Method of producing a fibrous web

The present invention relates to a method of producing a fibrous web, comprising a filler, according to the preamble of Claim 1.

In a method such as this, a filler is added to a slush comprising vegetable fibres, after which the fibre slush forms a fibre web, which is dried at the paper or cardboard machine.

The present invention also relates to use according to Claim 14.

FI Patent Specification 100729 describes a filler which is used in paper-making and which comprises porous aggregates formed of calcium carbonate particles which have been precipitated onto the surface of the fines. According to the patent specification, characteristic of this new filler is that the calcium carbonate has been precipitated onto fines fibrils which are prepared by a process of refining from cellulose fibres and/or mechanical pulp fibre. The size distribution of the fines fraction is in most cases equivalent to the fraction P100 of the wire screen. In the following, this filler is also called by its product name “SuperFill”.

On the basis of the said patent specification, the calcium carbonate percentage of the paper can be increased by using the filler, in which case the grammage of the paper can be decreased. In product groups which have a relatively high fresh filler percentage, the presented product gives a considerable advantage.

In the context of this invention “Graphic papers” mean types of paper that are used for instance for posters, book-covers, advertisements, maps, illustrated books, and as archival papers. In other words, they are used for purposes where the smoothness, the gloss and the quality of the colour are important properties.

In coated graphic papers, the amount of the pigments in the broke is large, which limits the amount of the fresh filler, among others composite fillers such as SuperFill, too. Consequently, in these cases it is not possible to fully utilise the ability of, for instance, SuperFill to improve the light-scattering properties of the paper.

The purpose of the present invention is to eliminate the disadvantages associated with the known art and to generate a completely new way of producing paper and similar fibre products. In particular, the purpose of the present invention is to generate a method which can be used to produce fibre products possessing good mechanical and graphical properties. Another purpose of the present invention is to generate a solution for increasing the light-scattering coefficient of paper at low fresh filler percentages.

The present invention is based on the idea that the uniformity of the filler distribution is increased by dosing dispersant into the composite filler, in which case the agglomeration of the calcium carbonate crystals is reduced and thus the light-scattering is increased.

In filler applications, dispersants have earlier been used mainly to improve the pumpability of the filler slurries. The literature mentions that dispersant can be used to increase the uniformity of the filler distribution (Weigl, J. and Ritter, E., “Die Bedeutung der Teilchengrösse und-form von Füllerstoffen sowie deren Verteilungen bei der SC-Papierherstellung, Wochenblatt für Papierfabrikation, 1995, no 17, pages 739-747.)

The properties of the composite fillers, in turn, have been modified by using several pigments. Accordingly, EP Published Patent Application 0 892 019 describes how CaCO₃, or another white pigment which turns insoluble, is precipitated onto the fine fibres (fibrils). After that, a further pigment is added in order that the precipitated pigment will bind the second pigment and the fine fibre together, with the result that a composite pigment is generated, one which binds better to paper than does the expensive pigment (TiO₂) by itself.

When dispersants are used together with conventional fillers, much of the retentive property of the filler is lost. Unexpectedly, it has now been found that this is not the case when acting according to the present invention. The reason is that a composite filler, such as SuperFill, can be retained on the paper (or corresponding fibrous web) also when it has been dispersed. In this respect, too, it seems to clearly differ from conventional fillers.

Consequently, according to the present invention, a composite filler is used as a filler in papers and cardboards in order to improve the light-scattering properties of the fibrous web. This filler is brought into contact with the dispersant before it is mixed with the fibre slush. More specifically, the fibrous web (for instance paper or cardboard), according to the present invention, is mainly characterized by what is stated in the characterization part of Claim 1.

The use according to the present invention is, in turn, characterized by what is stated in Claim 14.

Considerable advantages are achieved with the invention. Thus, the present invention can considerably improve the light-scattering properties of paper and cardboard, without decreasing the retentive properties of the filler. According to the present invention, it is possible to have the filler uniformly distributed in all three directions, whereas using the dispersant in the solution described above (where use of filler in particulate form was discussed) has mainly led to improvement of the distribution in the z-direction.

The solution presented here gives improved light-scattering properties particularly in those qualities of papers in which the percentage of the filler, particularly the fresh filler, is relatively small. Products of this kind are, among others, the above-mentioned graphic papers: poster papers, book covers and paper and cardboard used in advertisements, map paper, paper for illustrated books and archival paper.