Paper making and fiber liberation – Processes and products – With coating after drying
Reexamination Certificate
1998-09-16
2002-05-21
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
With coating after drying
C162S142000, C162S147000, C162S148000, C162S055000, C162S070000, C162S071000
Reexamination Certificate
active
06391154
ABSTRACT:
The present invention relates papermaking. In particular the invention concerns a method for producing a paper web. According to a method of this kind, a fibrous raw material is slushed to form a stock, a web is formed from the stock and the web is dried.
FIELD OF THE INVENTION
Description of Related Art
With the aid of the present invention it is possible to produce a base paper which is particularly well suited to the manufacture of fine paper. The surface weight of a base paper of this kind is generally 20 to 200 g/m
2
.
High-quality printing matters, such as brochures, advertising materials and catalogues, are made from fine papers which have good opacity, an even surface structure and high brightness.
Traditionally, fine papers have been manufactured from cellulosic hardwood or softwood pulps or mixtures thereof. The problem associated with the known art is that it is not possible at low grammage to reach sufficiently high opacity for the pulp or the paper manufactured therefrom. The formation of the chemical pulp and the paper made from it remains rather poor when high opacity is aimed at.
Light, coated paper qualities containing mechanical pulp are also known in the art. These are manufacture from a mechanical pulp made from spruce and they usually contain about ⅓ to ¼ softwood pulp which reinforces the pulp and improves the strength properties of the paper.
The particular advantages of mechanical pulps in comparison to chemical pulps are their lower production costs and greater yield. The coarse, stiff fibers of the pulp lead, however, to fiber coarsing, which shows in offset printing Further, the disadvantages of groundwood of spruce include its poor dewatering at low drainability and the large energy consumption of the whole pulping process. A problem of known papers based on groundwood is also their low brightness and poor brightness stability. They are not either shelf stable if spruce GW has been used.
Paper qualities containing mechanical pulps and combinations of mechanical pulps and chemical pulps, respectively, have not been used for fine papers. Instead, said types of papers (e.g. LWC) are primarily used as magazine papers.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems of the prior art and provide a solution for producing a base paper suitable for the manufacture of fine papers. It is also an object of the present invention to provide a fine paper of a novel kind, having the high brightness and good smoothness of traditional fine papers and which further exhibits the good opacity and excellent printability due to the good formation characteristic for mechanical printing papers.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the concept of combining groundwood of hardwood and chemical pulp of softwood and of producing a base paper from a mixture of the mechanical and the chemical pulp. In connection with the present invention it has been found that a mechanical pulp (in particular Pressure Ground Wood, PGW) manufactured from aspen and other wood species of the Populus family contain a great amount of short fibers which improve the traditionally insufficient bulk and light scattering of fine paper. Although the strength properties of aspen GW, as regards, for example ScottBond strength, are not entirely sufficient, by combining aspen GW with a chemical pulp produced from softwood, it becomes possible to produce a basepaper which exhibits excellent opacity at high brightness and an even surface and good strength. Due to the good bonding strength of softwood, aspen GW can be used in an amount of up to 30 to 70% of the dry weight of the pulp.
More specifically, the solution according to the present invention is mainly characterized in that the stock is formed from a mechanical pulp of a wood raw material of the Populus family and a bleached chemical softwood pulp, the amount of mechanical pulp being 20 to 70wt-% and the amount of bleached softwood pulp being 80 to 30 weight-% of the dry matter of the stock.
Considerable advantages are obtained by means of the invention Thus, using the base paper according to the invention it is possible to obtain better opacity on the same level of brightness as that exhibited for traditional fine papers. The paper contains more fines and its bulk and opacity are greater, which gives it good printing properties, Surprisingly, we have found that by means of the present invention it has become possible at lower grammage to produce a fine paper having an extremely high brightness. When the fine papers produced from the present base paper are compared with traditional fine papers, thanks to the low grammage, a yield gain of up to about 20% can be obtained; on the same opacity level the present paper will give more printing surface per weight unit than traditional fine papers.
The short-fibered aspen gives the paper good light scattering properties.
According to the invention it is therefore possible to provide a paper which fulfills all the quality requirements of coated fine papers consisting solely of chemical pulp but which, at the same time, provides high opacity and bulk and excellent printability properties.
In the following, the invention, its features and benefits will be examined in greater detail with reference to a detailed description and a number of working examples.
The fiber structure of aspen and wood species belonging to the same family differ from the fiber structures of the hardwood species most frequently used for pulp making, such as birch. The dimensions of the aspen fiber, the fiber length and width are smaller than for spruce and birch The tracheids of aspen are smaller (length 0.9 mm) than the tracheids of birch (1.0-1.1 mm). In both, the proportion of vasculum cells is about 25%. Traditionally, the tubular cells contained in aspen have been considered to cause runability problems on the paper machine and they have not been believed to provide for bonding. As a result of the short fibers and the poor bonding of the vasculum cells, dusting of the paper can occur on the paper machine and during posttreatment.
According to the present invention it has now surprisingly been found that by using a combination of mechanical pulp produced from aspen and chemical softwood pulp, the runability problems caused by the tubular cells can be avoided and a pulp can be produced which has impeccable strength properties. Since the aspen pulp has shorter fibers than the birch pulp and even much shorter than spruce, at a given grammage there are more aspen fibers than birch or spruce fibers. This leads to a greater light scattering coefficient and bulk in the present invention. Further, the advantageous fiber length distribution gives the paper an excellent formation i.e. variation of the grammage of paper on a small scale, typically <3 g/m
2
. The smoothness of the paper is also good.
Due to all these factors, it is now possible to achieve a base paper which can be coated for production of high-quality line papers, which have excellent printability properties
The advantages of the special aspen pulp in comparison to spruce groundwood comprise high brightness and brightness stability. The stability of the brightness is in particular due to the low lignin-content of aspen groundwood or corresponding mechanical pulp and to the low concentration of carbonyl groups compared to spruce groundwood. Further, it should be pointed out that a paper web produced from aspen has clearly better dewatering properties than a web produced from spruce. The shorter dewatering time and the higher dry matter content together give a sheet with more porosity.
The greatest advantage of the fiber distribution of aspen is obtained when the pulp has been beaten to the drainability of fine papers. It should be mentioned that spruce has to be refined to a higher degree of beating because of the stiff fibers contained therein. The long and stiff fibers of mechanical pulps produced from spruce cause a coarsing of the paper surface fibers during coating and, in particul
Leskelä Markku
Nygård Stina
Pitkänen Maija
Fortuna Jose
M-real Oyj
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