METHOD FOR RE-HUMIDIFICATION OF A PAPER TRACK, AND APPROVAL

Description

[0001] The invention relates to a method for re-moistening a web of paper, and a system for processing a web of paper.

[0002] Examples of plants for processing webs of paper include printing presses, for printing such a web, and corrugated board plants, for producing a web (more precisely, a corrugated board web) which is composed of several individual layers of paper.

[0003] In the processing of a paper web, drying is a regular procedure, for example, after printing in a printing press. During drying, significant moisture (i.e., water) is removed from the paper, which is then ideally added back in during re-wetting. However, a problem arises because paper has a limited capacity to absorb water, which is applied to the web during re-wetting. In some cases, the previously removed moisture cannot be completely returned. This regularly leads to defects such as wrinkling, a tin-can effect, roping, and similar issues. Such defects, in turn, pose problems when winding the web at the end of the line or when transferring the web directly to a downstream line in inline operation.

[0004] US Patent 5,596,930 A describes a method for moistening a moving web of printed material. A wetting agent can be applied to the web in several steps. Another example is known from WO0003086.

[0005] Against this background, an object of the invention is to improve the re-moistening of a paper web. For this purpose, a correspondingly improved method and an improved system are to be specified in particular.

[0006] The problem is solved according to the invention by a method with the features of claim 1 and by a system with the features of claim 15. Advantageous embodiments, further developments, and variants are the subject of the dependent claims. The descriptions relating to the method also apply mutatis mutandis to the system and vice versa. Where steps of the method are specified below, advantageous embodiments for the system result from the system being configured to perform one or more of these steps.

[0007] A key concept of the present invention is, in particular, a multi-stage re-moistening process in which the re-moistening of a paper web is divided into several stages, in each of which a moistening agent is applied to the web. This allows the effect to be utilized that a pre-moistened web subsequently absorbs further moisture significantly better than a web that is not pre-moistened, thus making it possible to introduce considerably more moistening agent, especially water, and therefore moisture into the paper.

[0008] The method according to the invention serves to re-moisten a paper web and is, in particular, also an operating method for a plant in which the re-moistening takes place. The following discussion, without limiting the generality of the invention, assumes a single-layer paper web, i.e., a paper web in the narrower sense; however, the invention is equally applicable to other, and in particular multi-layer, paper webs, e.g., a corrugated board web or a precursor for such a web, which is then composed of several individual layers of paper.

[0009] The web is guided through a system. For example, the web is unwound by a machine's unwinder, then processed, and finally rewound by a machine's rewinder, or alternatively, passed directly to a subsequent machine. For the sake of simplicity, it is assumed that the system is a printing press; however, the explanations given here apply generally to any system that processes a web of paper and, in connection with this, performs rewetting. A configuration in which the system is a corrugated board machine for the production of corrugated board is particularly advantageous, for example.

[0010] The system generally includes a processing unit for processing the web. Downstream of the processing unit, the system also includes a drying unit for drying the web (i.e., the processed web). The drying unit includes one or more hot air dryers and / or infrared dryers. The specific processing and drying methods are not initially relevant; what is more important is that, by its very nature, drying removes moisture from the paper web. This moisture is to be restored as completely as possible through re-humidification. For this purpose, the system includes several re-humidification units downstream of the drying unit, which re-humidify the web (i.e., the dried web) in multiple stages using a wetting agent.Accordingly, moisture is reintroduced to the web after drying by means of the wetting units, through the application of a wetting agent. In this case, the web is re-wetted in multiple stages: a first wetting unit performs pre-wetting (also called initial wetting) and a second unit, after a minimum dwell time, performs secondary wetting. Pre- and secondary wetting are carried out on the same side of the web, and in a suitable embodiment, even on both sides.

[0011] The wetting agent contains, in particular, water, preferably predominantly (i.e., ≥ 95% or even ≥ 99%) or even exclusively water. The water is suitably as soft and / or demineralized as possible. In a particularly advantageous embodiment, the wetting agent is a mixture of water and a wetting agent, preferably with a proportion of at most 1 vol%, particularly preferably at most 0.1 vol%, which is regularly sufficient for high-quality wetting agents to achieve a minimum surface tension. The wetting agent serves to reduce the surface tension of the water (e.g., from 72.7 mN / m (polar component: 51 mN / m, dispersed component: 21.7 mN / m) at 20 °C to 28.5 mN / m (polar component: 5.2 mN / m, dispersed component: 23.3 mN / m)) and thus further increases the absorption rate. Optionally, the humidifying agent is also mixed with air or another inert gas (e.g. nitrogen).

[0012] In this context, "A lies upstream of B" and equivalently "B lies downstream of A" mean that A lies ahead of B along the path, i.e., the path passes through A first and then B. Instead of "upstream / downstream", the pairs of terms "before / behind" and "before / after" are also used.

[0013] For the sake of simplicity, we will refer to "the web" as being treated (processed, dried, moistened, etc.). This means, in particular, that the web undergoes the appropriate treatment section by section, rather than being fully processed first, then completely dried, and only then moistened. Instead, the web is continuously (i.e., inline) guided through the entire system, with each section being treated at a specific point and then immediately passed on.

[0014] The minimum residence time is chosen such that, due to pre-wetting (by the first humidification unit), the web exhibits an increased absorption rate for the wetting agent in the second humidification unit. In a suitable, but not mandatory, embodiment, this is understood to mean an absorption rate in the range of 0.1 s to 1 s, particularly for a quantity of 3 g / m² of wetting agent (i.e., then 3 to 30 g / (s*m²)), and a corresponding absorption rate for other quantities. In other words, pre-wetting applies wetting agent to the web, and it is absorbed into the paper during the minimum residence time, resulting in increased hydrophilicity of the web after the minimum residence time.This process utilizes the fact that during pre-wetting, the wetting agent initially forms a film of liquid on the web. Due to the preceding drying, this film does not immediately and completely penetrate the paper, but remains at least partially and requires a certain amount of time to be fully absorbed. During this time, the web is essentially flooded with the wetting agent, rendering further additions pointless, as they cannot penetrate the paper. The liquid film acts as a barrier, limiting the paper's maximum absorption capacity. This maximum absorption capacity is also limited by the web's feed rate through the system and the available track length. Once the web is to be further processed or wound up, no excess wetting agent may remain on the web and must be skimmed off if necessary.

[0015] Accordingly, the re-humidification process is divided into several stages, namely at least two humidification units, which then constitute a first and a second stage of re-humidification. Pre-humidification does not yet use the actual target amount of humidifying agent, but rather a reduced amount. Additional humidifying agent is then added during the post-humidification stage with the second humidification unit. Since the web is now pre-moistened, the humidifying agent from the second unit penetrates the paper significantly faster than without pre-humidification (i.e., the absorption rate is increased), so that, overall, more humidifying agent can be introduced into the web over the same distance than with a single-stage re-humidification process. This advantage also applies analogously to re-humidification with more than two stages.

[0016] The minimum residence time serves primarily to ensure the most favorable time for re-wetting. As already mentioned, during pre-wetting, the wetting agent initially forms a liquid film on the web, which should ideally be largely dissipated by the time the web reaches the second wetting unit. As long as a liquid film is present on the web, the absorption rate for any further applied wetting agent is practically zero and only increases once the liquid film has been absorbed by the web. Therefore, the minimum residence time is preferably chosen such that the wetting agent applied with the first wetting unit has completely absorbed into the web by the time the web reaches the second wetting unit. It is specifically in this state and at this time that an increased absorption rate occurs.In principle, it is also possible to simply skim off any liquid film, e.g. with a squeegee, but this discards any moistening agent already applied, leaving it unused.

[0017] The absorption rate of the wetting agent into the paper is strongly dependent on the initial moisture content (i.e., starting moisture) of the paper. A dried paper, such as that found after the drying unit, absorbs water more slowly than pre-moistened paper. This is because the adsorbed water molecules on the paper's capillary walls increase its hydrophilicity. 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). According to this equation, a smaller contact angle results in a greater penetration depth per unit time, and therefore an increased absorption rate. Equivalent to an increased absorption rate are, in particular, an increased penetration depth, increased hydrophilicity, a reduced contact angle, and reduced surface tension.

[0018] A particularly problematic factor for the absorption rate of the paper web is a process known as keratinization. Paper contains fibers that absorb moisture and release it again during drying. Keratinization refers to any fiber change (i.e., an actual alteration of the paper fibers) that occurs during drying and rewetting processes. Examples of such fiber changes include irreversible pore closure (reduction of the specific surface area) due to the formation of hydrogen bonds between adjacent cell walls or the merging of microfibrils. Keratinization is irreversible and occurs regularly during drying in the drying unit. Re-introducing moisture from keratinized paper is particularly difficult, meaning the absorption rate is especially low.The method presented here does not aim to introduce as much moisture as possible within a given time and over a given distance using only a single humidification unit. Instead, it preferably begins by pre-humidifying to increase the absorption rate. Only once the absorption rate is significantly increased, ideally upon reaching a suitable threshold (e.g., from the range mentioned above), is further moisture introduced into the paper in a second stage via post-humidification. This post-humidification process is significantly more efficient than with keratinized paper, resulting in a greater overall moisture infusion over the same time and distance.The multi-stage re-moistening process and the resulting higher moisture content of the paper advantageously reduce or completely prevent keratinization of the paper in the system, especially during drying after re-moistening or drying between the two stages of re-moistening.

[0019] As a result, significantly more wetting agent, especially water, can be introduced into the web, thus achieving a particularly high moisture content. Similarly, a particularly large loss of moisture during drying can be advantageously compensated for completely or, if necessary, even overcompensated. This reduces or completely prevents defects such as roping, wrinkling, and the tin-can effect during web conveying and processing in the system. The absence of wrinkles in the web is particularly important here, and this is ensured by the multi-stage re-wetting process, both within the system and at the end of the system. Furthermore, the multi-stage re-wetting process positively influences web shrinkage within the system, especially by significantly reducing the effect of web shrinkage during drying through the subsequent re-wetting.Furthermore, multi-stage re-moistening allows for a moisture profile difference of less than 2%, and in particular a maximum of 1.3%. This means that the difference in moisture content at any two points along the width of the web is less than 2% or a maximum of 1.3%. A difference of 2% or more typically leads to warping during further processing of the web in a corrugated board mill, i.e., the corrugated board web becomes distorted. Finally, multi-stage re-moistening also enables high machine dynamics, especially with regard to changes in web speed and job changes, as any shrinkage of the web is compensated for as needed at appropriate points. This can be demonstrated, in particular, using suitable sensors to measure the web width in the mill and is also noticeable at the end of the corrugated board mill as a precisely cut blank.

[0020] The minimum residence time is preferably in the range of 0.5 s to 30 s, particularly preferably in the range of 1 s to 30 s, although the upper limit of 30 s is essentially dispensable; i.e., the minimum residence time is preferably at least 0.5 s or at least 1 s. Investigations 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 the web travels between the first and second humidification units. The conversion between minimum residence time and minimum distance is performed 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 aforementioned range of 0.5 s to 30 s, a nominal minimum distance of 1 m to 225 m results at preferred track speeds of 100 m / min to 450 m / min; a minimum distance of at least 2 m is advantageous in any case. The maximum minimum distance is significantly less than 225 m, since the track length (i.e., the distance traveled by the track) through the entire system is typically much less than 200 m. A minimum distance of 25 m to 50 m, and particularly 35 m to 40 m, between the first and second humidification units is especially preferred. These distances are advantageously located entirely within the system.

[0021] This results in a minimum dwell time of approximately 4 s to 30 s or 5 s to 24 s for the aforementioned orbital speeds.

[0022] Preferably, the minimum residence time between the first and second humidification stages is selected for a maximum web speed (e.g., 450 m / min) of the system. Lower web speeds are typically unproblematic, as 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 concern is that the web is still moist when it reaches the second humidification stage and is certainly not completely dry, so that an increased absorption rate results for the re-humidification. Regardless of this, the amount of humidifying agent applied is expediently adjusted depending on the web speed such that, regardless of the web speed, the same amount of humidifying agent is always applied per unit of time and per unit area.

[0023] The amount of moisture paper can absorb typically depends on its basis weight (e.g., in the range of 100 g / m² to 300 g / m² or even more) and generally also on the paper type. The higher the basis weight, i.e., the heavier the paper, the more moisture it can absorb. Furthermore, paper with a higher basis weight (especially > 300 g / m²) typically dries out less (when considering the moisture content in %), so a correspondingly high degree of re-moistening is usually not necessary. Nevertheless, larger quantities (especially > 3 g / m²) than with lower basis weights are easily absorbed (i.e., without wrinkling). In In an advantageous embodiment, the amount of wetting agent applied to the web per unit of time by the wetting units is adjusted depending on the paper's basis weight, with the amount increasing for higher basis weights. For the reasons stated above, this relationship is not necessarily linear and does not necessarily continue up to very high basis weights. Alternatively or additionally, the amount is adjusted according to the paper type. This is based on the consideration that the binder in the paper, and possibly other additives, as well as the porosity and initial moisture content of the paper—in other words, the paper type in general—influence the absorption rate, i.e., the paper's ability to absorb moisture.

[0024] Preferably, each humidification unit applies a maximum of 3 g / m² of humidifying agent. This amount is optimal, particularly for a standard system size and the minimum residence time requirements mentioned above, and is also suitable for typical paper types and basis weights. Depending on the basis weight, a single humidification unit preferably applies an amount of 1 g / m² to 3 g / m² of humidifying agent, with a higher amount being used for higher basis weights, as already described. With at least two humidification units, a total of up to 6 g / m² of humidifying agent can be applied, which, due to the multi-stage re-humidification process, is completely absorbed by the paper, resulting in a correspondingly high moisture content.Especially for very high grammages (particularly >300 g / m²<) or generally, it is advisable to apply a quantity of up to 4, 5, or 6 g / m²< or even more with a single humidification unit. The total quantity of humidifying agent then increases accordingly.

[0025] Suitablely, at least 5 g / m² of humidifying agent is applied jointly by the humidification units, or the web is re-humidified to at least 4% (i.e., wt.%) residual moisture. Suitablely, a web of coated paper is re-humidified to at least 4% to 5% residual moisture using the humidification units, and / or a web of uncoated paper is re-humidified to at least 5% to 7% residual moisture. To achieve a target water content (equivalent to target residual moisture) by means of re-humidification, a target quantity of humidifying agent is suitablely specified at the end of the system. This quantity is then distributed among the humidification units so that, in total, they apply the target quantity to the web in multiple stages. Residual moisture indicates the amount of moisture in the web relative to the weight of the web.Depending on the basis weight, an amount of 5 g / m² to 8 g / m² is preferred, whereby – as already described – a higher amount is used for a higher basis weight. Alternatively or additionally, a residual moisture content in the range of 5% to 6% is preferred, particularly regardless of the basis weight. The drying unit regularly dries the web to a residual moisture content in the range of 2.5% to 3.5% (uncoated paper), and accordingly, 1.5% to 3.5% residual moisture is then added during re-moistening. This is typically not possible with a single-stage re-moistening process. Coated paper, compared to uncoated paper, regularly stores less water relative to the paper thickness 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 re-moistening to achieve a moisture content of 4% to 5%.

[0026] Preferably, the humidification units are designed as spray bars. These are characterized by their particularly small footprint and high efficiency. Furthermore, spray bars are advantageously not operated in recirculation mode, ensuring that only fresh humidifying agent is applied to the web at any given time. Other designs are also conceivable, such as dampening units with rollers or spray discs, but the latter are generally less economical and require significantly more space compared to spray bars. The space requirement is a particularly important aspect of multi-stage rehumidification as described here, since this necessitates the placement of a humidification unit at at least two different locations within the system.However, since the system is typically designed to be as compact as possible and many components are integrated into a small space, multi-stage rehumidification with large humidification units (e.g., spray plates) is not readily feasible. Retrofitting is often impossible with existing system designs without a fundamental redesign. In contrast, spray bars are particularly compact and can be easily retrofitted to existing systems at various points without having to alter the existing system design.

[0027] The use of spray bars also has the advantage that they typically have several spray nozzles arranged along a width of the web, so that by selectively controlling individual spray nozzles, e.g. switching them on and off or adjusting the amount of wetting agent dispensed, an inhomogeneous moisture profile can be easily balanced, i.e. homogenized.

[0028] There are several suitable configurations for the arrangement of the humidification units within the system, some of which are particularly preferred and are described below. These configurations can, in principle, also be combined with one another.

[0029] The system comprises at least one section containing the aforementioned processing unit and the drying unit. Depending on the design, the system may also include further sections through which the web is guided sequentially; that is, at the end of one section, the web is directly transferred to the next, allowing for multiple processing and / or general treatments of the web. A final section is preferably a rewinder, which winds the web into a roll, or a transfer section, over which the web is transferred directly to a subsequent system without being rewound.

[0030] 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.

[0031] In a further suitable embodiment, no drying of the web and, in particular, no processing takes place between the first and second humidification unit, but preferably only a conveying of the web.

[0032] Between the first and second humidification units, no further processing or drying unit is located; the humidifying agent is simply drawn into the paper. If the system has multiple sections, the first and second humidification units are therefore located between two sections, at the beginning or end of a section, or a combination thereof. The first and second humidification units, along with the minimum distance between them, are collectively referred to as a two-stage humidification 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. Downstream of each drying unit, at least one of the humidification units is arranged, so that the web is humidified at least once after each drying stage and, in particular, before the next drying stage 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 in a continuous line. This is based on the consideration that it is typically harmless if drying occurs between two humidification units, provided that the desired residual moisture content is achieved at the end of the system.Humidification between two drying cycles also advantageously prevents the described keratinization of the paper, so that even with a distance between two humidification units and any intervening drying unit that is long compared to the minimum distance, an increased absorption rate is present at the humidification unit located further downstream, produced by the humidification unit located further upstream.

[0034] 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 ink, preferably water-based ink and / or inkjet ink. In a digital printing press, the printing unit typically has several printing bars, especially 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 rewetting described here is particularly suitable for use in a printing press, since the web is regularly subjected to intensive drying to dry the print produced during ink printing. As a side effect, moisture is also removed from the paper, which is then replenished by the rewetting process.

[0035] In a suitable configuration, the system comprises a primer application unit and a primer drying unit upstream of the printing unit, and a coating application unit and a coating drying unit downstream of the printing unit. The primer application unit processes the web by applying a primer, which serves primarily as a base for the subsequent printing. The primer drying unit dries the applied primer. Similarly, the coating application unit processes the web by applying a coating, which serves primarily as a finish for the preceding printing. The coating drying unit dries the applied coating. The coating application unit is either analog, meaning the coating is applied, for example, via a roller or bath, or sprayed on, or digital, meaning the coating is printed on using one or more printing plates, similar to a digital printing unit.The primer application unit and the paint application unit are further processing units of the plant.

[0036] Advantageously, two humidification units are arranged between the primer drying unit and the printing unit, or alternatively, only one humidification unit. Alternatively or additionally, another humidification unit is suitably arranged between the printing drying unit and the coating application unit, or alternatively, even two humidification units. Alternatively or additionally, preferably, two further humidification units are arranged downstream of the coating drying unit, or alternatively, only one humidification unit. In a particularly preferred embodiment, the system has five humidification units, of which two are arranged between the primer drying unit and the printing unit, one further humidification unit between the printing drying unit and the coating application unit, and two further humidification units downstream of the coating drying unit.Which of these humidification units are the first and second humidification units mentioned above is essentially arbitrary and is expediently set depending on the specific operating mode and especially on the paper type and basis weight. 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. In this case, an advantageous multi-stage re-humidification process is implemented at the corresponding point, in particular a two-stage humidification section, even before the web is processed in the subsequent processing unit or before the web is wound up at the end of the system or transferred to a downstream system.

[0037] If the web is not wound up at the end of the system, but transferred to a subsequent system, e.g., a corrugated board plant, via a transfer line, it is advantageous to arrange a further, in particular a sixth, humidification unit along the transfer line. The transfer line and the additional humidification unit are then still considered parts of the system.

[0038] The operation of a humidification unit is not necessarily tied to the operation of the upstream processing unit. In a suitable configuration, humidification occurs independently of whether processing has taken place beforehand. This is particularly advantageous when the drying unit upstream of the humidification unit is active or continues to operate after a job change, even though the associated processing unit is inactive. In such cases, the web continues to be dried, and re-humidification remains beneficial. For example, the primer application unit is not required and activated for every job, meaning it may be inactive at certain times.In these cases, however, the primer drying unit remains active and dries the web, which is then expediently re-moistened with one or more subsequent humidification units, even before or while the web enters the printing unit.

[0039] In a suitable embodiment, the system has an analog coating application unit downstream of the printing unit and, downstream of the analog coating application unit, an additional digital coating application unit. Optionally, the system has a primer application unit and a primer drying unit upstream of the printing unit, as described above. The above descriptions apply to the analog and digital coating application units. In particular, the system has an associated coating drying unit downstream of each coating application unit, by means of which the applied coating is dried. Preferably, one of the humidification units is arranged between the printing unit and the analog coating application unit (and thus downstream of the printing drying unit). Another humidification unit is arranged between the analog coating application unit and the digital coating application unit (and thus downstream of the coating drying unit from the analog coating application unit).Another humidification unit is located downstream of the digital coating unit (more precisely, downstream of the coating drying unit from the digital coating unit). Thus, the system has a total of three humidification units. Which of these units are the first and second humidification units mentioned above is essentially arbitrary and is expediently set depending on the specific operating mode and, in particular, the paper type and weight. Using all three humidification units simultaneously 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 in itself is already advantageous.

[0040] In principle, it is advisable to place a humidification unit at as many points as possible in order to prevent excessive shrinkage of the track within the system and to maintain the dimensional accuracy of the track over as long a distance as possible within the system, from which the one or more processing operations then benefit accordingly.

[0041] The system according to the invention is used for processing a web of paper. The system comprises a processing unit for processing the web and a drying unit, which is arranged downstream of the processing unit, for drying the web. Furthermore, the system comprises several humidification units, which are arranged downstream of the drying unit and are designed to re-humidify the web in multiple stages with a humidifying agent. This is achieved by pre-humidifying the web with a first humidification unit and then, after a minimum residence time, by re-humidifying the web with a second humidification unit. The minimum residence time is selected such that, due to the pre-humidification, the web exhibits an increased absorption rate for the humidifying agent in the second humidification unit. The above descriptions apply analogously to the system.

[0042] Exemplary embodiments of the invention are explained below with reference to a drawing. Each drawing schematically shows: Fig. 1 a system, Fig. 2 the absorption of water in paper, Fig. 3 a variant of the system made of Fig. 1 , Fig. 4 another variant of the system made of Fig. 1 .

[0043] In the Fig. 1 , 3 and 4 Each variant for a system 2 is shown, for processing a paper web 4. Specifically in the Fig. 3 and 4 Without limiting the generality of the text, a digital printing press is assumed as Annex 2, in which a single-layer web 4 made of paper is printed. However, the statements made in this context also apply analogously to other Annex 2s, e.g., a corrugated board machine, and other webs 4, e.g., multi-layer webs 4 made of paper.

[0044] The track 4 is guided through the system 2 and, for example, unwound by a not explicitly shown unwinder of the system 2, then processed and finally rewound by a winder 6 of the system 2 or alternatively passed on directly to a subsequent system (not shown) via a transmission line 8.

[0045] Plant 2 generally comprises a processing unit 10, in which web 4 is processed. Furthermore, plant 2 has a drying unit 12 downstream of processing unit 10 for drying web 4. Drying unit 12 includes one or more hot air dryers 14 and / or infrared dryers 16 for drying. During drying, moisture is inherently removed from the paper of web 4, which is to be replenished as completely as possible by means of re-humidification. For this purpose, plant 2 has several humidification units 18, 20 downstream of drying unit 12, by means of which web 4 is re-humidified in multiple stages with a humidifying agent. In this case, web 4 is re-humidified in multiple stages with the humidifying agent by means of a first pre-humidification in a humidification unit 18 and a second humidification unit 20 after a minimum residence time T for post-humidification.

[0046] The humidifying agent consists predominantly of water and is also a mixture of water and a wetting agent to reduce the surface tension of the water. Optionally, the humidifying agent is also mixed with air or a similar substance.

[0047] The minimum residence time T is chosen such that, due to pre-humidification, web 4 in the second humidification unit 20 exhibits an increased absorption rate for the humidifying agent. This utilizes the fact that, during pre-humidification, the humidifying agent initially forms a liquid film on web 4, which, due to the preceding drying, does not immediately and completely penetrate the paper but remains at least partially and requires a certain amount of time to be fully absorbed. During this time, web 4 is essentially flooded with the humidifying agent, and further addition of humidifying agent is pointless, as it cannot penetrate the paper. This is in Fig. 2 The figure illustrates the absorption of the wetting agent into web 4 at different times T1, T2, and T3. The wetting agent applied during pre-wetting B1 and post-wetting B2 are labeled differently. The absorbency A of the paper during post-wetting at the different times T1, T2, and T3 is also shown (this absorbency is a measure of the absorption rate). At time T1, immediately after pre-wetting, the liquid film forms a barrier and limits the paper's maximum absorbency. The paper fibers 22 form a pore structure that is saturated with the wetting agent from pre-wetting B1, making post-wetting more difficult. The magnification in Fig. 2 Time T1 shows in detail the movement of water molecules through the pore structure via capillary transport and the subsequent diffusion into the paper fibers 22. At time T2, the wetting agent from the pre-wetting process has completely penetrated the paper, the pore structure is pre-wetted, and the liquid film has dissipated. 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-wetting. 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 slightly again.

[0048] Accordingly, the re-humidification process is divided into several stages, namely at least two humidification units 18 and 20, which then constitute a first and a second stage of re-humidification. The pre-humidification does not yet use the actual target amount of humidifying agent, but rather a reduced amount. Additional humidifying agent is then added only during the post-humidification stage. Since web 4 is pre-humidified, the humidifying agent from the second humidification unit 20 penetrates the paper significantly faster than without pre-humidification, so that, overall and over the same distance, more humidifying agent is introduced into web 4 than with a single-stage re-humidification process.

[0049] The minimum residence time T serves to wait for the most favorable time for re-wetting. As long as a liquid film is present on web 4, the absorption rate for further applied wetting agent is practically zero and only increases once the liquid film has been absorbed by web 4. Therefore, the minimum residence time T is chosen such that the wetting agent applied by the first wetting unit 18 has completely absorbed into web 4 (i.e., by times T2 and T3 in the Fig. 2 ), when lane 4 reaches the second humidification unit 20. Specifically here, an increased absorption rate occurs.

[0050] In the first stage, an increased absorption rate is achieved through pre-moistening. Only once the absorption rate is significantly increased, ideally upon reaching a suitable threshold, is further moisture introduced into the paper in a second stage through post-moistening, so that overall more moisture is introduced into the paper over the same time and distance.

[0051] The minimum residence time T, for example, ranges from 0.5 s to 30 s. The minimum residence time T is equivalent to a minimum distance traveled by track 4 between the first and second humidification units 18 and 20. The conversion between minimum residence time T and minimum distance is performed using the track speed, i.e., the speed at which track 4 is conveyed through system 2.

[0052] The amount of moisture that paper can absorb typically depends on its basis weight (e.g., in the range of 100 g / m² to 300 g / m²) and generally also on the paper type. The higher the basis weight, i.e., the heavier the paper, the more moisture it can absorb. Therefore, for example, the amount of humidifying agent applied to web 4 per unit of time by humidification units 18 and 20 is adjusted according to the paper basis weight, with the amount being increased for higher basis weights. Similarly, the amount is adjusted either alternatively or additionally depending on the paper type.

[0053] In the embodiments shown here, a maximum of 3 g / m² of wetting agent is applied by each respective humidification unit 18, 20. Depending on the basis weight, a single humidification unit 18, 20 applies an amount of 1 g / m² to 3 g / m² or more of humidifying agent. Furthermore, by way of example, at least 5 g / m² of humidifying agent is applied by the humidification units 18, 20 together, or the web 4 is re-moistened to at least 4% to at least 7% (i.e., wt%) residual moisture, depending in particular on whether the paper is coated or uncoated.

[0054] In the exemplary embodiments of the Fig. 3 and 4 The humidification units 18 and 20 are each designed as spray bars. In principle, other configurations are also conceivable, e.g., humidification units with rollers or spray discs.

[0055] There are several suitable configurations for the arrangement of the humidification units 18, 20 within the system 2, two of which are described in the Fig. 3 and 4 The examples shown are not exhaustive. Individual aspects of these designs can, in principle, be combined with one another in any way.

[0056] In general, Annex 2 has at least one section 24, which contains the aforementioned processing unit 10 and the drying unit 12. Depending on the design and specifically in the Fig. 3 and 4 However, the system 2 may also have further system sections 24 through which the track 4 is successively routed. The winder 6 and the transmission section 8 each constitute a final system section 24. Specifically in Fig. 4 An embodiment is shown in which both a winding unit 6 and a transmission line 8 are available, from which a selection is made as needed (alternatively or additionally, an inline operation is also implemented here).

[0057] In the Fig. 3 and 4 The humidification units 18 and 20 are each arranged downstream of a respective cooling unit 26 of system 2, which in turn is arranged downstream of a respective drying unit 12. The cooling unit 26 serves to cool the track 4 again after the drying unit 12. The humidification units 18 and 20 are not integrated into this cooling unit 26, but are arranged separately and at a different location.

[0058] Furthermore, especially in Fig. 3 It is evident that between some of the humidification units 18, 20, no drying of the web 4 nor any processing takes place, but only conveying. Therefore, no further processing unit 10 or drying unit 12 is arranged between the first and second humidification units 18, 20. Fig. 3 The first and second humidification 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 humidification units 18, 20, as well as the minimum distance between them, are collectively referred to as a two-stage humidification section, which is shown in system 2. Fig. 3 Accordingly, it has two such two-stage humidification sections, which are shown in Annex 2 in Fig. 4 None, however.

[0059] In the Fig. 3 and 4Each system 2 has several system sections 24, each with a processing unit 10 and a subsequent drying unit 12, and 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 system 2. In contrast to the two-stage humidification section, the humidification units 18, 20 are now distributed across system 2 and are not arranged in a continuous line; this is particularly evident in Fig. 4 that case, but basically also applies to Fig. 3 to, since several humidification units 18, 20 are also arranged there between the drying units 12 and at the end of the system 2.

[0060] In the exemplary designs shown, the Fig. 3 and 4Plant 2 is a digital printing press, and one of the processing units 10 is a printing unit 28 for printing web 4 with ink. The associated drying unit 12 is a print-drying unit 30 for drying the ink. Fig. 3 The pressure drying unit 30 includes, for example, five infrared dryers 16 and four hot air dryers 18. Fig. 4 The pressure drying unit 30 is not shown in detail.

[0061] In Fig. 3 Upstream of printing unit 28, system 2 has a primer application unit 32 and a primer drying unit 34, and downstream of printing unit 28, a coating application unit 36 ​​and a coating drying unit 38. The primer application unit 32 and the coating application unit 36 ​​are further processing units 10 of system 2. The primer application unit 32 applies a primer, which serves as the basis for the subsequent printing and is dried by the primer drying unit 34. Similarly, the coating application unit 36 ​​applies a coating, which serves as a finish for the previously applied printing and is dried by the coating drying unit 38. The coating application unit 36 ​​is located in Fig. 3 analog, i.e. the varnish is applied or sprayed on, e.g. via a roller or a bath; alternatively, a digital varnish application unit 40 is also suitable, i.e. the varnish is printed on with one or more printing bars, as with a digital printing unit.

[0062] Annex 2 according to Fig. 3 has five humidification units 18, 20, wherein two of the humidification units 18, 20 are arranged between the primer drying unit 34 and the printing unit 28, another of the humidification units 18, 20 is arranged between the printing drying unit 30 and the coating application unit 36, and two further of the humidification units 18, 20 are arranged downstream of the coating drying unit 38. Fig. 3 An example assignment is also given, indicating which of the humidification units 18 and 20 are the first and second humidification units 18 and 20, respectively. However, this assignment is fundamentally variable. 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 paint drying unit 38 are also a first and second humidification unit 18, 20 for pre-humidification and post-humidification as described. This provides a multi-stage re-humidification process at the corresponding point, specifically a two-stage humidification section, even before the web 4 is processed in the subsequent processing unit 10 or before the web 4 is wound up at the end of the system 2 or transferred to a downstream system. If the web 4 is not wound up at the end of the system 2, but is transferred to a downstream system via a transfer section 8, e.g.,In a corrugated board plant, an optional sixth humidification unit 18, 20 is arranged along the transmission line 8 (not shown here).

[0063] Annex 2 according to Fig. 4 Downstream of printing unit 28, there is an analog coating application unit 36, and downstream of that, an additional digital coating application unit 40. One of the humidification units 18, 20 is arranged between printing unit 28 and the analog coating application unit 36, another humidification unit 18, 20 is arranged between the analog coating application unit 36 ​​and the digital coating application unit 40. A further humidification unit 18, 20 is arranged downstream of the digital coating application unit 40. Thus, system 2 has a total of three humidification units 18, 20. Alternatively, one of these three humidification units 18, 20 can be omitted, so that system 2 then has only two humidification units 18, 20. Reference symbol list

[0064] 2 Plant 4 Web 6 Rewinder 8 Transfer section 10 Processing unit 12 Drying unit 14 Hot air dryer 16 Infrared dryer 18 (First) humidification unit 20 (Second) humidification unit 22 Paper fiber 24 Plant section 26 Cooling unit 28 Printing unit (Processing unit) 30 Printing-drying unit (Drying unit) 32 Primer application unit (Processing unit) 34 Primer drying unit (Drying unit) 36 Coating application unit (analog, Processing unit) 38 Coating drying unit (Drying unit) 40 Coating application unit (digital, Processing unit) A Absorption capacity B1 Humidifying agent applied with pre-humidification B1 B2 Humidifying agent applied with post-humidification B2 T Minimum residence time T1, T2, T3 Time

Claims

1. Method 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 the plant (2) comprises, downstream of the processing unit (10), a drying unit (12) for drying the web (4), - wherein the plant (2) comprises, downstream of the drying unit (12), a plurality of wetting units (18, 20) by means of which the web (4) is rewetted using a wetting agent in several stages, by a first of the wetting units (18) pre-wetting the web (4) and a second of the wetting units (20) rewetting the web (4) after a minimum dwell time (T), - characterized in that the minimum dwell time (T) is selected such that, in the second wetting unit (20), the web (4) has an increased absorption rate for the wetting agent due to the pre-wetting.

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

3. Method according to claim 1 or claim 2, wherein the minimum dwell time (T) is in the range from 0.5 s to 30 s.

4. Method according to any of claims 1 to 3, wherein an amount of wetting agent applied to the web (4) by means of the wetting units (18, 20) per unit of time is adjusted according to a grammage of the paper, wherein, for a higher grammage, the amount is also increased.

5. Method according to any of claims 1 to 4, wherein a maximum of 3 g / m2 of wetting agent is applied by each wetting unit (18, 20).

6. Method according to any of claims 1 to 5, wherein, by means of the wetting units (18, 20) collectively, at least 5 g / m2 of wetting agent is applied, or the web (4) is rewetted to at least 4% residual moisture.

7. Method according to any of claims 1 to 6, wherein the wetting agent is a mixture of water and a wetting additive.

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

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

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

11. Method according to any of claims 1 to 10, wherein the plant (2) comprises a plurality of plant portions (24), each having a processing unit (10) and a downstream drying unit (12), wherein at least one of the wetting units (18, 20) is arranged downstream of each drying unit (12), such that the web (4) is wetted after each drying process.

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

13. Method according to claim 12, wherein the plant (2) comprises 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 wetting units (18, 20) are arranged between the primer drying unit (34) and the printing unit (28), wherein another of the wetting units (18, 20) is arranged between the print drying unit (30) and the varnish application unit (36, 40), wherein two more of the wetting units (18, 20) are arranged downstream of the varnish drying unit (38).

14. Method according to claim 12, wherein the plant (2) has an analog varnish application unit (36) downstream of the printing unit (28) and a digital varnish application unit (40) downstream of the analog varnish application unit (36), wherein one of the wetting units (18, 20) is arranged between the printing unit (28) and the analog varnish application unit (36), wherein another of the wetting units (18, 20) is arranged between the analog varnish application unit (36) and the digital varnish application unit (40), wherein another of the wetting units (18, 20) is arranged downstream of the digital varnish application unit (40).

15. Plant (2) for processing a web (4) made from paper, - having a processing unit (10) for processing the web (4), - having a drying unit (12), arranged downstream of the processing unit (10), for drying the web (4), - having a plurality of wetting units (18, 20) which are arranged downstream of the drying unit (12) and are designed to rewet the web (4) using a wetting agent in several stages, by a first of the wetting units (18) pre-wetting the web (4) and a second of the wetting units (20) re-wetting the web (4) after a minimum dwell time (T), - characterized in that the minimum dwell time (T) is selected such that, in the second wetting unit (20), the web (4) has an increased absorption rate for the wetting agent due to the pre-wetting.