METHOD AND ARRANGEMENT FOR SURFACE TREATMENT OF A PAPER AND/OR BOARD WEB

Abstract: The present invention relates to a method and an arrangement for surface treatment of a paper and/or board web (W). In accordance with the invention, the arrangement is formed of a yankee cylinder (2) and a calendering unit (5) placed one after the other in the machine direction, by means of which the web (W) is first glazed and dried and after that immediately calendered.

Description: Method and arrangement for surface treatment of a paper and/or board web The present invention relates to a method according to the preamble of claim 1 and to an arrangement according to the preamble of claim 5 for surface treatment of a paper and/or board web. Calendering is a method by means of which the properties, in particular smoothness and gloss, of a web-like material, such as a paper web, are improved after a drying unit in a paper or board machine. In calendering the paper web is passed into a nip which is formed between rolls pressed against each other and in which the paper web is deformed by the action of temperature, moisture and nip pressure, in which connection the physical properties of the paper web can be affected by controlling the above-mentioned parameters and the time of action. Extended-nip calendering has been found to be a good means of producing low-gloss paper grades by calendering. When higher gloss is required, the nip pressure of extended-nip calendering does not necessarily suffice to provide gloss. With ever-increasing running speeds, calendering is becoming a bottleneck in the papermaking process, and satisfactory quality is not achieved by today's machine calender units. Some of the drawbacks of the present papermaking process are also that the loss of bulk increases when gloss and smoothness are improved, and that in order to provide gloss and smoothness of sufficient quality, it is necessary to use webs with an abundance of coating and/or to use off-line calendering, in particular multi-nip supercalendering and/or soft calendering. Machine calendering means here and hereafter calendering in a calender unit in which nips are formed between metal rolls. The width of the nip in a machine calender is typically very small depending on the width of the rolls and the thickness of the paper web to be calendered, wherefore the nip load is relatively high. Supercalendering, which provides in off-line operation in practice the best result quality wise, means above and hereafter calendering in a calender unit in which nips are formed between a metal or chilled thermoroll and a paper or polymer roll provided with a resilient surface, in which connection a nip of a substantial width is formed. Soft calendering means above and hereafter calendering in a calender unit in which nips are formed in a manner similar to that of a supercalender between a metal or chilled roll and a roll having a resilient surface, in which connection a nip of a substantial width is formed. In soft calenders, each nip is formed between separate roll pairs, so that the nip load can be adjusted in each individual nip. Shoe and/or extended-nip calendering means above and hereafter calendering in a calender unit in which a nip is formed between a roll provided with a flexible mantle, the mantle of said roll being made, for example, of polyurethane, and a press roll or shoe roll which has a rigid mantle and is provided with an inside loading shoe and which is made of metal, such as steel. One extended-nip concept marketed by the applicant is called OptiDwell™, which includes two different extended-nip calenders: — OptiDwell Shoe™ calender based on shoe press technology, — OptiDwell Belt™ calender based on roll/belt technology. A yankee cylinder represents a drying and glazing method known in the art for a long time, and the aim of the yankee cylinder is primarily to improve gloss but not to evaporate water. Yankee cylinders are employed mainly on paper and board machines which manufacture high-quality folding boxboards and envelope paper. On the yankee cylinder, the moisture of the web is about 65 % at the most when it sticks to the surface of the cylinder and about 7—10 % at the most when it is separated from the surface of the cylinder. The main problem with the yankee cylinder is, thus, its speed dependence. The evaporation capacity of the yankee cylinder is limited, so when the speed is increased, the surface of the web is no longer glazed to a satisfactory degree. The primary object of the present invention is to provide an improvement in this detrimental speed dependence which is characteristic of the yankee cylinder and, with the improvement, to make it possible to increase speed without the quality, i.e. gloss and smoothness of paper or board suffering. This objective is achieved according to the invention by means of a method and an arrangement of the kind mentioned at the beginning, the principal special features of the method being set forth in the characterizing part of the accompanying independent claim 1 , and the principal special features of the arrangement being set forth in the characterizing part of the accompanying independent claim 5. Thus, the invention is based on the new and inventive basic idea that, after the web has been dried and glazed on a yankee cylinder, the web is glaze-calendered by using a shoe and/or extended-nip calendering unit as a calender placed after the yankee cylinder in accordance with an embodiment of the invention which is regarded as advantageous. In accordance with an embodiment of the invention considered to be particularly advantageous, the method employs and the arrangement comprises a combination in which there are disposed in the machine direction first a yankee cylinder and then a shoe calendering unit which serves as a glazing calender. With respect to the other characteristic features peculiar to the invention and the advantages attainable by them reference is made to the dependent claims of the accompanying set of claims. The invention and the advantages which can be attained by it are described below by way of example by means of an embodiment of the invention regarded as advantageous with reference to the accompanying drawing, the figure of which schematically shows an arrangement according to an advantageous embodiment of the invention for surface treatment of paper and/or board. The arrangement according to the embodiment shown in the figure for surface treatment of a paper and/or board web W comprises a yankee cylinder 2 and a calendering unit 5 placed after it in the direction of processing of the web W, said calendering unit being in the embodiment shown in the figure a single nip glazing- calendering unit whose extended nip is formed between a metal or chilled thermoroll 51 provided with a rigid mantle and a shoe roll 52 provided with a flexible mantle. The shoe roll 52 comprises an inside glide shoe which, when supported on the inside frame structure of the shoe roll 52, presses the flexible mantle of the shoe roll 52 or a roll/belt structure placed around the shoe roll (not shown in the figure) against the thermoroll 51 having a rigid mantle. When it is intended to achieve a given paper or board quality, the difference between the running speed used in the machine and the running speed allowed by the evaporation capacity of the yankee cylinder can be compensated for by means of the glazing calender unit 5, i.e. the use of the glazing calender unit 5 enables the speed of the machine to be increased without the quality, that is, gloss and smoothness of paper or board suffering. The web W to be treated is passed as a supported or free draw into a nip defined between the yankee cylinder 2 having a smooth surface and a wire or press roll 1 having a rubber surface, as shown in the figure, or into nips defined between the yankee cylinder 2 and two wire or press rolls placed one after the other in the machine direction. Said nip/nips has/have no primary dewatering function but their primary function is glazing, and in addition thereto, the web W sticks to the surface of the yankee cylinder 2 by means of the wire or press roll 1. After the nip between the yankee cylinder 2 and the wire or press roll 1 , the web W runs onwards on the surface of the yankee cylinder 2, on which a steam pressure of about 350 kPa acts on the outer surface of the web inside a hood 6, said pressure producing an evaporation capacity of 30—50 kg/m 2 , which dries the web from an incoming moisture which is typically 65—50 % but preferably not more than 70—80 % to a final moisture which is typically 7—10 % . The web W is separated from the surface of the yankee cylinder 2 by means of a doctor device 3 and a take-out roll 4. After the doctor device 3 and the take-out roll 4, the web W is guided into the extended nip between the rigid-mantle roll 51 and the flexible-mantle roll 52 of the shoe or extended-nip calendering unit 5. The shoe calendering unit 5, disposed after the yankee cylinder 2 in accordance with the invention, functions in the arrangement of the invention as a glazing calender which allows, without the desired quality of paper or board suffering because of the limited evaporation capacity of the yankee cylinder 2, the running speed used in the machine to be raised to a higher level than said maximum running speed dependent on the evaporation capacity of the yankee cylinder 2. Advantageously, the "gloss surface" of the web glazed on the yankee cylinder 2 is glazed in this calendering unit 5. The web W can be wound after the glazing carried out by means of the shoe calender 5. Based on the trial run results of the arrangement according to the invention, it has been possible to establish generally that the quality values of the web are improved, i.e. the bending resistance of the web in relation to the thickness of the web remains unchanged, its PPS roughness decreases and Hunter gloss improves when the yankee cylinder is followed by shoe calendering, and further that the improvement of the quality values is the clearer, the higher the linear load in the nip of the shoe calen- der. Some quality values as a function of the shoe calender are further shown below in table form. Table: attained quality values Above, the invention has been described only by way of example by means of some of its embodiments regarded as advantageous. This is of course not intended to limit the invention and, as is clear to a person skilled in the art. many alternative arrangements and modifications are feasible within the inventive idea and its scope of protection defined in the accompanying claims.

Claims: Claims 1. A method for surface treatment of a paper and/or board web (W), characterized in that the web (W) is glazed and dried first by means of a yankee cylinder (2), after which the web is immediately calendered. 2. A method according to claim 1, characterized in that the web (W) is calendered by means of a shoe or extended-nip calendering unit (5). 3. A method according to claim 1 , characterized in that, when it is intended to achieve a given paper or board quality, the difference between the running speed used and the maximum running speed dependent on the evaporation capacity of the yankee cylinder (2) is compensated for by means of calendering, i.e. calendering after the yankee cylinder enables the running speed of the yankee cylinder to be increased without the quality, that is, gloss and smoothness of paper or board suffering. 4. A method according to claim 1 , characterized in that the surface of the web (W) glazed by means of the yankee cylinder (2) is glazed in the calendering unit (5). 5. An arrangement for surface treatment of paper and/or board, characterized in that the arrangement is formed of a combination in which there disposed in the machine direction first a yankee cylinder (2) and then a calendering unit (5). 6. An arrangement according to claim 5, characterized in that the calendering unit is formed by a shoe or extended-nip calendering unit (5). 7. An arrangement according to claim 5, characterized in that, when it is intended to achieve a given paper or board quality, the difference between the running speed used and the maximum running speed dependent on the evaporation capacity of the yankee cylinder (2) is compensated for by means of the calendering unit (5), i.e. the calendering unit placed after the yankee cylinder enables the running speed to be increased without the quality, that is, gloss and smoothness of paper or board suffering. 8. An arrangement according to claim 5, characterized in that the calendering unit (5) glazes the surface of the web (W) glazed by means of the yankee cylinder (2).

Inventors: KOIVUKUNNAS PEKKA, LARES MATTI, KOIVUKUNNAS, PEKKA, LARES, MATTI
Applicants: KOIVUKUNNAS PEKKA, LARES MATTI, VALMET CORP, KOIVUKUNNAS, PEKKA, LARES, MATTI, VALMET CORPORATION
Application year: 2000
Main classification: D21H19
IPC codes: D21F5, D21G1, D21H25