Irene
Katarina EK.
Bo
Ragnar
Irene
Katarina EK.
Bo
Ragnar
| 1. | A method of manufacturing soft paper from cellulose fibres, comprising wetforming of a first fibre layer, c h a r a c t e r i z e d i n that airborne dry fibres are deposited directly on the wetformed layer while this is still wet, so that a second fibre layer is formed on the first one, where¬ by fibre bindings between the layers arise. |
| 2. | A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the dryformed layer is deposited 2 to a grammage of 220 g/m . |
| 3. | A method as defined in claim 1 or 2, c h a r a c t¬ e r i z e d i n that the forming of the layers takes place at a speed of 5002000 m/min. |
| 4. | A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the wet forming of the first layer takes place on a running fourdrinier wire, and the second layer is dryformed on the first layer while it is still on the wire. |
| 5. | A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the wet formed layer has a dry solids content of 2 % at the application of the airborne dry .'fibres thereon. |
| 6. | A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that airborne dry fibres are deposited directly also on the opposite side of the wetformed layer while this is still wet, so that a third fibre layer is formed on the first one. |
This invention relates to the making of soft paper from cellulose fibres.
Soft paper is a common denomination for paper used for absorbing purposes, such as tissue, drying cloth, nap¬ kins and handkerchiefs. Soft paper competes with woven fabrics, but in contrast to them *.is intended for one-way use. The properties desired are rapid and effect¬ ive absorption, soft and smooth structure and good strength also in wet state.
Soft paper is manufactured mainly by wet forming, i.e. a fibre suspension in water is caused to flow out on a running wire where it is dewatered and then dried. High-
-speed machines with wire speeds of between 500 and 2000 m/min are used. The grammage is 20-30 g/m . For giving the paper necessary extensibility and softness, criping is carried out usually by means of a so-called Yankee cylinder where the paper web is scraped off after drying.
A small amount of soft paper is manufactured by dry- -forming, i.e. dried papermaking pulp is fluffed where¬ after air-borne fibres without addition of water are depos¬ ited on an air-pervious wire. Binding is effected by means of suitable chemical binding agents. Soft paper man¬ ufactured in this way is very bulky, i.e. has a very loose structure. The machine speed must be low, about 50 m/min. The manufacturing cost is very high, and the products have to be sold at a high price.
Dry-formed soft paper, thus, has a higher bulk than wet-formed paper. The reason of this is that the fibres in the dry-formed paper have not been softened in water and, therefore, are not bent down into the paper plane. Further¬ more, no water has to be drained through the structure and, therefore, no capillary forces contract the structure at the removal of water by drying'. At dry-forming the fibres
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deposit at random in all directions both in the plane of the -paper and perpendicularly "t_h-er to 3 -while at wet— -forming the fibres substantially deposit in the plane of the paper.
Dry-forming yields almost unbound fibres. In order to obtain necessary strength without water addition, chem¬ ical binding agents are used. This implies, that the binding level in the final product can be controlled, and no cr§ping is required. Binding agents, however, are expensive, and the amount required thereof costs as much as the fibre raw material.
The high manufacturing cost .for .dry- .αrmed so paper is the main-reason-' hy this paper has such a small share of the market.
The present invention is a combination of wet-forming and dry-forming whereby the advantages of both methods have been utilized. According to the invention, air-borne fibres are deposited directly on a wet-formed layer while the layer is still wet. Its dry solids content preferably should be 5-25 % . The dr -formed layer should not have too, great .a th ckness. Preferably every -dry- ormed fibre should meet the we -formed layer. This implies in praet-
2 ice a grammage of 2-20 g/m . Between the two layers fibre bindings arise which ensure good cohesion of the layers.
A particularly advantageous quality of soft paper is obtained by depositing air-borne άr % fibres on both sides of the -wet-formed- laye .
Owing to the invention, the manufactured soft paper has a soft and smooth surface, higher bulk than wet-formed paper and higher strength than dry-forned paper without addition of chemicals. The method, ccording to the invent¬ ion also -results in -high internal bond strength (S-strengt in spite of the absence of special binding agents. The machine speed for wet-forming, i.e. 500-2000 m/min, can be maintained.
The invention is described in greater detail in the foll¬ owing by way of an embodiment thereof, with reference to the accompanying Figure showing schematically an arr¬ angement for the manufacture of soft paper.
From a head box 1 a fibre suspension flows out onto a running wire 2 thereby forming a first fibre layer in a wet manner on the wire. Dewatering takes place through suction boxes 3 located beneath the wire 2.
Air-borne dry fibres are deposited directly on the wet- -for ed layer,while it is still wet, by means of a form¬ ing box 4 located above the wire 2 and a vacuum box 5 located beneath the wire. The dry fibres thereby form a second fibre layer on the first one. Between the layers fibre bindings arise. The dry fibres are exposed in a defibering device 6, for example a hammer mill or coarse shredder, followed by a refiner for fluffing. The fibres are transported by means of a fan 7 to the forming box H , which can be of the type shown in patent application SE 85 05 918-6. Reject discharged from the forming box 4 through a conduit 8 can be re-cycled, possibly after renewed defibering.
The wet-formed layer shall have a gram age of 10-100
2 g/m . The wet-formed layer, at:-.the application of the air-borne dry fibres, shall have been dewatered to a dry solids content of 5-25 % • The dry fibres preferably should have been taken from chemical pulp in order to yield a surface of high softness.
At the dry-forming the fibres shall be well dispersed in the air. For ensuring this, the flow rate in the inlet to the forming box shall exceed 100 m/s.
The distribution between the reject flow through the conduit 8 and the fibre flow dry-formed on the wet-formed layer shall be so that between 25 % and 100 % of incom¬ ing fibres are deposited on the wet-formed layer. hεn
the dry fibres adhere on the wet-formed layer, the flow rate should be 10 -m , and ' the ibre.coneen- trazion in the air flow should not exceed 10 % .
In the Figure wet-forming on a fourdrinier wire is shown, but alternatively the forming can be carried out by means of twin wire, in such a way that the dry fibres are deposited-when one wire has left the wet-formed lay¬ er.
At the manufacture of soft paper with dry-formed fibre layers on both sides of the wet-formed layer, dry fibres can be deposited on one side of the wet-formed layer while it is on --.the .forming wire, so: hat a second fibre layer arises. Thereafter the web thus formed is transferred to a second wire whereby dry fibres are de¬ posited also on the rear side of the wet-formed layer while it is still wet, so that a third fibre layer is formed in the same way as the second fibre layer.
The invention is not restricted to the embodiments de¬ scribed, but can be varied within the scope of the invent¬ on .idea.