Process for producing tissue paper

This is a continuation of PCT application No. PCT/EP2007/010165, entitled “METHOD FOR THE PRODUCTION OF TISSUE PAPER”, filed Nov. 23, 2007, which is incorporated herein by reference.

1. Field of the Invention

The invention relates to a process for producing a tissue web, which is produced from a stock suspension including fibers.

The invention also relates to a process for producing a stock suspension for use in particular for the production of tissue webs.

2. Description of the Related Art

Tissue products are currently mainly produced from fully cellulosic materials, in particular kraft pulps.

Mechanically produced fibrous materials find only limited use, since here the tendency to yellowing and the poor strength properties of the stocks prevent widespread use.

Common mixture ratios between long fiber and short fiber stocks lie in the region of 50:50.

The porosity and the permeability of the tissue paper are determined critically by the freeness of the fibers in the stock suspension from which the tissue paper is produced.

Here, a high freeness necessitates a high content of fines in the suspension, which leads to lower porosity and permeability.

Furthermore, a high freeness causes a high water retention value for the fibers of the stock suspension, which means that the tissue paper is difficult to dewater during its production.

At high machine speeds, the poor dewatering ability often results in too low a dryness during production.

For instance, before the Yankee drying cylinder a certain dryness is needed in order to prevent lifting of the tissue paper web as a result of its contact with the hot circumferential surface of the tissue drying cylinder.

In addition, the tissue must be tear-resistant. The tearing strength is determined both by the production process and by the freeness of the fibers. In order to increase the tearing strength, the tissue paper must be consolidated during its production. In order to obtain a high tearing strength, the proportion of fines must also be high.

The requirements on the tearing strength thus contradict the requirements on the water absorption capacity, the absorbency and the dewatering ability.

What is needed in the art is the production of tissue paper with a high specific volume, as high a tearing length as possible, with the lowest possible freeness.

The present invention provides that the stock suspension contains lignocellulosic fibrous material made of wood or annual plants which has a tearing length of more than 6.0 km at 12°SR (°SR being the Schopper Riegler value) or a tearing length of more than 7.5 km at 15°SR and a lignin content of at least 15%, based on the oven-dry fibrous material, for coniferous wood in the unbleached state, or a tearing length of more than 4.5 km at 20°SR and a lignin content of at least 12%, based on the oven-dry fibrous material, for deciduous wood in the unbleached state, or a tearing length of more than 3.5 km at 20°SR and a lignin content of at least 10%, based on the oven-dry fibrous material, for annual plants in the unbleached state.

The fibers already exhibit high strength values at a freeness which is far lower as compared with fibers used hitherto. The fibrous material according to the invention is already capable of forming good bonds with neighboring fibers at a lower freeness and therefore also with a lower expenditure of refining energy.