EP0779393B1 - Calander for treating a paper web and application for this calander - Google Patents

Abstract

A calender for treating a web of paper includes a stack of hard rollers and soft rollers. The stack includes a first set of processing nips and a second set of processing nips. Each processing nip is formed by a juncture of one of the hard rollers and one of the soft rollers. One side of the web of paper contacts against the hard rollers in the first set of processing nips and contacts against the soft rollers in the second set of processing nips. At least a last soft roller in the stack, which delimits a last processing nip that the web of paper passes through in the calender, has a lower surface roughness Ra than a surface roughness Ra of at least a first soft roller in the stack. The first soft roller delimits a first processing nip that the web of paper passes through in the calender. The surface roughness R. of the last soft roller is less than 0.35 mu m.

Description

The invention relates to a calender for the Treatment of a paper web, the first one and passes through second working column, each between a hard roller and an elastic roller are, with one side of the web in the first working columns on the hard rollers and in the second Working gaps are present on the elastic rollers, as well to an application of this calender.

Calenders of the type considered here are widely known. Attempts have been made to give the paper web a high gloss or high smoothness on both sides by first pressing one surface elastically against the smooth hard rollers in the first working nips and then elastically pressing the other surface against the hard rollers in the second working nips. For this purpose, the calender has a roll stack which has an alternating gap between the first and the second working nips, or it consists of two or more stacks which are successively passed through by the paper web (DE-U-295 04 034, US-A -5 237 915). However, it was difficult to achieve approximately the same gloss and smoothness values on both sides. This applies in particular if, for the purpose of reducing the number of calender rolls, the compressive stress in the working gap to values above 45 N / mm ² , the surface temperature of the heated hard rolls to at least 100 ° C and, if necessary, the moisture of the paper web to be fed to 5% and more elevated.

The invention has for its object a calender of the type described at the beginning with which it is possible to approximate the paper web on both sides to give the same high gloss or smoothness values.

This object is achieved in that at least the elastic roller delimiting the last working gap has a lower surface roughness than at least one elastic roller delimiting a first working gap and this lower surface roughness R _a <0.35 μm.

This proposal is based on the surprising finding that the surface of the elastic roller not only an abutment forms around the opposite paper web side for the purpose of gloss and smoothness against the hard roller and possibly due to a Micro slip between this surface and the Paper web to support the formation of gloss and smoothness, but that its surface condition in similar to how the hard roller acts on the paper. This is especially true if the paper due to higher temperature, higher pressure load and / or higher humidity is easier to plasticize.

Elastic rolls usually have a surface roughness of at least R _a = 0.5 µm. If this roughness profile works in the second working columns, it reduces the gloss and smoothness values generated in the first working columns with the help of the smooth, hard rollers. This is not the case if elastic rollers with the claimed lower surface roughness R _a <0.35 µm are used. They do not impair the gloss or smoothness values already achieved in the first working columns or even correct these values upwards if they have deteriorated due to a rough plastic cover. It is therefore necessary according to the invention that the elastic roller at least of the last working gap has the aforementioned low surface roughness. However, the fine machining required for this involves higher costs. Therefore, where the roughness is irrelevant, that is to say in the area of the first working gap, it is quite possible to use elastic rollers of a conventional type, that is to say with a higher roughness of R _a ≥ 0.5 μm.

It is favorable that all the elastic rolls delimiting the second working gaps have a lower surface roughness R _a <0.35 µm. A reduction in the gloss and smoothness values achieved in the first working columns can be completely avoided.

The smaller the lower surface roughness, the better the effects. Values R _a = 0.2 to 0.3 µm are recommended.

The surface roughness R _a relates to the mean roughness value, that is to say the arithmetic mean of the absolute amounts of the distances between the actual profile and the central profile. The elastic rollers of lower surface roughness advantageously have an average roughness depth R _z <2.5 µm. The mean roughness depth is the arithmetic mean of the individual roughness depths of five adjacent individual measurement sections into which the roughness reference section is divided, and is usually R _z = 4.5 to 5.0 µm.

In addition, it is recommended that all elastic rollers delimiting the first working gap have a greater surface roughness than the elastic roller delimiting the last working gap. This leads to optimal results with the lowest processing costs. It should also be taken into account that at least in the last working gap the ratio of the surface roughness R _a of the elastic roller and the hard roller is <7. In practice, it is never possible to match the surface roughness of the elastic roller to that of the hard roller. However, the smallest possible ratio should be striven for.

To achieve particularly high gloss and smoothness values, care should be taken to ensure that at least the hard roller delimiting the last working gap has a surface roughness R _a = 0.04 to 0.05 µm.

It is also advantageous that the hard roller has a surface layer made of a wear-resistant material, such as tungsten carbide. This surface layer can also be produced with a high degree of smoothness. It is also due to its wear resistance suitable for putting a scraper on them can without affecting the surface quality of the roller becomes.

For the construction of the elastic roller with low surface roughness it is recommended that they have an elastic Plastic cover wears.

In particular, the elastic cover made of a fiber-reinforced Plastic base layer and a fiber-reinforced Plastic cover layer exist. The base layer can do this with special consideration the elasticity and the top layer under particular Considering the high smoothness can be chosen.

It is also favorable that the elastic roller one Cover made of a plastic with such great resilience has the depth of permanent set after passing through a defect up to 1 mm thick due to the working gap at most 5% of the thickness of the defect is. The large resilience means that the high surface smoothness of the elastic rollers also not lost in the event of defects.

In a preferred embodiment, it is ensured that that the first and second working columns in one through the paper web from top to bottom Arranged stack and through an alternating gap, the is bounded by two elastic rollers, from each other are separated. In such a stack of rollers are the second working column delimiting rollers because of the Especially the weight of the rollers above heavily burdened. The second ones are correspondingly high Working gaps occurring compressive stresses. As a result affects the low roughness of the last elastic roller particularly cheap.

This is especially true if the elastic roller of the last working gap the bottom roller of the stack forms. It creates on one side of the paper web a similarly high gloss or smoothness value as it does the opposite side of the paper web from the hard Roller is awarded.

It has turned out to be particularly favorable that the stack consists of eight rollers and has four first working gaps and two second working gaps. A very small number of rollers can be used with this construction. Two second working gaps are sufficient to achieve the desired gloss and smoothness value. The fact is exploited that even in the first four working gaps there is a certain smoothing of the paper web surface lying against the elastic rollers, even if elastic rollers with a usual surface roughness R _a ≥ 0.5 µm are provided there.

An alternative ensures that the top one and bottom roller of the stack by a heatable hard roller is formed.

Again, the stack can consist of eight rollers, with the alternating gap in the middle of the stack located.

The invention also relates to the application of the previously mentioned calender for producing a coated Paper with almost the same gloss on both sides from at least 75% according to Gardner and above, in particular Gardner over 80% gloss.

Fig. 1 schematisch einen erfindungsgemäßen Kalander,

Fig. 2 einen Teilschnitt durch eine elastische Walze,

Fig. 3 die Rauhigkeit in vergrößertem Maßstab und

Fig. 4 eine zweite Ausführungsform eines Kalanders.

The invention is explained in more detail below with reference to preferred exemplary embodiments illustrated in the drawing. Show it:

1 schematically shows a calender according to the invention,

2 shows a partial section through an elastic roller,

Fig. 3 shows the roughness on an enlarged scale and

Fig. 4 shows a second embodiment of a calender.

The calender 1 illustrated in FIG. 1 has one Roll stack on, the one from the top down an elastic Top roller 2, a heatable hard roller 3, one elastic roller 4, a heatable hard roller 5, a elastic roller 6, an elastic roller 7, a heatable hard roller 8 and an elastic lower roller 9 includes. The elastic rollers 6 and 7 form between an alternating gap 10. There are first working columns 11, 12, 13 and 14 and below second working column 15 and 16. A paper web 17 by a Unwinding device 18 (off-line) or a paper machine (in-line) goes through first first working column 11 to 14, one side 19 the paper web with the hard rollers 3 and 5 in contact comes, and following the alternating gap 10 the second working column 15 and 16, in which the other Page 20 of paper web 17 with the hard roller 8 in Touch comes before the web to a winder 21 arrives. The deflection of the paper web 17 takes place in the usual way via guide rollers 22.

A control unit 23 places a signal line 24 with which force P the roller stack is loaded becomes. Top roller 2 and bottom roller 9 are deflection adjustment rollers, their degree of deflection via control lines 25 and 26 is affected. The three heatable hard rolls 3, 5 and 8 are supplied with heating energy H, for which control lines 27, 28 and 29 are provided are. If necessary, there is also a moistening device available, which the paper web with a enter the predetermined moisture F into the calender 1 leaves.

The hard rollers are steel rollers that are covered with tungsten carbide. This layer has a surface roughness of R _a = 0.04 to 0.05 and is extremely wear-resistant.

The elastic roller 9 consists of a hollow jacket 30, which carries an elastic cover 31. This reference consists preferably of a fiber-reinforced plastic base layer 32, which mainly for the Elasticity is responsible, and a fiber-reinforced Plastic top layer 33, the main is responsible for the smoothness of the surface 34. The fibers come primarily from plastic, such as aramid fibers ("Kevlar"). The Reference plastic is, for example, an epoxy resin. The top roller 2 has the same structure. The elastic Rollers 4, 6 and 7 are constructed similarly to that Difference that instead of the hollow cloak a full one or filled core is present. As an example of such an elastic cover 31 is a reference "TopTec 4" from Scapa Kern, Wimpassing / Austria, called.

It is important for the invention that the elastic roller 9 delimiting the lowest working gap 16, better still all elastic rollers 7 and 9 delimiting a second working gap, has a surface roughness R _a <0.35 μm, preferably 0.2 to 0.3 μm have. This value R _a is shown in FIG. 3 as follows: the actual profile I is delimited on the upper side by the reference profile A and on the underside by the basic profile B. The middle profile C is selected so that the sum of the material-filled areas D1 enclosed above it by the actual profile I is equal to the sum of the material-free areas D2 enclosed by the actual profile I below it. The mean roughness value R _a then results as the arithmetic mean of the absolute amounts of the distances hl of the actual profile I from the central profile C.

In contrast, the averaged roughness depth R _{z is formed} as the arithmetic mean of the individual roughness depths (h2) of five adjacent individual measurement sections into which a roughness reference section is divided. This value should be 9 <2.5 μm on the surface 34 of the elastic roller 9.

The upper elastic rollers 2, 4 and 6 need not have this high degree of surface quality. It is sufficient if they have a surface roughness R _a ≥ 0.5 µm in the usual way. This value can be achieved without difficulty by cutting material, for example by grinding, which is possible at relatively low cost. In order to achieve the surface roughness R _a <0.35 µm, on the other hand, an additional treatment is required in the materials known today, for example leveling the surface with the aid of a counter-roller, which is preferably in contact with low slip, with both rollers overlapping.

In the embodiment according to FIG. 4 there is Roll stack from a hard top roll 35, an elastic Roller 36, a hard roller 37, an elastic Roller 38, an elastic roller 39, a hard one Roller 40, an elastic roller 41 and a hard one Bottom roller 42. The changing nip 43 is located between the elastic rollers 38 and 39. A paper web 44 passes through the first working column before the alternating gap 43 and behind the alternating gap 43 second working column.

With the elastic rollers with low surface roughness numerous attempts were made. In all cases showed that on both sides 19 and 20 the Paper web 17 high gloss values and almost the same Size. Depending on the paper weight, the compressive stress in the working gaps, the temperature of the hard rolls, the dwell time in the working columns and from the moisture of the Paper showed gloss values of 76 to 83% Gardner and smoothness values up to 2,500 according to Bekk (s).

Average compressive stresses in the lowest working gap of more than 45 to 60 N / mm ² , surface temperatures of the hard rollers from 100 to 150 ° C. and paper moisture of 5 to 7% have proven to be favorable. The preferred ranges were 50 to 55 N / mm ² , 110 to 125 ° C and 6% moisture. The dwell time in the working columns should be between 0.1 and 0.9 ms, preferably between 0.2 and 0.5 ms.

The calender was particularly suitable for coated papers with a weight of 100 to 150 g / m ² . It made no great difference whether the screen side of the paper came into contact with the hard rollers or with the elastic rollers.

In order to demonstrate the superiority of the configuration according to the invention, a test was carried out with the calender of FIG. 1, the elastic roller 2 and the elastic roller 7 having a roughness R _a = 0.5 μm, while the elastic rollers 4, 6 and 9 had a reduced roughness R _a = 0.3 µm. The paper web 17 was first moved out and measured after the fourth working gap 14. The upper side, which had touched a hard roller four times, had 81% gloss, while the underside, which had previously only touched elastic rollers, already had 74% gloss. This value remained almost unchanged after the change gap 10 with 80/75%. After the second working gap 15, the gloss was 75/80%. This means that the upper side lost five percentage points due to the contact with the rough and therefore matt elastic roller with R _a = 0.5 µm, whereas the lower side gained a total of five percentage points in gloss due to the first contact with the heated hard roller 8. After going through the last working gap 16, the gloss result was 82/82%. This means that the top surface has increased in gloss by seven percentage points due to contact with the lowest smooth elastic roller.

From this it must be concluded that the contact of an already glossy paper web side with an elastic roller, which has a surface roughness R _a ≤ 0.5 µm, can result in a loss of gloss of five percentage points, or even more with two contacts. If, on the other hand, it is ensured that the glossy paper web surface produced in the first working gaps 11 to 14 by contact with the hard rollers only or at least in the last working gap comes into contact with a very smooth elastic roller, it is possible to achieve the desired gloss. or to get smoothness values on both sides. It should therefore be ensured that after the alternating gap, no elastic covers of higher roughness come into contact with the smoothed surface of the paper, or at least that the last processing takes place with a very smooth elastic roller, which compensates for any deterioration of the surface .

For further details of the calender and its Operation is based on the specification of DE-U-295 04 034 referred. They run in combination with the elastic Roller of low roughness, as is the case here are claimed, to particularly good results.

Claims (18)

1. Calender for the treatment of a paper web which passes successively through first and second work gaps (11 to 16) which are each formed between a hard roll (3, 5, 8; 35, 37, 40, 42) and an elastic roll (2, 4, 6, 7, 9; 36, 38, 39, 41), one side of the web being in contact with the hard rolls (3, 5; 35, 37) in the first work gaps (11 to 14) and with the elastic rolls (7, 9; 39, 41) in the second work gaps (15, 16), characterised in that at least the elastic roll (9; 41) defining the last work gap (16) has a lower surface roughness than at least one elastic roll (2, 4, 6; 36, 38) defining a first work gap (11 to 14) and this lower surface roughness R_a < 0.35 µm.
2. Calender according to claim 1, characterised in that all the elastic rolls (7, 9; 39, 41) defining the second work gaps (15, 16) have a lower surface roughness R_a < 0.35 µm.
3. Calender according to claim 1 or 2, characterised in that the lower surface roughness R_a = 0.2 to 0.3 µm.
4. Calender according to any of claims 1 to 3, characterised in that the elastic rolls with lower surface roughness have an average peak-to-valley height R_z < 2.5 µm.
5. Calender according to any of claims 1 to 4, characterised in that all the elastic rolls (2, 4, 6; 36, 38) defining the first work gaps (11 to 14) have a higher surface roughness than the elastic roll (9; 41) defining the last work gap (16).
6. Calender according to any of claims 1 to 5, characterised in that at least in the last work gap (16) the ratio of surface roughness R_a of elastic roll and hard roll is < 7.
7. Calender according to any of claims 1 to 6, characterised in that at least the hard roll (8; 42) defining the last work gap (16) has a surface roughness R_a = 0.04 to 0.05 µm.
8. Calender according to any of claims 1 to 7, characterised in that the hard roll (8; 42) has a surface layer of a wear-resistant material such as tungsten carbide.
9. Calender according to any of claims 1 to 8, characterised in that the elastic rolls carry an elastic covering (31) made of plastic.
10. Calender according to claim 9, characterised in that the elastic covering (31) consists of a fibre-reinforced plastic bottom layer (32) and a fibre-reinforced plastic top layer (33).
11. Calender according to claim 9 or 10, characterised in that the elastic roll (9) has a covering (31) made of a plastic with such a high recovery capacity that the depth of permanent set after passage of a flaw up to 1 mm thick through the work gap (16) is not more than 5% of the flaw thickness.
12. Calender according to any of claims 1 to 11, characterised in that the first and second work gaps are arranged in a stack through which the paper web (17) passes from top to bottom, and separated from each other by a change gap (10; 43) which is defined by two elastic rolls (6, 7; 38, 39).
13. Calender according to claim 12, characterised in that the elastic roll (9) of the last work gap (16) forms the lowermost roll of the stack.
14. Calender according to claim 13, characterised in that the stack consists of eight rolls and has four first work gaps (11 to 14) and two second work gaps (15, 16).
15. Calender according to claim 12, characterised in that the uppermost and lowermost roll of the stack is formed by a heatable hard roll (35, 42).
16. Calender according to claim 15, characterised in that the stack consists of eight rolls and the roll gap (43) is located in the middle of the stack.
17. Use of the calender according to any of claims 1 to 16 for producing a coated paper with almost the same gloss of at least 75% according to Gardner on both sides.
18. Use according to claim 17, characterised in that the gloss according to Gardner is over 80% on both sides.

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