This application is 371 of PCT/FI00/00190 filed 10 Mar. 2000.
The present invention relates to a method and apparatus for coating webs of paper and board or for surface sizing the same in order to improve their printability, strength or other qualities.
In order to improve the qualities of a paper or board sheet, the base web of the paper or board sheet is treated in different ways. The goal of each treatment is to improve the strength or printability properties of the produced grade. Strength improvement is principally accomplished by way of surface sizing, wherein the web surface is coated with a strength-improving sizing agent such as a starch solution. Coating is applied, among other reasons, for such purposes as better product brightness, surface impermeability or smoothness, while calendering serves to improve the surface smoothness and gloss.
Conventionally, web treatment is performed after base web formation so that a dry web is treated in separate off-line equipment or, alternatively, in online equipment connected directly after the paper- or boardmaking machine. Herein, the web is dried at least essentially close to its final degree of moisture content, whereby the web must be moistened and redryed particularly during surface sizing and coating, which increases the machine length and energy consumption. As modern paper/boardmaking machines are already equipped with efficient dewatering and drying sections, it would be advantageous to have the web-wetting operations such as surface sizing and coating moved as close as possible to the headbox so as to take place within the wire section or press section, whereby the dewatering and drying of the base sheet having the surface treatment agent applied thereto can be accomplished at least partially simultaneously. Surface sizing and coating performed at the press or wire section would also offer substantial quality benefits inasmuch the penetration of the surface sizing agent into the web takes place in a manner entirely different from that when the treatment agent causing web wetting is applied to an already dried base sheet. The quality of calendering is improved if this step is carried out on a web of higher moisture content, whereby also the outcome of calendering for the most common paper and board grades would benefit from being performed at the press section of a paper/boardmaking machine.
The history of having the base sheet formation and finishing phases chained in two separate steps can be traced to two major factors. Firstly, the formation of the base sheet and its subsequent finishing have traditionally been considered extremely autonomous production phases to be implemented independently from each other. Secondly, paper webs in particular and even board webs are very fragile prior to their drying close to the final solids content, whereby it has not been possible to execute such treatments that cause wetting of the web without jeopardizing web runnability.
From the art is known an embodiment in which surface sizing is carried out using a film-transfer applicator. In a film-transfer application apparatus, a coating film metered very accurately on a rotating film-transfer roll is transferred from the roll to the surface of the running web. Although a film-transfer applicator offers very good runnability and causes a minimal stress on the web, the water permeating the web anyhow weakens its strength. Since the web will not be passed directly from the film-transfer roll onto a supporting element such as a wire, an unsupported gap remains between the film-transfer roll and the subsequent supporting element. Hence, the web is always subjected to stresses in the cross-machine direction and particularly in the machine direction. For instance, variations in the moisture content profile may cause stress peaks on the web that readily break the wet and fragile web.
In addition to the technique of film-transfer application, use of spray application has been proposed in the art, wherein the surface size or coating mix is applied to the web by means of an array of spraying nozzles staggered in the cross-machine and/or the machine direction.
In U.S. Pat. No. 3,146,159 is described an embodiment in which application is performed on a wet web by coating one side of the web and simultaneously supporting the web during application from its other side by a fabric. Coating on a calibrating press is also described.
U.S. Pat. No. 4,793,899 describes spray-coating and short-dwell application techniques, wherein the web support arrangement is more advanced than that of the above-cited patent, however, not even this embodiment is free from unsupported web travel passages and the applicator still has a web-supporting fabric therein.
U.S. Pat. No. 5,152,872, there is described an embodiment free from unsupported web travel passages. In this arrangement, the coating mix is first metered on the outer surfaces of rolls and therefrom directly to the web, yet having a felt running in the nip.
It is an object of the present invention to provide a method suited for treating a paper or board web with a wetting substance or, alternatively, calendering the web prior to the cylinder dryer section of a paper- or boardmaking machine meanwhile the solids content of the web is still very low, typically 10–60%.
It is a further object of the invention to provide a method in which the web can be passed fully supported from the wire section of the paper- or boardmaking machine, when desired, up to the winder, thus utilizing the quality improvement benefits offered by a controlled management of web moisture content and wetting.
The goal of the invention is achieved by way of passing the web supported by a transfer belt through at least one surface treatment section such as a coater station or a calender prior to taking the web to the first dryer cylinder group of the paper- or boardmaking machine.
According to one advantageous embodiment of the invention, at least one nip supported by the web-transferring belt is used for dewatering simultaneously with the application of a web treatment agent to the web. The dewatering nip may be formed between a transfer belt and the wire of the wire section or the felt of the press section of the machine. As a matter of convenience, dewatering in a nip between the transfer belt and the wire is defined to comprise a portion of the press section, whereby reference to a nip of the press section in the following text may also be understood to include a nip formed between a wire and a web transfer belt.
The invention offers significant benefits.
One of the most important benefits of the invention is that the invention allows a coated or surface-sized board or paper to be manufactured in a machine of vastly simplified or shorter construction over those known in the prior art, because the web surface treatment and drying steps can be carried out in a single or almost single space and with the same equipment that in the prior art served for dewatering on the press section and the dryer. Accordingly, the machinery is principally comprised of existing sections. If the machinery is equipped with a calender section, it may be located at the most advantageous point along the line in respect to the web moisture content and treatability of the paper grade being manufactured. The efficiency of the web drying process is improved, because drying can be performed only once without the need for rewetting an already dried web. Water removal is also performed more cost-effectively from a very wet web than from a dry web. If the surface sizing or coating application step is carried out, e.g., in the press section nip so that the web is supported from the side to be treated by the transfer belt and from the other side by a felt or wire, water is removed from the web toward the felt or the wire, thus allowing the treatment substance to penetrate into the web. In the best case, the entire volume of water corresponding to that of the treatment substance is subsided from the wet web into the felt, whereby the drying capacity needed for web dewatering is not increased. In this manner, the invention can provide so good a web surface smoothness that soft-calendered qualities of paper or board can be made with an acceptable quality. Water transport and removal as well as the calendering effect may be augmented by heating the web transfer belt or its support roll. By means of a heated belt, it is possible to control the temperature profile of the web, whereby the moisture content or smoothness profile of the web, for instance, may be varied. Additionally, the adjustment of the size metering can be used for controlling the moisture content profile.
The support belt helps to form a tapering nip between the belt and the web, thus allowing a large amount of surface size to be applied which is advantageous particularly in the manufacture of boxboard. Particularly a shoe press is capable of providing an excellent penetration in the web. Typically a shoe press is also suitable for use in the manufacture of grades having a high bulk and/or improved strength of the paper or board web. By way of applying the surface size on a wet paper or board, the number of hydrogen bonds that principally determine the web strength is increased. Also the swelling of fibers that occurs during the wetting of a dry web is eliminated, whereby the web surface quality is improved. The method according to the invention is suitable for making a great number of paper or board grades with a competitive-edge quality or for producing a base paper of excellent finish for conversion into high-quality coated grades.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
In the following, the invention will be examined in greater detail by making reference to the appended drawings in which
In the following description, a surface sizing process adapted to operate in different manners in conjunction with the press section of a papermaking machine is elaborated by way of example. Obviously, the same or essentially similar embodiments can be used in a boardmaking machine and others serving to apply a coating or other treatment substance on the surface of a web.
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From the shoe press 13, the web travels on the transfer belt 9 out from the shoe press nip. The web is picked from the transfer belt 9 onto the dryer wire 15 of the dryer cylinder group by a third suction roll 16. Adherence of the web to the felt is secured by means of a suction box 17, and the final drying of the web is carried out by means a dryer cylinder group 1, whereupon the web is wound into machine rolls or, altenatively, is taken to further processing at finishing equipment connected to the paper- or boardmaking machine. After the web has left the transfer belt 9, the belt can be cleaned if necessary with the help of water jets 18 and a scraper 19.
In the embodiment shown in the diagram, if the press nip is adapted to operate in conjunction with the wire section, the felt 4 is replaced by a transfer belt, and the roll 5 has no suction facility but instead is advantageously adapted to operate with a backing roll. The coating is metered with the help of a spray applicator in front of the ingoin ide of the nip formed between the roll 5 and its backing roll. In slow machines, the web can be passed directly after the press nip between the rolls 7 and 8 to the cylinder dryer section. In this case, the roll 7 is advantageously a shoe roll, while the roll 8 can be a suction roll.
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Next, the web is taken to a second press which advantageously is a shoe press as in the illustrated embodiment. The transfer belt 39 of the first shoe press 40, 41 travels over a shoe roll 42 of the second shoe press thus passing the web into the press nip. A backing roll 43 is adapted to press against the shoe roll 42 and a felt 44 passes over the backing roll 43. Also in this nip, water is removed from the web and the water removal takes place in the direction of the felt 44. Next, the web is passed supported by the transfer belt 39 onto a wire or felt 46. The web is adhered to the felt 46 with the help of a suction roll 45 mounted at the tangential meeting point of the transfer belt 39 with the felt 46. Depending on the paper or board grade to be produced, the web is next passed to a dryer cylinder group, an assembly of the kind described above for treating the other side of the web or to some other type of web treatment apparatus.
In addition to those described above, the invention may have alternative embodiments.
Obviously, the above-described embodiments may be modified in a plurality of ways. Particularly, the number and location of spray applicators may be varied as required from those illustrated herein. Instead of spray application, it is also possible to use so-called jet application, wherein a jet nozzle assembly as wide as the entire web width to be coated is used for ejecting a freely discharged jet of the treatment substance. As the jet applicator discharges the coating in a uniform jet free from droplet formation, the nuisance of coating mist formation is avoided. The jet can be directed to the surface of a belt, roll, web or directly into a nip. In the spirit of the invention, however, it is essential that a treatment substance is applied in at least one transfer-belt-supported nip to that side of a web which faces the impervious belt. Preferredly, the nip should have a impervious belt on the coated side of the web and a felt permeable to water on its other side. The pressing force in the nip may be imposed either by means of rolls or, most advantageously, using a shoe press. This kind of nip achieves efficient water removal from the web and simultaneously subjects the web surface to application or calendering.
The web treatment substance such as a coating mix or surface size may also be applied to the surface of the transfer belt using, e.g., applicator devices similar to those employed in the film-transfer application technique. E.g., the assemblies elucidated in the exemplifying embodiments may also be modified by having a plurality of surface treatment and dewatering stations arranged to operate in-line so that the transfer belt is alternately facing the opposite sides of the web. The invention is also adaptable to multilayer application, wherein at least one coating or web treatment layer is applied using the method according to the invention.
Although the technique according to the invention is most advantageously implemented without having any unsupported passages in the machinery so that the web is at all times passed supported by at least one endless loop of a support element, a roll or a cylinder, it is possible in some special arrangements to transfer the web over an unsupported passage or supporting the passage by air jets to the next support element. An air-impingement dryer, infrared dryer or the like may be used when so desired for drying the web after its treatment while the web is still travelling supported on a belt or felt, before the web enters the next nip and before the treated side of the web becomes the supported side of the web or prior to the entry of the web into the dryer section. While the composition and state of the web treatment substance do not directly affect the function of the method according to the invention, they may require the use of certain types of applicator apparatuses or changes in the physical layout of the machinery. The web treatment substance may be in the form of a liquid, solution, dispersion, emulsion or foam, or any other kind of substance which is sufficiently easy to meter and apply.
The methods according to the invention may be complemented with coat weight measurement performed while the web is still supported on the belt or wire. Obviously, measurement equipment that operate from both sides of the web are unsuitable. One applicable technique for measuring the base sheet solids content and the coat weight, for instance, is the x-ray fluorescence method when CaCO3 pigments are used. Any conventional technique of basis weight and moisture content measurement may then be combined with the x-ray fluorescence method, whereby a number of quality variables can be computed from the measurement data thus obtained.
Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices described and illustrated, and in their operation, and of the methods described may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
Number | Date | Country | Kind |
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990557 | Mar 1999 | FI | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FI00/00190 | 3/10/2000 | WO | 00 | 1/17/2002 |
Publishing Document | Publishing Date | Country | Kind |
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WO00/55423 | 9/21/2000 | WO | A |
Number | Name | Date | Kind |
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4793899 | Skaugen | Dec 1988 | A |
4931143 | Karvinen et al. | Jun 1990 | A |
5256257 | Schiel | Oct 1993 | A |
5575891 | Trokhan et al. | Nov 1996 | A |
Number | Date | Country |
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0 881 329 | Dec 1998 | EP |
459125 | Dec 1936 | GB |