Paper Manufacturing Process

Description

The present invention relates to a method for manufacturing a web comprising wood-based fibres and man-made fibres.

To improve the strength and/or dimensional stability of webs made of wood-based fibres, it is possible to admix man-made fibres, i.e. synthetic fibres or regenerated fibres, with wood-based fibres. While the wood-based fibres used have an average fibre length of 1.2 mm, the fibre length of man-made fibres used is usually in the order of 5 mm. In the manufacture of the fibre, problems are caused by different fibre lengths and also various other fibre properties. One problem is the flocculation of the fibres in the headbox, resulting in poor formation of the web. Long fibres adhere to each other, and they also capture short fibres in the fibre bundles of the long fibres.

The flocculation tendency can be alleviated by reducing the consistency of the pulp, but the production parameters may thus become such that the production is not possible with conventional machinery and/or the machine speeds must be dropped so low that the production is not profitable. Attempts have also been made to manufacture multi-layered paper, wherein diluted pulps can be run at a relatively high speed. Furthermore, paper qualities of similar types have been shifted to be manufactured by wet web formation with machinery for non-woven fabrics. To sum up, it has been problematic to run webs comprising man-made fibres in paper machines, and attempts have been made to solve this problem for about 30 years.

The method according to the invention is characterized in that a polymer is added to a fibre suspension comprising wood-based fibres and man-made fibres, to improve the web formation and to slow down the removal of water in the wire section of the paper machine.

The above-mentioned problems can be reduced or eliminated by using a polymer mixed in the pulp to improve the web formation and to slow down the removal of water in the wire section of the paper machine. The web can be manufactured by utilizing a normal paper machine construction, because there is no need to lower the consistency of the pulp, and thus the size of the slice is sufficient for dosing the pulp onto the wire. Typically, the consistency is about 1% and the normal running speed is 800 m/min.

The improvement in the web formation is manifested by the smoothness of the web. As the polymer slows down the exit of water, mechanical means can be used to have an effect on the placement of the fibres in the web. The mechanical means include, for example, the intrinsic improvement in the web formation during dewatering, due to the nature of the dewatering, because the fibres tend to move to locations with lower flow resistance and shear forces due to the speed difference between the headbox discharge and the wire. Other possible improving mechanisms include shearing forces generated by shaking of the wire and/or oscillation of the surface of the pulp layer. Further mechanical means include the effect of suction boxes underneath the forming wire, or it is possible to use a roll on the upper surface of the forming wire to prevent the formation of fibre bundles.

The polymer to be used in the method according to the invention has a high molecular weight and is relatively unbranched. The polymer is preferably anionic.

Suitable polymers to be used include, for example, anionic polyacrylamide, CMC, guargum, or compounds of these. An anionic polyacrylamide suitable for the purpose has a molecular weight of at least 10,000,000 g/mol and an electric charge density of about 30%, and it is dosed 0.2 to 2 kg per ton of fibre. CMC has a molecular weight of at least 750,000 g/mol, and it is dosed 2 to 8 kg per ton of fibre. During the process, the dosage of the polymer can be reduced, because the polymer hardly adheres to the fibre and is thus returned to use.

The pH value, at which the method works, is neutral or alkaline. The pH of the fibre suspension is adjusted to at least 6.5, for the polymer to function in the process. On the other hand, the polymer does not function at too high a pH value either. The polymer is added either into the pulp mixing tank, before the machine screen, or after the machine screen.

The method according to the invention is used primarily in the manufacture of wallpaper, particularly in connection with so-called paste-the-wall wallpapers (the adhesive agent is applied onto the wall to be papered and the dry wallpaper is placed onto the adhesive). In wallpaper, the content of man-made fibres with a typical fibre length of 5 mm is often 10 weight-%, but in a wallpaper made by the method according to the invention, the content may be 15 to 50 weight-%. Normally, the man-made fibre to be used is polyester, because it has a density suitable for manufacturing with pulp fibres. In the manufacture of wallpaper, the method is cost-efficient, as the paper can be made according to the conventional paper machine concept.

In addition to wallpaper, the method according to the invention can be applied in products which are presently made by the wet web formation technique, such as filter papers, dust bags for vacuum cleaners, wet-strength towels, bed sheets and linen papers.

In the following, the invention will be described with reference to an example and a figure. The figure illustrates the formation (Ambertec formation) as a function of the consistency in the headbox.

An experiment was made to find out the effect of polyacrylamide and CMC on the formation at different consistencies. The reference was a sample to which no polymer had been added to improve the formation and to slow down the dewatering.

The dosage of anionic polyacrylamide was 1 to 2 kg per ton of fibre. The dosage of CMC was 5 to 10 kg per ton of fibre. The pH of the fibre suspension in connection with acrylamide was from 7 to 8. The pH of the fibre suspension in connection with CMC was from 7 to 8.

The invention is not restricted to the description above, but it may vary within the scope of the claims.