. The minimum residence time is equivalent to a minimum distance that the web travels between the first and the second moistening unit. The conversion between minimum residence time and minimum distance is carried out using the web speed, i.e. the speed at which the web is conveyed through the system. The web speed may vary depending on the order.For the stated range of 0.5 s to 30 s, at preferred web speeds of 100 m / min to 450 m / min, this results in a nominal minimum distance in the range of 1 m to 225 m; in any case, a minimum distance of at least 2 m is advantageous. In particular, the maximum minimum distance is significantly less than 225 m, since the web run (i.e. the distance covered by the web) through the entire system is typically significantly less than 200 m. A minimum distance between the first and second moistening units in the range of 25 m to 50 m, in particular 35 m to 40 m, is particularly preferred. These ranges are advantageously still completely within the system. This then results in a minimum residence time of approximately 4 s to 30 s or 5 s to 24 s for the stated web speeds.

[0024] The minimum residence time between the first and second moistening stages is preferably selected for a maximum web speed (e.g. 450 m / min) of the system. Lower web speeds are then typically unproblematic, since a longer residence time does not produce a significant change in the moisture content of the paper. Accordingly, there is initially no upper limit for the minimum residence time; the primary factor is that the web is still moist when it reaches the second moistening unit and in any case is not completely dry, resulting in an increased absorption rate for re-moisturizing. Irrespective of this, the amount of moistening agent applied is expediently adjusted depending on the web speed in such a way that, with varying web speeds, the same amount of moistening agent is always applied per unit of time and per unit area.

[0025] The amount of moisture that the paper can absorb typically depends on the grammage (e.g. in the range of 100 g / m 2 up to 300 g / m 2 or even more) of the paper and generally also on the paper type. The higher the grammage, i.e. the heavier the paper, the more moisture it can absorb. In addition, paper with a higher grammage (especially > 300 g / m 2 ) are typically less dried out (when considering the moisture content in %), so that a strong re-moistening corresponding to the high grammage is not regularly required, nevertheless larger quantities (especially > 3 g / m 2) than with lower grammages is possible without any problems (i.e. in particular without the formation of wrinkles). In an advantageous embodiment, the amount of moistening agent that is applied to the web by means of the moistening units per unit of time is set as a function of the grammage of the paper, with the amount being increased for higher grammages. For the above reason, the relationship is not necessarily linear and in particular does not necessarily continue up to very high grammages. Analogously, alternatively or additionally, the amount is set as a function of the paper type. This is based on the consideration that a binder in the paper and possibly other additives as well as the porosity and initial moisture content of the paper, i.e. generally the paper type, influence the absorption rate, i.e. the ability of the paper to absorb moisture.

[0026] Preferably, a maximum of 3 g / m 2 Humidifying agent is applied. This amount is optimal for a typical system dimensioning and the ranges mentioned above for the minimum residence time, and is also suitable for typical paper grades and grammages. Depending on the grammage, a quantity of 1 g / m is preferably applied with a single humidification unit. 2 up to 3 g / m 2 of moistening agent is applied, whereby - as already described - a higher grammage also requires a higher amount. With at least two moistening units, a total of up to 6 g / m 2 Apply a moistening agent, which is completely absorbed by the paper due to the multi-stage remoistening process, thus achieving a correspondingly high moisture content. Especially for very high grammages (especially >300 g / m 2) or generally, a quantity of up to 4, 5 or 6 g / m 2 or more is applied. The total amount of moisturizer then increases accordingly.

[0027] Suitably, at least 5 g / m 2Humidifying agent is applied or the web is remoistened to at least 4% (i.e. here and otherwise wt.%) residual moisture. Suitably, a web of coated paper is remoistened to at least 4% to 5% residual moisture using the humidifying units and / or a web of uncoated paper is remoistened to at least 5% to 7% residual moisture. To achieve a target water content (equivalent to target residual moisture) by means of remoistening and at the end of the system, a target quantity of humectant is suitably specified, which is then distributed among the humidifying units so that they apply the target quantity to the web in several stages. The residual moisture indicates the amount of moisture in the web relative to the weight of the web. Depending on the grammage, a quantity of 5 g / m 2 up to 8 g / m 2preferred, whereby - as already described - a higher amount is used for a higher grammage. Alternatively or additionally, a residual moisture content in the range of 5% to 6% is preferred, particularly regardless of the grammage. The drying unit regularly dries the web to a residual moisture content in the range of 2.5% to 3.5% (uncoated paper); accordingly, 1.5% to 3.5% residual moisture is then added during remoistening; this is typically not possible with just one-stage remoistening. Coated paper regularly stores less water relative to the layer thickness of the paper than uncoated paper and is then typically dried to 1% to 1.5% (from an initial moisture content of 5%), so that 2.5% to 4% residual moisture is added during remoistening in order to achieve a moisture content of 4% to 5%.

[0028] Preferably, the humidification units are each designed as spray bars. These are characterized in particular by their particularly small installation space requirements and high cost-effectiveness. Furthermore, spray bars are advantageously not operated in recirculation mode, so that only fresh humidification agent is always applied to the web. In principle, other designs are also conceivable, e.g. dampening units with rollers or spray discs, although the latter in particular are less economical than spray bars and also require significantly more installation space. The installation space requirement is a particularly important aspect in the multi-stage rehumidification system described here, because this requires that a humidification unit be installed at at least two different locations within the system.However, since systems are usually designed to be as compact as possible and many components are combined in a small space, multi-stage rehumidification with large humidification units (e.g. spray discs) is not easily possible. Especially with existing systems, retrofitting is not possible without fundamentally redesigning the system. Spray booms, on the other hand, are particularly compact and can be easily retrofitted at various points in existing systems without having to abandon the previous system design. The use of spray booms also has the advantage that they typically have several spray nozzles arranged along the width of the web. This means that even an inhomogeneous moisture profile can be easily compensated, i.e. homogenized, by selectively controlling individual spray nozzles, e.g. switching them on and off or adjusting the amount of humidifying agent discharged.

[0029] There are several suitable configurations for arranging the humidification units within the system, some of which are described below. These configurations can also be combined with one another.

[0030] The system has at least one section containing the aforementioned processing unit and the drying unit. Depending on the design, the system may have additional sections through which the web is guided sequentially. This means that at the end of one section, the web is passed directly to a subsequent section, allowing multiple processing and / or general treatments of the web. A final section is preferably a winder, with which the web is finally wound into a roll, or a transfer line, via which the web is transferred directly to a subsequent system without being wound up.

[0031] In a suitable embodiment, the humidification units are arranged downstream of a cooling unit of the system, which is located downstream of the drying unit. The cooling unit serves to cool the web again after the drying unit. The humidification units are not integrated into this cooling unit, but are arranged separately and at a different location, in particular further downstream of the cooling unit.

[0032] In a further suitable embodiment, the web is not dried and in particular not processed between the first and second moistening units, but preferably only conveyed. Accordingly, no further processing or drying unit is arranged between the first and second moistening units; the moistening agent is simply drawn into the paper. If the system has multiple sections, the first and second moistening units are arranged between two sections, or at the beginning or end of a section, or a combination thereof. The first and second moistening units, as well as the minimum distance between them, are collectively referred to as a two-stage moistening section.

[0033] In a further suitable embodiment, the system comprises several sections, each with a processing unit for processing the web and a subsequent drying unit for drying the web. At least one of the humidification units is arranged downstream of each drying unit, so that the web is humidified at least once after each drying cycle, and in particular before the next drying cycle or before the end of the system. In contrast to the two-stage humidification section, the humidification units are now distributed throughout the system and are not arranged contiguously.

[0034] This is based on the consideration that it is typically harmless if drying takes place between two humidification units, provided that the desired residual moisture content is achieved at the end of the system. Humidification between two drying units also advantageously prevents the described keratinization of the paper, so that even if the distance between two humidification units and, if applicable, a drying unit in between is longer than the minimum distance, the absorption rate at the humidification unit further downstream is increased, created by the humidification unit further upstream.

[0035] In a preferred embodiment, the system is a printing press, in particular a digital printing press. Additionally or alternatively, the processing unit is suitably a printing unit for printing the web with an ink, preferably a water-based ink and / or inkjet ink. In a digital printing press, the printing unit typically has several printing bars, in particular for different colors (e.g. CMYK). In a printing press, the drying unit is then a print-drying unit for drying the ink. The multi-stage remoistening described here is particularly suitable for use in a printing press, since the web is regularly dried intensively in order to dry the print created during printing with the ink. As a side effect of this, moisture is also removed from the paper, which is then added again during remoistening.

[0036] In a suitable embodiment, the system has a primer application unit and a primer drying unit upstream of the printing unit, and a varnish application unit and a varnish drying unit downstream of the printing unit. The primer application unit processes the web in such a way that a primer is applied, which serves in particular as the basis for subsequent printing. The primer drying unit dries the applied primer. Analogously, the varnish application unit processes the web in such a way that a varnish is applied, which serves in particular as a finish for the previous printing. The varnish drying unit dries the applied varnish. The varnish application unit is either analogue, i.e. the varnish is applied or sprayed using a roller or a bath, for example, or digital, i.e. the varnish is printed using one or more printing bars, as in a digital printing unit.The primer application unit and the paint application unit are further processing units of the system.

[0037] Two of the humidification units are expediently arranged between the primer drying unit and the printing unit, or alternatively only one of the humidification units. Alternatively or additionally, another of the humidification units is suitably arranged between the printing drying unit and the paint application unit, or alternatively even two of the humidification units are arranged. Alternatively or additionally, two further humidification units are preferably arranged downstream of the paint drying unit, or alternatively only one of the humidification units. In a particularly preferred embodiment, the system has five humidification units, of which two of the humidification units are arranged between the primer drying unit and the printing unit, one further humidification unit between the printing drying unit and the paint application unit, and two further humidification units downstream of the paint drying unit.Which of these humidification units are then the above-mentioned first and second humidification units is essentially arbitrary and is expediently set depending on the specific operating mode and specifically depending on the paper type and grammage. It is generally advantageous if the two humidification units between the primer drying unit and the printing unit are a first and second humidification unit for pre-humidification and post-humidification as described and / or if the two humidification units downstream of the varnish drying unit are a first and second humidification unit for pre-humidification and post-humidification as described. Then, an advantageous multi-stage rehumidification is implemented at the corresponding point, in particular a two-stage humidification section, even before the web is processed in the respective subsequent processing unit or even before the web is wound up at the end of the system or transferred to a subsequent system.

[0038] If the web is not wound up at the end of the system, but is transferred to a subsequent system via a transfer line, e.g., a corrugator, it is advisable to arrange a further, particularly sixth, moistening unit along the transfer line. The transfer line and the further moistening unit are then still part of the system.

[0039] The operation of a particular humidification unit is not necessarily tied to the operation of the upstream processing unit; rather, humidification occurs in a suitable configuration regardless of whether processing has previously taken place. This is particularly advantageous if the drying unit upstream of the humidification unit is active or continues to operate during a job change even though the associated processing unit is inactive, because then the web continues to be dried, and re-humidification remains advantageous. For example, the primer application unit is not required and activated for every job, so it may be inactive at certain times.In these cases, however, the primer drying unit remains active and dries the web, which is then conveniently remoistened with one or more subsequent moistening units before or while the web enters the printing unit.

[0040] In a suitable embodiment, the system has an analogue coating applicator downstream of the printing unit and, downstream of the analogue coating applicator, an additional digital coating applicator. Optionally, the system has a primer applicator and a primer drying unit upstream of the printing unit, as already described above. The above explanations apply to the analogue and digital coating applicators. In particular, the system also has an associated coating drying unit downstream of the respective coating applicator, by means of which the applied coating is dried. Preferably, one of the humidification units is arranged between the printing unit and the analogue coating applicator (and downstream of the printing drying unit). Another of the humidification units is arranged between the analogue coating applicator and the digital coating applicator (and downstream of the coating drying unit to the analogue coating applicator).Another humidification unit is located downstream of the digital coating application unit (more precisely, downstream of the coating drying unit to the digital coating application unit). Thus, the system has a total of three humidification units. Which of these humidification units, the aforementioned first and second humidification units, is essentially arbitrary and is conveniently set depending on the specific operating mode and, in particular, the paper type and grammage. Using all three humidification units together is also advantageous. Alternatively, one of the three aforementioned humidification units can be omitted, so that the system then has only two humidification units, which is already advantageous in itself.

[0041] In principle, it is advisable to install a moistening unit at as many locations as possible to prevent excessive shrinkage of the web within the system and to maintain the dimensional stability of the web over as long a distance as possible within the system, which then benefits the one or more processing steps accordingly. The system according to the invention is used to process a paper web. The system has a processing unit for processing the web and a drying unit, which is arranged downstream of the processing unit, for drying the web.The system further comprises several humidification units located downstream of the drying unit and designed to re-moisten the web with a humidifying agent in multiple stages. A first of the humidification units pre-moistens the web and a second of the humidification units post-moistens the web after a minimum residence time. The minimum residence time is selected such that the web in the second humidification unit exhibits an increased absorption rate for the humidifying agent due to the pre-moistening. The above statements also apply analogously to the system.

[0042] Exemplary embodiments of the invention are explained below with reference to a drawing. The drawings schematically show:

[0043] Fig. 1 a system,

[0044] Fig. 2 the absorption of water in paper,

[0045] Fig. 3 a variant of the system from Fig. 1 ,

[0046] Fig. 4 shows a further variant of the system from Fig. 1 .

[0047] 1, 3 and 4 each show a variant of a system 2 for processing a paper web 4. In particular, in Figs. 3 and 4, a digital printing press is assumed as system 2, without loss of generality, in which a single-layer paper web 4 is printed. The statements in this context also apply analogously to other systems 2, e.g. a corrugated cardboard system, and other webs 4, e.g. multi-layer paper webs 4. The web 4 is guided through the system 2 and, for example, unwound by an unwinder (not explicitly shown) of the system 2, then processed and finally wound up again by a rewinder 6 of the system 2 or, alternatively, passed on directly to a subsequent system (not shown) via a transfer path 8.

[0048] The system 2 generally comprises a processing unit 10 with which the web 4 is processed. Furthermore, the system 2 has a drying unit 12 downstream of the processing unit 10 for drying the web 4. The drying unit 12 has one or more hot air dryers 14 and / or infrared dryers 16 for drying. During drying, moisture is fundamentally removed from the paper of the web 4, which is to be replenished as completely as possible by means of remoistening. For this purpose, the system 2 has several moistening units 18, 20 downstream of the drying unit 12, by means of which the web 4 is remoistened in several stages with a moistening agent. In the present case, the web 4 is remoistened with the moistening agent in several stages, in that a first moistening unit 18 pre-moistens and a second moistening unit 20, after a minimum residence time T, post-moistens.

[0049] The humectant in this case contains predominantly water and is also a mixture of water and a wetting additive to reduce the surface tension of the water. Optionally, the humectant can also be mixed with air or a similar substance.

[0050] The minimum residence time T is selected such that the web 4 in the second moistening unit 20 has an increased absorption rate for the moistening agent due to the pre-moistening. This utilizes the knowledge that the moistening agent initially forms a liquid film on the web 4 during pre-moistening, which, due to the previous drying, is not immediately completely absorbed into the paper, but at least partially remains and in any case requires a certain amount of time to be completely absorbed by the paper. During this time, the web 4 is, so to speak, flooded with the moistening agent and a further supply of moistening agent is pointless because it cannot absorb into the paper. This is illustrated in Fig. 2, which shows the absorption of the moistening agent into the web 4 at different times T1, T2, T3.The moistening agent applied during pre-moistening B1 on the one hand and post-moistening B2 on the other hand are labeled differently. Also shown is the absorption capacity A of the paper during post-moistening at the various times T1, T2, and T3 (this absorption capacity is a measure of the absorption rate). At time T1, immediately after pre-moistening, the liquid film forms a corresponding barrier and limits the maximum absorption capacity of the paper. The paper fibers 22 form a pore structure flooded with the moistening agent from pre-moistening B1, making post-moistening difficult. The enlargement in Fig. 2 at time T1 shows in detail the movement of the water molecules through the pore structure by capillary transport and the subsequent diffusion into the paper fibers 22.At time T2, the moistening agent from the pre-moistening has been completely absorbed into the paper, the pore structure is pre-moistened, and the liquid film has been broken down. The contact angle is, in particular, 0, and the absorption rate is increased compared to time T1 and also compared to the absorption rate before pre-moistening. At time T3, the absorption rate is still increased; due to swelling of the paper fibers 22, the pore structure is somewhat more closed, and the absorption capacity does not decrease again slightly.

[0051] Accordingly, in this case, remoistening is divided into several stages, namely at least two moistening units 18, 20, which then form a first and a second stage of remoistening. Pre-moistening is not carried out with the actual target amount of moistening agent, but rather with a reduced amount. Additional moistening agent is then only added during post-moistening. Since web 4 is pre-moistened here, the moistening agent of the second moistening unit 20 penetrates the paper significantly faster than without pre-moistening, so that overall and over the same distance, more moistening agent is introduced into web 4 than with a single-stage remoistening. The minimum residence time T serves to wait for the most favorable time possible for post-moistening.As long as a liquid film is present on the web 4, the absorption rate for further applied moistening agent is practically zero and only increases once the liquid film has been absorbed by the web 4. Therefore, the minimum residence time T is selected such that the moistening agent applied by the first moistening unit 18 is completely absorbed into the web 4 (ie, at times T2 and T3 in Fig. 2) when the web 4 reaches the second moistening unit 20. This results in an increased absorption rate.

[0052] In this case, an increased absorption rate is initially achieved in a first stage by pre-moistening. Only once the absorption rate has been significantly increased, ideally upon reaching a suitable threshold value for the absorption rate, is further moisture introduced into the paper in a second stage by post-moistening, so that overall more moisture is introduced into the paper over the same time and distance.

[0053] The minimum residence time T is, for example, in the range from 0.5 s to 30 s. The minimum residence time T is equivalent to a minimum distance traveled by the web 4 between the first and second humidification units 18, 20. The conversion between the minimum residence time T and the minimum distance is performed using the web speed, i.e., the speed at which the web 4 is conveyed through the system 2.

[0054] The amount of moisture that the paper can absorb typically depends on the grammage (e.g. in the range of 100 g / m 2 up to 300 g / m 2 ) of the paper and generally also on the paper type. The higher the grammage, i.e., the heavier the paper, the more moisture the paper can absorb. Therefore, for example, the amount of moistening agent applied to the web 4 per unit of time by means of the moistening units 18, 20 is adjusted depending on the paper grammage, with the amount also being increased for a higher grammage. Analogously, alternatively or additionally, the amount is adjusted depending on the paper type.

[0055] In the embodiments shown here, a maximum of 3 g / m 2 Humidifying agent is applied. Depending on the grammage, a single moistening unit 18, 20 applies a quantity of 1 g / m 2up to 3 g / m 2 or more of moistening agent is applied. In addition, at least 5 g / m 2 Humidifying agent is applied or the web 4 is remoistened to at least 4% to at least 7% (ie wt.%) residual moisture, particularly depending on whether the paper is coated or uncoated.

[0056] In the embodiments of Figs. 3 and 4, the humidification units 18, 20 are each designed as spray bars. In principle, other configurations are also conceivable, e.g., dampening units with rollers or spray plates.

[0057] There are several suitable configurations for the arrangement of the humidification units 18, 20 within system 2, two of which are shown in Figs. 3 and 4. Individual aspects of these configurations can, in principle, be combined with one another as desired.

[0058] In general, the system 2 has at least one system section 24, which contains the aforementioned processing unit 10 and the drying unit 12. Depending on the configuration, and specifically in Figs. 3 and 4, the system 2 may, however, have further system sections 24 through which the web 4 is guided successively. The winder 6 and the transfer path 8 each represent a final system section 24. Specifically, Fig. 4 shows a configuration in which both a winder 6 and a transfer path 8 are present, from which one can then select as needed (alternatively or additionally, inline operation is also implemented here).

[0059] In Figs. 3 and 4, the humidification units 18, 20 are each arranged downstream of a respective cooling unit 26 of the system 2, which is arranged downstream of a respective drying unit 12. The cooling unit 26 serves, after the drying unit 12, to cool the web 4 again. The humidification units 18, 20 are not integrated into this cooling unit 26, but are arranged separately and at a different location.

[0060] Furthermore, it can be seen specifically in Fig. 3 that between some of the moistening units 18, 20, the web 4 is not dried or processed, but merely conveyed. Accordingly, no further processing unit 10 or drying unit 12 is arranged between the first and second moistening units 18, 20. In Fig. 3, the first and second moistening units 18, 20 are arranged at the end of a system section 24, but other positions are possible and equally suitable. The first and second moistening units 18, 20 and the minimum distance between them are collectively referred to as a two-stage moistening section; system 2 in Fig. 3 accordingly has two such two-stage moistening sections, whereas system 2 in Fig. 4 has none.

[0061] 3 and 4, the respective system 2 has a plurality of system sections 24, each with a processing unit 10 and a subsequent drying unit 12. Downstream of each drying unit 12, at least one of the humidification units 18, 20 is arranged, so that the web 4 is humidified at least once after each drying and before the next drying or before the end of the system 2. In contrast to the two-stage humidification section, the humidification units 18, 20 are now distributed over the system 2 and are not arranged contiguously; this is particularly the case in Fig. 4, but basically also applies to Fig. 3, since there too, a plurality of humidification units 18, 20 are arranged between the drying units 12 and at the end of the system 2.

[0062] In the exemplary embodiments shown in Figs. 3 and 4, the system 2 is a digital printing press, and one of the processing units 10 is a printing unit 28 for printing the web 4 with an ink. The associated drying unit 12 is a printing / drying unit 30 for drying the ink. In Fig. 3, the printing / drying unit 30 has, for example, five infrared dryers 16 and four hot-air dryers 18. In Fig. 4, the printing / drying unit 30 is not shown in detail.

[0063] In Fig. 3, the system 2 has a primer application unit 32 and a primer drying unit 34 upstream of the printing unit 28, and a varnish application unit 36 ​​and a varnish drying unit 38 downstream of the printing unit 28. The primer application unit 32 and the varnish application unit 36 ​​are further processing units 10 of the system 2. The primer application unit 32 applies a primer, which serves as the basis for the subsequent print and is dried with the primer drying unit 34. Similarly, the varnish application unit 36 ​​applies a varnish, which serves as a finish for the previous print and is dried with the varnish drying unit 38. The varnish application unit 36 ​​in Fig. 3 is analog, ie the varnish is applied or sprayed on, for example, via a roller or a bath; alternatively, a digital varnish application unit 40 is also suitable, ie the varnish is printed on with one or more printing bars as in a digital printing unit.

[0064] The system 2 according to Fig. 3 has five humidification units 18, 20, with two of the humidification units 18, 20 being arranged between the primer drying unit 34 and the printing unit 28, another of the humidification units 18, 20 being arranged between the printing drying unit 30 and the paint application unit 36, and two further humidification units 18, 20 being arranged downstream of the paint drying unit 38. Fig. 3 also shows an example of which of the humidification units 18, 20 are a first and second humidification unit 18, 20. However, this assignment is fundamentally variable. In Fig.3, the two humidification units 18, 20 between the primer drying unit 32 and the printing unit 28 are then a first and second humidification unit 18, 20 for pre-humidification and post-humidification as described, and the two humidification units 18, 20 downstream of the varnish drying unit 38 are also a first and second humidification unit 18, 20 for pre-humidification and post-humidification as described. As a result, a multi-stage re-humidification is realized at the corresponding point, specifically a two-stage humidification section, even before the web 4 is processed in the respective subsequent processing unit 10 or even before the web 4 is wound up at the end of the system 2 or transferred to a subsequent system. If the web 4 is not wound up at the end of the system 2, but is transferred to a subsequent system by means of a transfer section 8, e.g.a corrugated board plant, a further, sixth moistening unit 18, 20 is optionally arranged along the transfer path 8 (not shown here).

[0065] System 2 according to Fig. 4 has an analogue varnish application unit 36 ​​downstream of the printing unit 28 and, downstream of this, an additional digital varnish application unit 40. One of the humidification units 18, 20 is arranged between the printing unit 28 and the analogue varnish application unit 36, and another of the humidification units 18, 20 is arranged between the analogue varnish application unit 36 ​​and the digital varnish application unit 40. Yet another of the humidification units 18, 20 is arranged downstream of the digital varnish application unit 40. Thus, system 2 has a total of three humidification units 18, 20; alternatively, one of these three humidification units 18, 20 is omitted, so that system 2 then has only two humidification units 18, 20.

[0066] List of reference symbols

[0067] 2 Appendix

[0068] 4 lane

[0069] 6 rewinders

[0070] 8 Transmission path

[0071] 10 processing unit

[0072] 12 drying units

[0073] 14 hot air dryers

[0074] 16 infrared dryers

[0075] 18 (first) humidification unit

[0076] 20 (second) humidification unit

[0077] 22 Paper fiber

[0078] 24 Plant section

[0079] 26 Cooling unit

[0080] 28 Printing unit (processing unit)

[0081] 30 Pressure-drying unit (drying unit)

[0082] 32 Primer application unit (processing unit)

[0083] 34 Primer drying unit (drying unit)

[0084] 36 Paint application unit (analog, processing unit)

[0085] 38 Paint drying unit (drying unit)

[0086] 40 Coating unit (digital, processing unit)

[0087] A Absorbency

[0088] B1 Humectant, which is applied with the pre-moistening B1

[0089] B2 Humectant, which is applied with the re-moisturizing B2

[0090] T Minimum residence time

[0091] T1, T2, T3 time

Claims

Claims 1 . Method for remoistening a web (4) of paper, - wherein the track (4) is guided through a system (2), - wherein the system (2) comprises a processing unit (10) with which the web (4) is processed, - wherein the system (2) has a drying unit (12) downstream of the processing unit (10) for drying the web (4), - wherein the system (2) has a plurality of moistening units (18, 20) downstream of the drying unit (12), by means of which the web (4) is remoistened with a moistening agent in several stages, in that a first of the moistening units (18) pre-moistens the web (4) and a second of the moistening units (20) re-moistens the web (4) after a minimum residence time (T), - wherein the minimum residence time (T) is selected such that the web (4) in the second moistening unit (20) has an increased absorption rate for the moistening agent due to the pre-moistening.

2. Method according to claim 1, wherein the minimum residence time (T) is selected such that the moistening agent applied by the first moistening unit (18) is completely absorbed into the web (4) when the web (4) reaches the second moistening unit (20).

3. Process according to claim 1 or 2, wherein the minimum residence time (T) is in the range of 0.5 s to 30 s.

4. Method according to one of claims 1 to 3, wherein an amount of moistening agent which is applied to the web (4) by means of the moistening units (18, 20) per unit of time is adjusted depending on a grammage of the paper, For a higher grammage the quantity is also increased.

5. Method according to one of claims 1 to 4, wherein with a respective humidification unit (18, 20) a maximum of 3 g / m 2 Humectants are applied.

6. Method according to one of claims 1 to 5, wherein the humidification units (18, 20) together at least 5 g / m 2 Humidifying agent is applied or the web (4) is re-moistened to at least 4% residual moisture.

7. A method according to any one of claims 1 to 6, wherein the humectant is a mixture of water and a wetting additive.

8. Method according to one of claims 1 to 7, wherein the humidification units (18, 20) are each designed as spray bars.

9. Method according to one of claims 1 to 8, wherein the humidification units (18, 20) are arranged downstream of a cooling unit (26) which is arranged downstream of the drying unit (12).

10. Method according to one of claims 1 to 9, wherein no drying of the web (4) takes place between the first and the second moistening unit (18, 20).

11. Method according to one of claims 1 to 10, wherein the plant (2) has several plant sections (24), each with a processing unit (10) and a subsequent drying unit wherein at least one of the moistening units (18, 20) is arranged downstream of a respective drying unit (12), so that the web (4) is moistened after each drying.

12. Method according to one of claims 1 to 11, wherein the system (2) is a printing machine, in which the processing unit (10) is a printing unit (28) for printing the web (4) with an ink, and wherein the drying unit (12) is a printing-drying unit (30) for drying the ink.

13. The method according to claim 12, wherein the system (2) has a primer application unit (32) and a primer drying unit (34) upstream of the printing unit (28) and a varnish application unit (36, 40) and a varnish drying unit (38) downstream of the printing unit (28), wherein two of the moistening units (18, 20) are arranged between the primer drying unit (34) and the printing unit (28), wherein a further one of the moistening units (18, 20) is arranged between the printing drying unit (30) and the varnish application unit (36, 40), wherein two further moistening units (18, 20) are arranged downstream of the varnish drying unit (38).

14. The method according to claim 12, wherein the system (2) has an analogue varnish application unit (36) downstream of the printing unit (28) and a digital varnish application unit (40) downstream of the analogue varnish application unit (36), wherein one of the humidification units (18, 20) is arranged between the printing unit (28) and the analogue varnish application unit (36), wherein a further one of the humidification units (18, 20) is arranged between the analogue varnish application unit (36) and the digital varnish application unit (40), wherein a further one of the humidification units (18, 20) is arranged downstream of the digital varnish application unit (40).

15. Plant (2) for processing a web (4) of paper, - with a processing unit (10) for processing the web (4), - with a drying unit (12) arranged downstream of the processing unit (10) for drying the web (4), - with a plurality of moistening units (18, 20) which are arranged downstream of the drying unit (12) and are designed to remoisten the web (4) with a moistening agent in several stages, in that a first of the moistening units (18) pre-moistens the web (4) and a second of the moistening units (20) after a minimum residence time (T) re-moistens the web (4), - wherein the minimum residence time (T) is selected such that the web (4) in the second moistening unit (20) has an increased absorption rate for the moistening agent due to the pre-moistening.

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