Presses – Methods – With heating or cooling
Reexamination Certificate
2002-10-23
2004-07-06
Ostrager, Allen (Department: 3725)
Presses
Methods
With heating or cooling
C100S075000, C100S161000, C102S206000
Reexamination Certificate
active
06758135
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
When calendering advantageously in a paper machine it is useful to raise the moisture content of the surface layer of paper so as to be as high as possible and to keep the middle layers of paper optimally dry. By this means, in a nip
ips of a calender or in another contact under pressure there occurs a permanent deformation in the moist surfaces of paper and the dry middle part recovers like an elastic material when the compression load ends. Such a state of the web leads to the fact that the caliper of paper is better preserved when the same surface quality is sought to be achieved (compared with a situation in which moisture is uniformly distributed in paper).
The surface of paper is affected during calendering by external compressing pressure, temperature, and the moisture contained in the web. The smoothening of the paper surface in a calender nip is based on a quick pulse generated by heat and pressure on the surface of paper: the polymers of paper exceed their glass transition temperature because of raised temperature and moisture, and soften so as to be mouldable. By the action of pressure, the surface of a hot and smooth steel roll is copied to the softened and deformable fibres. At the outlet of the nip, the temperature of the fibres falls below the glass transition temperature of polymers, the fibres cool and solidify in their smoothed structure. In addition thereto, the properties of the web are reduced because of compression when the caliper of the web is reduced.
Through calendering, attempts are made to improve the quality values of paper that have already been achieved or, at a standard quality level, to achieve a higher speed or a better bulk of the paper. It is known that the plasticity of paper, i.e. its readiness of being moulded, can be increased by raising the temperature and/or the moisture content of the paper. A considerable change takes place in the plasticity of paper when the temperature of the polymers contained in the paper rises to or beyond the so-called glass transition temperature. Then, the paper can be moulded more readily than below the glass transition temperature. An increase in the moisture content of paper lowers the glass transition temperature. Most commonly, the paper web is heated in a calender nip by means of a heatable roll, a so-called thermo roll and, in addition to this, possibly by means of steam treatment before the calender. The steam treatment also increases the moisture content of the paper, thereby lowering the glass transition temperature and thus enhancing the mouldability of paper.
Thus, when paper is calendered, the effect of calendering on the paper is highly dependent on the moisture content and temperature of the fibres contained in the paper at the time of calendering: the mouldability of the fibres is increased sharply even as if by a jump when their temperature reaches the so-called glass transition temperature, which is directly proportional to the moisture content of the fibres, or when their temperature is above the glass transition temperature. Above the glass transition temperature, it is easy to produce permanent deformations in the fibres, and below said temperature the deformations tend to be reversible. In order to ensure the permanence of the effects of calendering, the web must be moisturized to lower the glass transition temperature and, in addition, very high calendering temperatures and high pressures must be used, in which connection the entire web readily exceeds the glass transition temperature and, thus, the deformation takes place evenly through the entire cross section of the web in the cross direction.
With respect to the prior art relating to the invention, reference is also made to FI Patent 98 230 (corresponding U.S. Pat. No. 5,524,532), which discloses a method and a device of calendering a paper or board web, wherein the web is passed through a calendering nip. The calendering nip is formed of a heated roll and another roll or of a heated roll and an extended-nip roll or of a heated roll and a belt arrangement, by means of which the web is pressed against the heated roll, wherein, before the web enters the calendering nip, a moisture distribution is produced between the surfaces of the web in the thickness direction of the web such that the moisture is higher on that side of the surface of the web which is to be calendered than in the web interior. A temperature difference is produced between the surfaces of the web such that the web surface to be calendered will be on the side of lower temperature, and evaporation of moisture from the web is prevented during the moisture transfer process when the moisture present in the web is transferred towards the surface on the side of the lower temperature.
It is also known from the prior art that by controlling the imbition time water can be caused to stay in the very surface layers of paper. With respect to the prior art relating to this, reference is made to the publication A. Heikkinen, P. Linnonmaa & M. Diebel “
Practical Aspects Concerning Moisture Gradient Calendering”
, Wochenblatt f{dot over (u)}r Papierfabrikation, Jahrgang 127, 1999, Nr. 10, pp. 680-685. It is essential that the moisturizing device has a sufficiently small droplet size in order that the small water amounts which are applied shall form a film and not drops here and there on the surface of the paper. The droplet size shall typically be less than 200 &mgr;m, less than 20 &mgr;m on average, advantageously 10 to 20 &mgr;m.
The essential gradient control variables include:
Moisturizing delay
Water temperature
Water quantity
Number of applications
Even though this description mainly refers to paper, it shall be understood that the disclosure also relates to the calendering of board.
One multinip calendering method known in the state of the art is supercalendering, which is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll coated with a resilient cover, such as a polymer roll. The resilient-surface roll conforms to the surface contours of paper and presses the opposed side of paper evenly against the smooth-surface press roll. Today the supercalender typically comprises 10 to 12 nips and for the purpose of treating both sides of the web, the supercalender comprises a so-called reversing nip, in which there are two resilient-surface rolls against each other. The linear load increases in the calender from the top nip to the bottom nip because of the force of gravity.
The problems in calendering today are mainly due to the following factors.
a. Initial moisture content, the number of steam treatments and the temperature of calendering are mainly determined by the final moisture content after calendering such that
i. when the final moisture content becomes too low, the web absorbs moisture, which results in deterioration of the achieved gloss in the form of afterroughening, and
ii. when the final moisture content becomes too high, the drying of the web effectively destroys the quality values that have been attained.
b. On the other hand, the determination of the initial moisture content of calendering is affected by desired optical properties and the level of blackening. When the final moisture content becomes too high, the opacity, i.e. translucence of the web deteriorates, which appears in a finished paper product as an increase in print-through values, and the blackening level rises, which reduces the selling value of paper in the form of diminished brightness and a poor visual impression.
Because of these factors, the real control variables of a modern calender are relatively limited and the operating window of an individual calender has become rather narrow with the increasing drying capacity of the calender. Today the improvement of quality is thus successfully accomplished in practice only by increasing the number of nips in the calender. In connection with this, t
Heikkinen Antti
Kyytsönen Markku
Linnonmaa Pekka
Moisio Pekka
Vuorijärvi Ville
Metso Paper Inc.
Ostrager Allen
Stiennon & Stiennon
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