Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2002-04-16
2004-08-10
Walls, Dionne A. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C106S461000, C427S359000, C427S372200, C427S428010, C427S356000, C162S204000
Reexamination Certificate
active
06773550
ABSTRACT:
The present invention relates to coating of papers and cardboards. In particular the invention concerns a method according to the preamble of claim 1 for coating fibrous webs, such as base papers of fine papers.
According to a method of the present kind, a coating colour containing pigments is applied to the surface of a web and dried in order to form a coated web.
A disadvantage of known coating colours and pigments contained therein is the uneven distribution of the coating material, i.e. poor coverage. In particular with small amounts of the coating colour, the poor coverage gives rise to bad printability and patchy brightness of the paper. As a remedy, large amounts of coating have been used. Attempts have been made to improve the coverage also by producing a so-called structurized coating colours. This means that a destabilization of the coating mixture has been aimed at by e.g., a cationic substance. The problem of structurization is for example poor runability and poor surface hardness which create problems during printing. Large coating amounts lead to poor opacity, bulk and cracking problems in particular with light paper qualities.
High-speed coating with the film press method is hampered by mist-forming in the coating nip which interferes with runability and impairs the quality of paper. The problem arises when the film splitting in the nip is not under control and a part of the film does not accompany the paper web or the coating roller but is directly flung out from the nip. Uncontrolled film splitting may be caused by insufficient immobilization of the coating colour before splitting. The problem can be solved by raising the immobilization point of the coating colour by increasing the dry matter content of standard coating colour. This solution to the mist-forming, however, leads to another problem. Since the amount of coating is dependent on the dry matter content of the coating colour, the feed thickness of the film will have to be reduced. The thickness of the film on the coating roll is regulated with a rotating rod. The thickness of the film can to some extent, but not sufficiently, be regulated by varying the thickness and the rotational speed of the rod. When the rod load is increased too much, which happens when the dry matter is too high, the pasta film will, however, break between the rod and the coating roll. This phenomenon is called drop formation. The coating colour flies in the form of big drops to the coating roll and big lumps are thus transferred to the paper.
As will appear from the above, also when coating is carried out with the film transfer method at high speeds it is difficult to obtain sufficient coverage. Further, at high speeds two difficult problems relating to film press coating will emerge, namely mist-formation and drop-formation. These problems lead to both defects in quality and to poor coverage.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems of the prior art and to provide an entirely novel solution for coating of paper webs, cardboard webs and similar fibrous webs.
The present invention is based on the concept of increasing the immobilization point of the coating colour by using in the coating colour a pigment, the proportion of smallest particles of which (<0.2 &mgr;m and <0.5 &mgr;m) is approximately the same or slightly smaller than conventional pigments. Preferably less than about 10% of the particles are smaller than 0.2 &mgr;m and a maximum of 35% are smaller than 0.5 &mgr;m. According to the invention, the proportion of mid-size pigment particles having a diameter of 0.5 to 2 &mgr;m is clearly larger than for conventional pigments, typically this proportion is over 20% greater. Within the scope of the present invention, this particle size distribution is called “steep”. We have found that when the distribution is steep a good coverage and simultaneously even a better surface strength is obtainable.
The above mentioned numerical values of the particle sizes hold for spherical or approximately spherical particles measured by a Sedigraph apparatus.
The above-mentioned coating mixture is used in particular for film transfer coating at high speed which exceed 1450 m/min, when aiming at small coating amounts.
More specifically, the process according to the present invention is mainly characterized by what is stated in the characterizing part of claim
1
.
The invention will provide considerable advantages. Thus, by means of the invention a product can be obtained, having excellent surface properties, excellent coverage and still good structural and optical properties. It is essential for the invention that the coating pigment which has a steep particle size distribution yield a coating colour, a paste, which immobilizes at a much lower dry matter content than traditional mixtures. In this way it becomes possible to control the aimed coating amounts at high speed without any runability and quality problems. In particular, it is possible to avoid the problems appearing during film press coating at high speeds; the coating colour immobilizes so rapidly that film splitting takes place controllably without mist-forming. Since the immobilization point can be raise without increasing the dry matter content, no drop formation occurs.
In the following the invention will be discussed more closely with the aid of a detailed description and a number of working examples.
BRIEF DESCRIPTION OF THE FIGURES
The attached drawings depict the normal and steep particle size distributions.
FIG. 1
depicts the normal and steep particle size distribution of gypsum.
FIG. 2
depicts the normal and steep particle size distribution of carbonate.
FIG. 3
shows the cumulative particle size distribution determined by laser diffraction for the carbonates 1 to 3 used in Example 3.
DETAILED DESCRIPTION OF THE INVENTION
Within the scope of the present invention, the term web stands for a material comprising paper or cardboard a corresponding cellulosic substance, which is derived from wood or annual or perennial plants. Said material can be wood-free or wood-containing and it can be prepared from mechanical, semimechanical (chemimechanical) or chemical pulp. The chemical pulp can be bleached or unbleached. The material can also comprise recycled fibers, in particular reclaimed paper or cardboard. According to a particularly preferred embodiment the web is produced from a mixture of a mechanical pulp and a chemical pulp, the proportion of the mechanical pulp being 80 to 30%. This mixture may comprise pulp produced from hardwood or softwood by mechanical defibering methods, such as GW, PGW, TMP or CTMP pulp. The raw material used can be spruce. A preferred product is obtained by coating a base paper produced from a mixture of chemical pulp and a mechanical pulp of aspen or another wood species of the Populus family. Examples of wood species of the Populus family are
P. tremula, P. tremuloides, P balsamea, P. balsamifera, P. trichocarpa
ja
P. heterophylla
. Aspen (trembling aspen,
P. tremula
; Canadian aspen
P. tremuloides
), and aspen varieties known as hybride aspens produced from different base aspens by hybridizing as well as other species produced by recombinant technology, and poplar are considered particularly advantageous. The chemical pulp can be produced by any suitable method from hardwood or softwood, in particular from softwood. The thickness of the material web is typically in the range of 30 to 250 g/m
2
, preferably it is about 30 to 100 g/m
2
when coated paper is produced.
A preferred embodiment of the invention comprises coating a base paper manufactured from mechanical spruce pulp and chemical softwood pulp in order to produce LWC paper and coating a base paper manufactured from mechanical aspen pulp and chemical softwood pulp in order to produce fine papers.
The coating colours according to the invention can be used for single coating and as so called pre-coat and surface-coat colours. Preferably the material is double-coated, first with a precoating and then with a surface coati
Leskelä Markku
Nygård Stina
Pitkänen Maija
Kubovcik & Kubovcik
M-Real Oyj
Walls Dionne A.
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