Paper making and fiber liberation – Apparatus – With coating or impregnating means
Reexamination Certificate
2002-11-01
2004-05-11
Walls, Dionne A. (Department: 1731)
Paper making and fiber liberation
Apparatus
With coating or impregnating means
C162S265000, C162S361000, C100S333000, C100S331000, C100S161000
Reexamination Certificate
active
06733632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the manufacture of paper having a high smoothness, wherein the paper arriving from the paper machine is supplied on-line to a supercalender (multinip soft calendar), wherein the paper travels through a plurality of nips for achieving the desired and smoothness properties.
The present invention also relates to an apparatus for carrying out the process.
2. Description of the Related Art
Gloss and smoothness are characteristics of a paper web which not only influence the appearance of the paper web but also the possibilities of further processing of the paper web. In certain types of applications, gloss and/or smoothness values are desired which are to be reproducible as uniformly as possible.
Gloss and smoothness of the paper web are usually increased by conducting the paper web following the dry end of the paper machine into a smoothing unit which is composed of one or more nips, wherein the gloss and/or the smoothness of the paper web is increased by the pressure in the nip and the temperature of the rolls forming the nip. However, this makes it only possible to influence the gloss and smoothness of the paper web to a limited extent because, when the pressure in the nip is increased too much, the paper web is compressed too much and a volume loss of the paper web occurs. In this connection, there is the additional danger that the paper web loses stiffness. There are also limits with respect to the increase of the roll temperature because this increase requires a large amount of energy. For example, for achieving roll temperatures of 200° C., significant quantities of energy must be supplied because the rolls are continuously cooled by the paper web travelling past the rolls.
Therefore, it has already been attempted, for example, in silicon papers, to influence the gloss and smoothness of the paper web by the moisture of the paper web. However, this has the disadvantage that after the treatment the supplied moisture has to be removed at least partially which requires additional process steps which, in turn, increase the time required for the treatment of the paper web and the apparatus is more complicated.
Basically, there are essentially two types of smoothing units. So-called supercalenders have a plurality of rolls arranged one above the other and nips provided between the rolls through which the paper web travels. The large number of nips produces a high degree of overlap or contact and a good distribution of the glazing work between pressure and temperature. Supercalenders are usually provided off-line, i.e., the paper web arriving from the paper machine is initially wound onto a reel-spool and is transferred together with the reel-spool to the supercalender, wherein the paper web travels through the supercalender at a significantly lower speed than the paper machine speed. The off-line installation has the advantage that the paper web can level out or equalize prior to entering the supercalender, so that the operation in the supercalender does not have to be carried out under the requirements of the paper machine which is influenced by many factors. However, the installation requirements are significantly higher. A supercalender classically has heated steel rolls, on the one hand, and paper rolls or rolls covered with cotton, on the other hand. More recently, also so-called multinip soft calendars are used in which the paper rolls are replaced by rolls having polymer covers. These rolls have a different elastic behavior than the paper rolls, so that it is possible to operate with a lower nip pressure.
Secondly, there are so-called machine calendars or soft calendars which can be connected on-line to a paper machine and through which, consequently, the paper web travels with the paper machine speed. However, machine calendars only have a small number of nips, so that the operation is carried out with higher pressure and temperature and, thus, the paper web is stressed to a greater extent. A significant disadvantage of the soft calendars is that not all types of paper can be upgraded to high qualities. In particular, it is not possible to manufacture highly compacted SC-A paper on-line in a soft calendar. It was possible recently to achieve the printing properties of a natural gravure paper supercalendared in 11 nips in a soft calendar with only four nips; however, this requires relatively high roll temperatures and compressive stresses in the nips. Also, these qualities can only be achieved in a range of speeds which corresponds to the glazing speed in the supercalender which is usual for this paper (see: Rothfuss, Ulrich: Inline- und Offline-Satinage von holzhaltigen, tiefdruckfähigen Naturdruckpapieren in: Wochenblatt für Paperfabrikation 1993, No. 11/12, pages 457-466). Consequently, such qualities can only be achieved with the off-line installation of the soft calendar.
DE 43 01 023 C2 discloses in connection with machine calendars a process which provides to apply steam to the paper web immediately prior to the nip and to conduct the paper web through the nip before the temperature and moisture have equalized in the paper web. This not only results in moistening of the paper web, but a temperature increase is achieved simultaneously. The heat contained in the steam is transferred during the condensation onto the paper web, so that this measure produces a paper web which has at the surface thereof the necessary temperature and the necessary moisture. When this paper web is guided through the nip, the nip primarily influences the surface portion of the paper web, while the middle and bottom portions are influenced significantly less. Consequently, no changes worth mentioning occur in the thickness direction in the middle (and bottom) portions. The volume of the paper web is maintained to a greater extent, although the surface quality is significantly improved. However, the on-line manufacture of SC-A papers is not possible with machine calendars of this type.
SUMMARY OF THE INVENTION
Therefore, it is the primary object of the present invention to manufacture paper having a high smoothness on-line.
In accordance with the present invention, the paper web arriving from the paper machine and supplied on-line to a supercalender (multinip soft calendar) is moistened with spray mist and the spray mist is applied onto the paper web about 0.6 to 1.2 s, preferably 0.8 to 1 s, before the paper web travels through the first nip.
The moistening of paper webs by means of nozzle-type moistening units is basically known in the art. A spray head suitable for this purpose is known, for example, from DE 38 19 762 C2. However, since during spraying individual droplets are applied onto the paper web, the paper web is inevitably moistened unevenly. Therefore, this type of moistening has in the past only been used in the off-line treatment of paper webs, so that the moisture has sufficient time to distribute uniformly over the paper web. The on-line moistening has in the past been carried out by means of steam blow boxes, wherein by definition the formation of droplets in the spray mist is to be prevented. However, we have recognized that the selection of an appropriate distance between the spray mist moistening and the first nip of the supercalender makes possible a uniformly moistened paper web even during the on-line installation and on a paper web conveyed with the paper machine speed. It was found that a duration of 0.6 to 1.2 s, preferably 0.8 to 1 s, is sufficient for obtaining, in addition to a uniform moistening across its surface, a paper web that is uniformly moistened in the Z-direction, i.e., across its cross-section or thickness. Accordingly, the selection of the precise position of the moistening device depends significantly on the paper machine speed. At a usual paper machine speed of up to 1,400 m/min and a desired time of application of 0.7 s, the spatial distance of the moistening unit from the first nip is about 16 m. In paper machines with appropriate deflections of the p
Diebel Manfred
Mann Rudolf
Winheim Stefan
Kueffner Friedrich
V. I. B. Systems GmbH
Walls Dionne A.
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