Paper making and fiber liberation – Processes and products – Multi-layer waterlaid webs or sheets
C162S183000, C162S135000, C162S205000, C162S158000
FIELD OF THE INVENTION
The invention relates to a method and devicie for manufacturing a paper or board web, and in particular, a method and device for producing an uncoated web which substantially corresponds in its properties to a precoated web.
BACKGROUND OF THE INVENTION
The method and the paper or board machine according to the invention can be applied very widely in the manufacture of different paper and board grades in which the conventional method would comprise improving the quality of paper or board by precoating. Typical such writing and printing papers are fine paper, LWC base paper or improved newsprint known by a person skilled in the art. The coating of different board grades has also increased in recent years. Hereafter, the invention will, however, be described mainly in connection with the manufacture of fine paper. However it is appreciated that the invention is not confined to this to this area
In this description, the use of the term fine paper is intended to mean uncoated fine paper and coated fine paper. The basis weight of uncoated fine paper is usually 40 to 230 g/m
and that of coated fine paper 60 to 250 g/m
. Typical pulp for manufacturing fine paper comprises chemical fibres: short fibres which are obtained, for example, from birch and an eucalyptus tree, and a long-fibre material obtained from softwood trees is generally added thereto.
The proportion of mechanical pulp is generally below 10%. About 15 to 30% of filler is added to the pulp, and the filler may be stone, calcium carbonate, kaolin and/or other suitable mineral pigments. Recently, in the manufacture of fine paper, increasing use has also been made of recycled fibres.
As mentioned above, by means of the method and the paper machine in accordance with the invention, it is possible to manufacture profitably paper that corresponds to precoated fine paper. In this application, the aim of conventional precoating has been to provide the surface of the web with certain properties which are beneficial in the further processing of the web. These properties include, among other things, a desired porosity of and pore size distribution in the surface as well as a desired oil absorption level, for example, measured by the Cobb-Unger method. The coverage degree of pigment is also one property that is monitored. The process of manufacturing fine paper according to the invention, which does not apply conventional precoating, thus enables said properties to have values which correspond to those attained when using a conventional manufacturing process that includes a precoating stage.
Corresponding to coated fine paper, the invention can also be applied advantageously to paper grades containing mechanical pulp, which paper grades are coated at the final stage of the manufacturing process. Typical such paper grades are LWC and MWC.
The method and the device according to the invention also make it possible to manufacture better newsprint than before without adding a surface sizing unit to a line. The significance of the invention in this application will be the greater, the faster the newspaper machines will be and the thinner the newsprint will become. Four-colour printing requires a surface that is stronger and denser than before. As known, it has been taught previously that this is attained by surface-treatment of the web.
It is required more and more often that boards shall have a better printing surface than before. Our invention offers good possibilities for that without costly investments at the dry end of the board machine. When multi-layer boards are manufactured, our invention also offers an advantageous solution for improving the printing properties of webs forming the surface layers of multi-layer board. The bonding strength of multi-layer board can also be increased by feeding, for example, starch into the surface of the webs.
In paper or board machines known in prior art, the short circulation and other stock systems are most commonly built such as to mix fibres, fillers, fines and additives to form a stock that is as homogeneous as possible in order to be supplied into a headbox of a paper or board machine. In multi-layer web forming, it is also known to use several different stock systems for feeding different fibre suspensions either into one or more headboxes. The headbox spreads the formed pulp suspension evenly on a wire section, in which dewatering and couching of the web begin. In prior art there are known several different types of wire sections, or formers, known in themselves to a person skilled in the art; fourdrinier formers, hybrid formers, and gap formers. Board machines may include even several wire units. From the standpoint of the invention, the most preferable former is a gap former, in which a slice jet produced by a headbox is fed between two wires and the bulk of the water is removed between said wires in two directions. One advantageous gap former arrangement has been described in the paper L. Verkasalo: Efficient Forming at High Speeds, XI Valmet Paper Technology Days 1998. In the arrangements known from prior art, the fibre and filler distribution in the thickness direction of the web can be controlled only to a limited degree, for example, by placement and vacuums of the dewatering elements of the former. The fillers often accumulate on the surfaces of the web in dewatering stages.
In prior art there are known in themselves multi-layer headboxes, some of them being described, for example, in the paper M. Odell: Multilayering, Method or Madness?, XI Valmet Paper Technology Days 1998 and in FI patent 92 729, and one of them being also described in the paper P. Ahonen: Challenges for Digital Printing Paper, XI Valmet Paper Technology Days 1998. Multi-layer headboxes allow desired layer structures to be produced in the web by feeding stock in layers between wires. Instead of multi-layer headboxes, in the case of some board grades in particular, several different headboxes and wire units have been used in order to provide a layered structure.
The web is passed from the wire section to a press section where water is removed from the web by pressing it against one or two felts. A person skilled in the art knows several different press arrangements from prior art, for example, a press based on roll nips, marketed by the applicant under the trademark SymPress™II. Recently, instead of roll nips, ever-increasing use has been made of an extended nip known in itself in prior art because of its higher dewatering capacity and/or its ability to retain the bulk of the web.
The dryer section in prior-art paper and board machines has most commonly been formed of a dryer section which uses a conventional single- and/or twin-wire draw and in which drying takes place mainly as cylinder drying while the wire presses the web against a heated cylinder surface. At high running speeds, single wire draw through the entire dryer section has become common in recent years. As the most recent arrangement, for example, the patent application PCT/FI98/00945 has proposed combining impingement drying with cylinder drying in order to provide a higher evaporation rate and a shorter dryer section.
In some prior-art arrangements, the paper web is passed from a dryer section to a precalender, which in known arrangements may be a calender with hard or soft nips, in which the paper web is passed through the nip between rolls to provide smoothness to the surface of the paper web. In the precalender, loose fibres and other components of stock are also fixed to the surface of the web, but, at the same time, differences in density may also be caused in the base paper and some of the bulkiness of the web important to many grades may be lost. Precalendering is particularly important before the coating of the web when blade coating is used in order to avoid so-called blade lines.
In surface sizing, the surfaces of the web are treated with a starch or pigment solution in a film size press, for example, by means of an applicator device marketed by the applicant under the trade name SymSizer™. Surface sizing
Fortuna Jos'e A.
Metso Paper Inc.
Steinberg & Raskin, P.C.
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