Corrugated cardboard is known to have numerous applications. By altering the number and thickness of layers as well as the shape of corrugation of the corrugated layer, it is possible to achieve corrugated cardboard with very varying properties and therefore suitable for very varying applications, although particularly well suited corrugated cardboard is as material for various packaging containers. When corrugated cardboard is used as a packaging material, its primary purpose is to protect the packaged products against external impact and thus provide mechanical protection to the product to be packaged.

Summary of the invention The intention of the present invention is to provide a corrugated cardboard in which the properties of corrugated cardboard have been successfully expanded in a simple and effective manner. This is achieved in a corrugated cardboard according to the invention by having a layer formed by dry laying as at least one of its layers. This

layer formed by dry laying can be either the corrugated layer or equally the smooth layer of corrugated cardboard. Because of its manufacturing technique, a layer formed by dry laying can be given various properties different from the properties of conventional corrugated cardboard. Thus, a layer formed by dry laying can contain fibres or particles absorbing liquid or odours. The fibres of a layer formed by dry laying can also be dyed to give a desired tone of colour to the layer in question. A layer formed by dry laying can also contain fibres or particles emitting odours.

Furthermore, a layer formed by dry laying can be made to contain fibres or particles conducting electricity or insulating from electricity. Thus the properties of a layer formed by dry laying can be very widely varied according to the desired application in each case.

A corrugated cardboard according to the invention can further contain at least one barrier layer on at least one surface of at least one cardboard layer. The barrier layer is typically a gas-and/or liquid tight thermoplastic layer attached to the cardboard layer either by extrusion or lamination.

List of figures In the following, a corrugated cardboard according to the invention is described in more detail with reference to the attached drawing in which

Fig. 1 shows a first example embodiment of a corrugated cardboard according to the invention, and Fig. 2 shows a second example embodiment of a corrugated cardboard according to the invention.

Detailed description of the invention In Fig. 1 of the attached drawing, a first example embodiment of a corrugated cardboard according to the invention is shown. It contains a smooth cardboard layer 1 and, attached to it conventionally by glueing, a corrugated layer 2. This corrugated layer 2 is produced by dry laying, i. e., by so-called dry-forming technique. This means that a web is formed on the wire of a dry laying machine with the help of one or several forming boxes, i. e. formers. Into the formers reaching transversely across the entire width of the wire, a mixture of air and fibre is blown which is mixed and screened to form an even material layer on the wire moving underneath according to the known technique. There can be as many formers placed above the wire as the desired layer thickness requires, when one layer after another is formed on the same production line until the desired thickness is achieved. From different formers different fibre mixtures can be blown to the web to provide the web with the desired properties.

In a corrugated cardboard according to the invention, the layer formed by dry laying

can be composed of primarily wood fibres which are preferably relatively long-fibre mechanical pulp and of plastic fibres which can be of any suitable thermobonding quality. After the forming step, the web is bonded with heat in, for instance, a flow- through oven.

The properties of a layer achieved by dry laying used in a corrugated cardboard according to the invention can vary significantly according to the intended application of the corrugated cardboard in each case. In the embodiment according to Fig. 1 in which the layer formed by dry laying is corrugated, the layer formed by dry laying must, as a rule, be relatively rigid. This is achieved by appropriately selecting the web fibres and the fibre composition as well as the bonding agent, which can be either fibrous or liquid. A relatively rigid web is required to allow the web to be corrugated into a corrugated web 2 shown in Fig. 1 so that it maintains this corrugated shape after the corrugation. In corrugation, the originally relatively rigid web softens preferably in other respects as well making it very suitable for use in various packaging and softening purposes.

As an example, a layer formed by dry laying suitable for forming the corrugated layer of a corrugated cardboard according to the invention can be shown to have the following properties: basis weight 70 g/m2, thickness when four layers 2.6 mm, dry tensile strength in machine direction 0.40 kN/m and in cross direction 0.32 kN/m,

dry elongation in machine direction 10 % and in cross direction 8 %, wet tensile strength in machine direction 0.20 kN/m and in cross direction 0.18 kN/m, and absorption 520 g/m2. The rigidity of this type of a layer defined as bending length is on average 61 mm when defined with the method EDANA 50.3-93 with which the <BR> bending of a rectangular material strip (25 1 mm x 250 1 mm) caused by its own weight to a certain angle (7.1°) is defined.

When using organic fibres, such as wood fibres, as material for a dry laid layer, the web will naturally absorb liquids efficiently and liquids are also spread in it efficiently. This property can obviously be encouraged by adding to the dry laid product other absorbent materials, such as superabsorbents either in fibrous or particle form. Similarly agents absorbing odours can be added to the web either in fibrous or particle form. Furthermore, it is possible to modify the electrical properties of the web by adding to it either fibres or particles conducting electricity or alternatively fibres or particles insulating from electricity. This way the electrical properties of a packaging material can be varied according to the requirements of the intended application in each case.

In addition to that a layer formed by dry laying can be given the different properties described above, it can also be dyed as desired by forming it from dyed fibres.

Consequently the web becomes dyed throughout and the dyeing will not affect the

web properties in other ways. Similarly the heat insulating properties of a web can be varied according to the above methods.

Figure 2 of the attached drawing shows another example embodiment of a corrugated cardboard according to the invention. Here, layer 20 of cardboard is corrugated and the smooth layer 10 attached to it is a layer formed by dry laying. In this embodiment, the layer formed by dry laying can be given the desired properties exactly in ways described above. In this embodiment, however, the dry laid layer does not need to be particularly rigid since no corrugation is required.

A corrugated cardboard according to the invention can also be made to contain various barrier layers. Preferably these barrier layers are attached to a cardboard layer which in the embodiment of Fig. 1 is marked with a reference number 1 and in the embodiment of Fig. 2 with a reference number 20. Such a barrier layer creating gas and/or liquid tightness and typically made of thermoplastic can be located on either of the surfaces of the cardboard layer or also on both of its surfaces. If there are several cardboard layers, the barrier layer can naturally be located between these layers, as well.

Above, a corrugated cardboard according to the invention is described with the help of merely two example embodiments. It should be realized that to these

embodiments various changes can be made without deviating from the scope of protection defined by the attached claims. Thus, a corrugated cardboard can contain several corrugated layers as well as several smooth layers so that the corrugated cardboard resembles, for example, conventional so-called two-sided corrugated cardboard in which the corrugated layer is located between two smooth layers.

Furthermore, as already mentioned above, either the corrugated layer or the smooth layer, when made of cardboard, can be made of one or several webs, in which case a barrier layer or several barrier layers, if such exist, can be located on any surface of any of the cardboard layers. There can also be several layers formed by dry laying so that such a layer formed by dry laying and corrugated is located on both sides of one smooth layer, or also so that on one or both free surfaces of two-sided corrugated cardboard is added a corrugated layer. More than one corrugated layers can, if desired, be added as various combinations into a corrugated cardboard according to the invention with a method known from conventional corrugated cardboard technique. Naturally, the shape of corrugation used can also be varied using the known method.