BACKGROUND OF THE INVENTION

This invention relates to a process for the production of a fibrous sheet material mainly consisting of cellulosic fibres.

Industrial paper-making, based on the wet process has been in commercial use for more than a hundred years.

The industrialisation and commercialisation of air-laid paper have been practiced for ten to twenty years and although this process is on its way to replacing the wet process in the production of certain soft products, there are drawbacks which have to be eliminated before all paper products can be commercially produced by the air-laying process.

Using the air laying process fibres are laid down dry, or practically dry, on the wire, e.g. by using the apparatus described in US patent No. 3,575,749 and in patents in other countries.

It is a fact that only very few types of fibrous products (mostly the softer varieties and

Formica products) are made industrially using the air-laying process.

The air-laying process has advantages over the wet-laying process regarding energy, space for equipment, pollution, manpower and plant cost, and it would be extremely profitable for the paper industry if the two processes could be combined.

The wet process normally requires the use of 100 times more water than fibres. The dry process has the disadvantage that it does not produce hydrogen bonds, which is a necessity for the manufacture of most types of paper. Sometimes expensive latex or heat activatable fibres must be used to compensate for the lack of hydrogen bonds.

To produce one ton of paper using the traditional wet process, one has to use 100,000

litres of water which predominantly serves to carry the fibres to the wire. However, this vast amount of water is not necessary for the creation of hydrogen bonds in the paper product. Out of the said 100,000 litres of water, about 99 % is only used for a fraction of a second on the wire, and is then sucked away, or pressed out from the fibrous layer using heavy and energy-consuming machinery.

Using the wet process it is impossible to make a 1-ply product of an acceptable quality - e.g. for use as toilet paper and facial tissues - because there is too much hydrogen bonding for such products, with the result that the paper is too stiff. Therefore when producing soft products using the wet process, it is necessary to produce 2 or 3 very thin plies of the fibrous products and to combine these plies to a final product by operating separate equipment. In other words, to make 1000 metres of a 2 or 3 ply paper product, like facial tissues or toilet paper etc. , it is necessary to produce 2 or 3000 metres of a 1-ply product, before the layers are combined.

The process of the invention comprises depositing a layer of defibrated cellulosic material onto a continuously moving support, adding to said layer an aqueous medium in an amount of from 0.3 to 1 part by weight of water per part by weight of the defibrated material causing hydrogen bonds (partly or totally) to be established within the fibrous layer, and processing the fibrous layer to form the desired fibrous sheet material.

DESCRIPTION OF THE DRAWING

In the drawing, Fig. 1 is a side view of the apparatus according to the present invention.

A particularly preferred embodiment of the invention is suitable for use in connection with a conventional wet-laying paper machine and the invention will now be described in further detail with reference to such a machine. It should be understood however, that the advantages obtained by the process of the invention can also be achieved by using an apparatus specifically designed for the production of fibrous sheet materials by said process.

By using the process of the invention in connection with a conventional wet-laying paper machine the head-box, or other wet-laying equipment normally used for uniformly distributing the fibres on the forming wire, is bypassed, closed down or replaced by an air-laying head, preferably placed in such a manner that there is suction underneath the wire. This suction might be obtained through the use of suction equipment normally employed in the wet process, or another simpler vacuum installation 4. The defibrated fibres can be distributed in a dry state or almost dry state (by means of said dry-forming distributor which thus functions as an "air-laying head- box").

The aqueous solution or water is preferably supplied to the fibrous layer with the vacuum still maintained under the wire. This suction maintains a subatmospheric pressure in a zone below this supply zone to improve the penetration of the aqueous solution into the fibrous layer. The amount of water to be supplied to the dry-laid paper is determined by taking into account the amount of water necessary for the required degree of hydrogen bonding. According to the invention, only a small fraction of the quantity of water normally necessary for the wet-laying process is added to the air-laid fibrous layer on the wire, to control the degree of hydrogen bonding. Thus, a fibrous felt is obtained suitable for use in the manufacture of practically all paper products including soft products, writing and printing paper or board products.

Hydrogen bond is produced by adding less than one part water per part fibre.

The aqueous solution is preferably added in an amount sufficient to obtain a water content in the fibrous layer of from about 0.4 to about 0.6 and more preferably for many products about 0.5 parts by weight of water per part by weight of fibrous material. The amount of water added to the dry-laid paper is thus far below the vast amounts used in a wet-laying process.

When the fibres have been distributed on the wire by the air-laying process and a certain amount of water has been added, the paper-felt is normally passed onto the

normal finishing equipment generally used in the wet paper industry. For instance, it is advantageous in the making of soft products to subject the fibrous material to a creping operation on a creping cylinder. Thus it is possible to obtain a great saving in the use of binders, because the process takes advantage of the hydrogen bonding without creating excessive hydrogen bonding. Thus paper (even with a weight per m2 of 30-100g) can be made as soft as required by controlling the amount of water supplied after the fibres are air-laid. On the other hand, it is possible to prepare harder products, such as writing paper, by using calender rolls in the conventional manner.

The dry-laid fibrous layer has a three dimensional structure whereas a fibrous layer prepared by wet-laying mostly is orientated two-dimensionally. When substituting the wet-laying head-box by a dry-laying head-box, the fibres become more entangled in one another, and when hydrogen-bonding is created, or a binder is added, this en¬ tanglement is maintained so as to form a product having an improved cross machine strength.

The invention also relates to an apparatus for carrying out the process described above.

This apparatus comprises a fibre distributor 1 mounted above a continously movable permeable support 2 with supporting rollers 3, so as to form a layer 7 of a defibrated cellulosic material on said movable permeable support 2. The apparatus also comprises means 5 for adding an aqueous solution to said layer in an amount of 0.3 to 1.0 part by weight of water per part by weight of the defibrated material.

In a preferred embodiment of the invention, a dry-laying fibre distributor 1 is mounted above the foraminous wire of a paper machine based on wet-laying, whereas the finishing of the paper product 7 takes place by conventional creping cylinder 6.

Thus it is possible to transform existing wet-forming plants making various paper products, (tissue products, board and normal writing paper etc.), just by placing an air-laying head-box above the wire where the wet-forming normally begins, and to use

such plants for production of their normal range of products.

By saving up to 99.75 % of the water used in wet-laying, it is possible to considerably reduce the amount of energy normally used for the fibre distribution and for the removal of the huge quantity of water from a wet-laid fibrous layer and also improve the quality of the products.