EP0449841B1 - Process for glazing paper webs - Google Patents

Abstract

Glazing is effected in the nip (3) of a calender (2) under the action of pressure, moisture and heat. The desired surface quality is obtained by bringing the surface of the paper or cardboard web (1) to be glazed in the nip (3) to a state above the second-order transition temperature curve of the material. Within 20 to 60 ms (milliseconds), the surface of the paper web (1) leaving the nip (3) first undergoes a transition step in a cooling device (4) in combination with a change in moisture content to a state below the second-order transition point of the material. The surface can be cooled by contact with a chilled surface (8) or by direct contact with a cooling, preferably inert gas. The increase in roughness of the paper web (1) after glazing is thereby reduced, so that the surface quality attained in the nip (3) remains essentially constant. Devices for implementing the process are disclosed.

Description

The invention relates to a method for smoothing a paper or cardboard web, which is described in the preamble of the claim.

When smoothing paper or cardboard, it is known that the structure achieved in the press nip of the smoothing unit changes after the paper web has emerged from the press nip. So there is a change in the paper web, whereby the desired smoothness of the surface, which has already been achieved in the press nip, decreases again, or the roughness of the surface increases. This happens through more or less strong and locally different back-swelling, especially when there are smoothing temperatures in the press nip which are below the glass transition point (this is the plasticizing temperature) of the material to be smoothed. A decrease in smoothness is usually also observed when smoothing temperatures in the press nip are above the glass transition point and the paper web gradually cools down after exiting the press nip at ambient temperature below the glass transition point of the material. It is said that the paper "works".

Often, the optimal surface properties desired for further processing, such as printing, writing, etc., of the smoothed paper or cardboard, as described in DE-OS 36 00 033, can only be achieved at high temperatures of the surfaces of the rollers used in the smoothing unit, or can only be achieved at high temperatures in the press nip. In many cases it is advisable to heat the paper or cardboard web at least in its surface layers above the glass transition temperature in order to achieve the plasticization of the material here. After the exit from the press nip comes the warm web thermodynamically gradually back into equilibrium with the environment, ie the differences in temperature and humidity in equilibrium with the environment are compensated. The material "works", and the higher the web temperature at the beginning, the more it works. This is a known long-term effect, whereby the micro-roughness of the smoothed surface can increase again. The above-mentioned elastic back-swelling additionally occurs, the non-plasticized inner layers of the web increasing in thickness compared to the state in the press nip. This is a known short-term effect, which is completed after about 100 ms to 2 min after the web emerges from the press nip. This swelling is different locally. It is particularly pronounced in places of high pressure, such as places with fiber flakes or places with a higher basis weight in the web. This also leads to an increase in roughness in the fiber flake area after smoothing ("macro roughness").

All of this can have the consequence that to achieve a desired high surface smoothness of the paper web the smoothing process several times in succession, e.g. in several press nips, under changing smoothing conditions with regard to the pressure, the moisture of the paper web and the temperature would have to be carried out, or it has to be driven more slowly or with a higher line force.

• a) wenn die zu glättende Papierbahn dick ist, d.h. ein hohes Flächengewicht hat, dann wird die im Preßspalt aufgenommene Wärme in das Bahninnere abgeführt, und es erfolgt eine schnelle Abkühlung der Oberfläche, vorausgesetzt, daß in den Oberflächenschichten höhere Temperaturen vorhanden sind als im Bahninneren und daß die mittlere Temperatur unter der Glasumwandlungstemperatur liegt;;
• b) wenn eine entsprechend hohe Bahnfeuchte vorliegt, dann wird die Papierbahn nach dem Austritt aus dem Preßspalt mit der Verdunstungskälte gekühlt, insbesondere wenn die Papierbahn in dem Preßspalt über 100° C aufgewärmt wurde;
• c) wenn der Kristallinitätsgrad des Materials hoch ist und damit auch die Glasumwandlungstemperatur in der Papierbahn, so daß dann bei hoher Temperaturdifferenz zwischen Papier und Umgebung eine hohe Wärmeabfuhr verbunden mit einer raschen Temperaturabsenkung der Papierbahn unter die Glasumwandlungstemperatur erfolgt.

According to current knowledge, the following listed operating parameters favor the process in which the paper web located in the press nip of the smoothing unit at a temperature above the glass transition point of the material is to be brought below the glass transition point of the material after it has emerged from the press nip of the smoothing unit:

• a) if the paper web to be smoothed is thick, ie has a high basis weight, then the heat absorbed in the press nip is dissipated into the interior of the web, and there is a rapid cooling of the surface, provided that in the Surface layers are at higher temperatures than inside the web and that the mean temperature is below the glass transition temperature;
• b) if a correspondingly high web moisture is present, then the paper web is cooled with the evaporative cooling after leaving the press nip, in particular if the paper web has been heated above 100 ° C. in the press nip;
• c) if the degree of crystallinity of the material is high and thus also the glass transition temperature in the paper web, so that a high heat dissipation combined with a rapid temperature decrease of the paper web below the glass transition temperature then takes place at a high temperature difference between the paper and the environment.

If these favorable conditions for the cooling of the paper web after smoothing do not exist, or cannot be achieved or cannot be achieved for technological reasons, then the quality of the surface or of the two surfaces of the paper web possibly achieved in the press nip of the smoothing unit passes Back-swelling, and here in particular due to locally different back-swelling, and thus also lost again due to an increase in the macro-roughness, but also due to an increase in the micro-roughness.

The invention had for its object to find a method for smoothing a paper or cardboard web of the type specified initially, which makes it possible that the quality of the smoothed surface achieved in the press nip of the smoothing unit, or the surfaces of the paper web after the exit of the paper web is at least partially maintained from the press nip of the calender. An increase in roughness of the already smoothed surfaces should at least be minimized.

This object is achieved by measures that are in the characterizing part of the Are specified, solved.

As a result of the conscious, targeted cooling which is carried out practically immediately after emerging from the press nip, possibly with a parallel reduction in the moisture of the paper or cardboard web, the structure achieved in the press nip of the smoothing unit, that is to say the quality of the surface, is "frozen" and thus at least partially maintained. A thermodynamically stabilized state below the glass conversion curve is achieved, whereby the forced "freezing" or solidification of the surface layers, in particular by preventing the stronger-elastic swelling at higher points in weight, also counteracts an increase in the micro-roughness. This achieves a higher level of smoothness. A repetition of the smoothing process can be omitted, or it can be driven faster or with lower line forces. This saves a previously necessary mechanical and energy expenditure. If you drive faster, production increases.

Some useful embodiments of the method according to the invention are described in the subclaims.

The subject matter of the invention and the advantages which can be achieved thereby are described and explained in more detail below. The description refers to a drawing in which:

1 to 8 show examples of devices for carrying out the method according to the invention.

9 to 15 are graphical representations of process examples.

Fig. 16 shows a special form of sealing.

The paper web 1 to be smoothed is guided into a smoothing unit 2. In the press nip 3 between Rollers 5 and 6, at least one of which is heated, are used to smooth the paper web in a known manner using pressure, moisture and heat. The heating takes place in such a way that at least one surface of the paper web is heated by contact with a heated surface to a temperature which is above the glass transition point of the material to be smoothed and which is dimensioned such that the average web temperature after the conversion step is below the glass transition temperature , the temperature inside the paper web should advantageously remain below the glass transition point. A desired structure and a desired quality of the surfaces of the paper web 1 are thus achieved in the press nip. After exiting the press nip 3, the paper web 1 is subjected to a conversion step by means of a cooling device 4 before a period of approximately 20 to 60 ms, wherein at least one smoothed surface of the paper web is cooled to a temperature below the glass transition point of the material. As a result of this practically immediate conversion of at least the surfaces of the paper web 1, which takes place before 20 to 40 ms has elapsed, its structure smoothed in the press nip 3 and its desired quality are almost retained.

There is hardly any swelling of the web damaging the surface quality, and there is no increase in roughness on the smoothed surfaces of the web.

In order to achieve a desired temperature for smoothing in the press nip 3 and to have a desired moisture content of the web 1 there, it is advantageous in some cases to heat the web 1 appropriately before entering the press nip 3 and / or to correct its moisture.

The conversion step is carried out by means of a cooling device 4 which is arranged at the exit of the paper web 1 from the press nip 3. Since in the case, for example FIG. 1, the paper web 1 is smoothed on both sides, the cooling device 4 is designed as a mirror image of the paper web 1. It shows an endless, with which Speed of the paper web 1 at the same speed and in the same direction, good heat-conducting tape 8. Cooling elements 9 are provided on the belt 8. In operation, the paper web 1 is guided between the belts 8, its surfaces being cooled in contact with the cooled surfaces of the belts 8 when a cooling medium is passed into the cooling bodies 9. Shielding walls 10 are advantageously provided on the heatable rollers 5 and / or 6 and are intended to prevent their heat from being radiated and convected at least in the direction of the cooling device 4. Through the spaces between the rollers and the shielding walls 10, the steam which emerges from the press nip at the exit of the paper web 1 can advantageously be removed or sucked in. A correspondingly intensive suction of this steam can be understood as the first stage of the cooling step with regard to the emerging paper web 1. This first step can also be initiated by means of a suction line 11 provided for this purpose, the arrangement of which is particularly shown in FIG. 3. Under favorable technological conditions for this, as far as the temperature and moisture of the paper web 1 to be smoothed is concerned, this first stage can already be achieved by evaporation of the moisture and thus cooling of the paper web 1 in order to quickly reach a state below the glass transition point and thus to stabilize those reached in the press nip Smoothness quality is sufficient. To remove condensation moisture, 8 scrapers 13 are arranged on the belts.

The paper or cardboard web 1 can also be converted by direct contact of its surfaces with a cooling gas. To start the cooling step as early as possible, 3 nozzles 12 can be arranged at the outlet from the press nip, which are used to blow a cooling gas onto the surfaces of the paper web 1.

Another example of a suitable cooling device 4 is shown in FIG. 2. This cooling device can also be a mirror image of the paper web 1 and thus act on both of its surfaces. It has a hood 14 and partitions 15 to limit the spaces that are open to the paper web 1. A cooling gas is passed into a space 16 via a feed line 17, through a subsequent space 18 into a space 19 open to it and discharged from it by means of a discharge line 20. In this way, the cooling gas comes into direct contact with the paper web 1, since the rooms 16, 18 and 19 are each open to the paper web 1. At the entry and exit of the paper web 1 to the hood 14, seals 21, for example sealing rollers or immersion strips, for example according to FIG. 15, are provided in order to avoid losses in the cooling gas as far as possible. In order to avoid oxidation, which could lead, for example, to the undesirable loss of whiteness of the paper web, it is advisable to use an inert gas, such as nitrogen or carbon dioxide, for cooling.

3 and 4 are one-sided smoothing, that is, smoothing predominantly one of the surfaces of the paper or cardboard web 1. In the example according to FIG. 3, the upper roller 5 is heated, the surface guided on this is smoothed more. A cooling device 4 with a hood 14 is assigned to this surface, just like a suction device 11 for sucking in the steam occurring at the outlet of the paper web from the press nip 3. In the example according to FIG. 4, the lower roller 6 is heated. The surface of the paper web 1 smoothed on this roller is fed to a cooled roller 7 for cooling. The steam emerging from the press nip 3 at the exit of the paper web 1 is drawn off by means of a suction device 11, with which a first conversion step takes place. A ventilation device 22, which generates further evaporative cooling, combined with a scraper 13, serves to remove condensate from the jacket of the cooling roller 7.

5 shows a smoothing unit 2 with endless belts 23 which can be driven at the same speed and in the same direction to the paper web 1. The use of pressure and the heating in the press nip 3 take place via known pressure elements 24 with pressure pockets open towards the band 23, which are acted upon by a heat-carrying medium. The approximately rectangular, space-saving guidance of the belts 23 enables the arrangement of the cooling device 4 directly at the exit of the paper web from the press nip 3.

A cooling of the paper web 1 in direct contact with a cooling gas can also be accomplished by means of cooling devices 4, which are shown in FIGS. 6 and 7. The jackets of the rollers 7 are perforated. The cooling gas is blown through a distribution box 26 through the surface of the paper or cardboard web that is guided past, and after it has passed through the paper web 1, this gas is sucked off through the perforated jacket of the roller 7 by means of a suction box 25. These boxes 25 and 26 can e.g. According to the model of the cooling device 4 described above with reference to FIG. 2 in the sense of countercurrent or cocurrent cooling and are accordingly connected to a supply or discharge line 17 or 20 for the cooling gas. In the example according to FIG. 7, the cooling gas is blown via a distribution box 27 through the perforation of the roller 7 onto the surface of the paper web 1 that is guided past. The gas is fed to the distribution box 27 through a supply line 17 and is removed from the suction box 28 through a discharge line 20. In most cases, guiding the gas from outside to inside will be more advantageous, so that the outer box serves as a distributor and the inner box as a suction box. A ventilation device 22 and a scraper 13 for discharging the condensate from the surface of the jacket of the roller 7 are each arranged on the perforated jackets of the rollers 7.

A cooling device 4 according to FIG. 8 has a cooling roller 7, the jacket of which is cooled by means of a heat sink 9. The cooling takes place in a circular segment-shaped gap between the jacket of the roller 7 and an endless, equally fast belt 8 which can be driven in the same direction as the jacket of the roller 7 and which partially wraps around the jacket.

• A. Vorbehandlung der Papierbahn 1 vor ihrem Eintritt in den Preßspalt 3 des Glättwerks durch Aufwärmen und/oder Befeuchten der Oberflächen der Bahn. Dadurch wird gegebenenfalls das Erreichen bzw. Überschreiten der Glasumwandlungstemperatur des Materials in dem beheizten Preßspalt 3 begünstigt oder überhaupt ermöglicht. Vorteilhafte Techniken dazu sind z.B. Oberflächenbefeuchtung mit Dampf, mit Wasserdüsen, mit Wasser-Schaber, mit Auftragen eines Wasserfilms über Walzen, bzw. Vorwärmung über Kontakt mit vorgelagerten Heizwalzen, mit Dampf, mit temperiertem Wasser, mit IR-Strahlung, Mikrowellen oder in einem beheizten, insbesondere verlängerten Preßspalt, z.B. wie gemäß Fig. 5.
• B. Gezielte Feuchtereduzierung durch Verdampfung im Preßspalt 3 (Kontakt der Papierbahn mit beheizten Walzen oder Bändern) und Abkühlung infolge Verdunstungskälte. Durch Verdampfung zumindest eines Teiles der Bahnfeuchte aufgrund der eingesetzten Temperatur im Preßspalt 3 und durch die damit verbundene und parallel verlaufende Bahnabkühlung mittels der Verdunstungskälte unmittelbar nach dem Preßspalt 3 kann ausgehend von einem Zustand oberhalb der Glasumwandlungskurve ein Zustand unterhalb der Glasumwandlungskurve erreicht werden. Dieser Vorgang kann sowohl in einem Preßspalt 3 zwischen Glättwalzen (siehe Fig. 1 bis 4) wie auch zwischen Glättbändern (siehe Fig. 5) stattfinden.
• C. Gezielte Abkühlung der Papierbahn 1 praktisch unmittelbar vor Ablauf von etwa 20 bis 60 ms nach Austritt aus dem Preßspalt 3 des Glättwerks von einer Temperatur oberhalb der Glasumwandlungskurve zu einer unterhalb der Glasumwandlungskurve. Dies kann erfolgen z.B. durch Verdunstung von der Feuchte der Bahn oder von zusätzlich aufgesprühter Flüssigkeit, wie flüssiger Stickstoff oder flüssige Luft, Alkohol, Azeton oder Keton und durch Absaugung der Dämpfe, durch Kontakt mit kalter Flüssigkeit, z.B. Wasser, durch Kontakt mit einer Eisleiste, z.B. aus Kohlendioxid- oder Wassereis oder durch Anwendung einer der im Zusammenhang mit Fig. 1 bis 8 als Beispiel beschriebenen Abkühlvorrichtung 4.

As can be seen from the foregoing, the method according to the invention the following measures applied individually or in combination:

• A. Pretreatment of the paper web 1 before it enters the press nip 3 of the calender by heating and / or moistening the surfaces of the web. As a result, reaching or exceeding the glass transition temperature of the material in the heated press nip 3 is promoted or even made possible. Advantageous techniques for this are, for example, surface moistening with steam, with water nozzles, with a water scraper, with application of a water film over rollers, or preheating via contact with upstream heating rollers, with steam, with tempered water, with IR radiation, microwaves or in a heated one , in particular extended press nip, for example as shown in FIG. 5.
• B. Targeted moisture reduction by evaporation in the press nip 3 (contact of the paper web with heated rollers or belts) and cooling due to evaporative cooling. By evaporation of at least a part of the web moisture due to the temperature used in the press nip 3 and by the associated and parallel web cooling by means of the evaporative cooling immediately after the press nip 3, a state below the glass conversion curve can be achieved starting from a state above the glass conversion curve. This process can take place both in a press nip 3 between smoothing rolls (see FIGS. 1 to 4) and between smoothing belts (see FIG. 5).
• C. Targeted cooling of the paper web 1 practically immediately before the end of about 20 to 60 ms after exiting the press nip 3 of the smoothing unit from a temperature above the glass conversion curve to a below the glass conversion curve. This can take place, for example, by evaporation from the moisture of the web or from additionally sprayed-on liquid, such as liquid nitrogen or liquid air, alcohol, acetone or ketone and by suction of the vapors, by contact with cold liquid, for example water, by contact with an ice bar, for example made of carbon dioxide or water ice or by using one of the in connection with FIGS. 1 to 8 as Cooling device 4 described in the example.

Depending on the method used, the humidity is reduced, increased or remains unchanged (Fig. 12, 15).

In all of the measures described under A, B or C, used individually or in combination, care must be taken that the glass transition temperature is exceeded in the press nip 3, and that the moisture content assumes the technologically desired value in parallel with the cooling below the glass transition temperature preferably corresponds thermodynamically to the equilibrium moisture (relative to the environment).

• 30 die Temperatur-Koordinate,
• 31 die Feuchtigkeits-Koordinate,
• 32 die Glasumwandlungskurve der Zellulose und Heimzellulose, und
• 33 die Glasumwandlungskurve des Lignins jeweils bei einer Kristallinität von 60 %.

9 to 15 graphs of the measures under A, B or C are shown individually or in their possible combinations. In the figures:

• 30 the temperature coordinate,
• 31 the moisture coordinate,
• 32 the glass conversion curve of cellulose and home cellulose, and
• 33 the glass conversion curve of the lignin in each case with a crystallinity of 60%.

The course of the individual measure steps is designated with letters A, B or C, whereby the temperature rise, the cooling and the changes in humidity are shown. In each case, 34 is the starting point in front of the press nip 3, 35 the entry point into the press nip 3, dashed lines the temperature profile in the press nip 3, 36 the exit point from the press nip 3 and 37 the final state after cooling and moisture reduction of the paper web.

In the example according to FIG. 9, the warming up takes place only in the press nip 3 and the cooling only by the evaporative cooling of the own moisture of the paper web 1 without its preheating and without pre-moistening before entering the press nip 3 (measure B).

In the example according to FIG. 10, the heating only takes place in the press nip 3 without preheating and prewetting, but the cooling takes place through the conversion step 4 immediately after the paper web 1 has emerged from the press nip 3 (measure C).

In the example according to FIG. 11, the heating in the press nip 3 takes place after pre-heating and pre-moistening and the cooling only by the evaporative cooling of the moisture of the paper web 1 (combination of measures A and B).

In the example according to FIG. 12, the heating in the press nip 3 takes place after preheating and pre-moistening beforehand and the cooling through the conversion step 4 immediately after the paper web 1 has emerged from the press nip 3 (combination of measures A and C).

In the example according to FIG. 13, the warming up in the press nip 3 takes place after preheating and pre-moistening and the cooling partly through evaporative cooling of the moisture of the paper web and through the conversion step 4 immediately after the paper web 1 exits the press nip 3 (combination of measures A, B and C).

The example according to FIG. 14 relates to the case in which both the glass transition temperature of the cellulose or home cellulose and the glass transition temperature of the lignin in the paper web when the method according to the invention is exceeded. After preheating and prewetting in front of the press nip 3, the paper web 1 in the press nip 3 is smoothed at a temperature above the higher glass transition temperature of the lignin. Cooling takes place by using cooling step 4 (combination of measures A and C). Measures B or C or combinations of measures A and B or A, B and C are also conceivable in this case.

Claims (8)

1. A method for smoothing a web (1) of paper or card in glazing rollers (2) using pressure, moisture and heat in a press nip (3) of the glazing rollers in order to achieve a desired surface quality on the glazed paper web (1),
   with at least the surface of the web (1) of paper or card which is to be glazed being treated during glazing in the press nip (3) at a temperature above the second-order transition point of the material,
   characterised in that once the web (1) of paper or card has emerged from the press nip before a period of approximately 20 to 60 ms (milliseconds) has elapsed the glazed surface of the paper web (1) is deliberately subjected to a transition step to bring it to a temperature and moisture content below the second-order transition point of the material.
2. A method according to Claim 1, characterised in that the transition step is performed by means of a cooling device (4) which is located at the exit of the paper web (1) from the press nip (3).
3. A method according to Claim 1, characterised in that the transition step takes place by heat-exchanging contact of the surface of the web (1) of paper or card to be cooled with a cooled surface, to which the web of paper or card is guided.
4. A method according to Claim 1, characterised in that the transition step is performed at least in part by drawing off the steam which is produced upon the emergence of the paper web (1) from the press nip (3).
5. A method according to Claim 1, characterised in that the transition step takes place with direct contact of the surface of the paper web (1) to be cooled with a cooling gas.
6. A method according to Claim 5, characterised in that the transition step is performed using an inert gas.
7. A method according to Claim 5, characterised in that the gas is blown on to the paper web (1) and is drawn by suction through the paper web (1).
8. A method according to Claim 1, characterised in that the paper web (1) is preheated and/or moistened before entering the press nip (3).

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