Background art Paper industry has only in recent years realised the possibility of providing kraft paper, such as for sacks, with embossed patterns, for example in order to increase the friction between filled sacks which are piled. An example of such an embossed wrapping paper is described in WO 90/14998. In this publication a preferred procedure for embossing is described, wherein the kraft paper is embossed using a drying cylinder. The result, however, is not found to be quite satisfactory, and further improvements are still left to be made.

In WO 99/02772 a method for making kraft paper is disclosed. According to a preferred embodiment, the paper web is subjected to embossing, with the aid of an embossing wire. This method is not found to be satisfactory either, since the wire leaves little opportunities for flexibility.

In the manufacturing process of kraft and sack kraft paper, it is desirable to carry out the converting, i. e. the permanent changing of the properties of the kraft paper, at higher machine speeds, in order for the production to become more effective and less time consuming.

Summarv of the Invention The object of the present invention is to improve the effect of embossments on kraft paper and to obtain a

more flexible approach to the manufacturing of kraft paper. This is accomplished by a kraft paper and method according to the accompanied claims.

It has now unexpectedly been found that patterns can successfully be embossed on kraft paper, even though the paper, rather than being moist, is substantially dry. The expression"substantially dry"is herein meant to be understood as a dry solids content of at least 80%.

Traditionally, the embossing on kraft paper is carried out at a dry solids content of 60-80% during the drying step. By providing the embossments after the drying step, it has satisfactorily been found that the embossments keep their shape to a very high extent. When, contrarily, a moist paper is drawn with increased force against e. g. a drying cylinder, there is a risk that the embossed pattern can be negatively affected, such as smoothed down. Also when the paper web is subjected to drying, it has a tendency to shrink, thus changing the shape of the embossments. By allowing the paper web to pass the drying section of the paper machine first, there will be less tendency for the embossed pattern to be smoothed down.

Preferably, the embossing process is carried out by means of embossing rolls. This provides a great advantage over the conventional embossing wires, in that practically any pattern can be achieved. Embossing wires have unfortunately been limited to a very small range of possible patterns.

It has unexpectedly been found that it is possible to reduce the grammage of kraft paper from 80 g/m2 to 60 g/m2 with maintained stiffness if it is provided with a well defined embossed pattern. In particular, tests have shown that a kraft paper of 60 g/m2 embossed with a pattern described in the present disclosure, has the stiffness of a not embossed kraft paper of at least 70 g/m2, suitably 80 g/m2. Such a pattern, which provides remarkable stiffening characteristics to a kraft paper,

would not have been possible to achieve with a conventional embossing wire.

Hence, this embossed kraft paper allows the converting to be carried out at higher machine speeds, and also the converter to operate with lower grammage.

Not only does this lower grammage achieve a more favourable working condition for the converter, but it also achieves a much desired and considerable reduction of packing weight and costs.

Thus, according to one aspect of the present invention a method for embossing kraft paper is provided.

The method involves allowing the paper web to pass the drying section of the paper machine, so that the paper is substantially dry. The paper is then embossed in this substantially dry condition.

Preferably, the method involves providing such a pattern of embossments that stiffens the kraft paper.

According to an advantageous aspect of the invention there is provided a possibility of varying the stiffness of the kraft paper in different directions.

Yet another aspect of the invention is the possibility of producing a kraft paper having reduced grammage, while maintaining the stiffness of a typical kraft paper with higher grammage.

According to another aspect of the present invention a kraft paper is provided with a pattern of embossments.

The pattern comprises embossments running in two directions substantially transverse relative one another, in such a way that selective stiffness variations are obtained and/or possibility of changing grammage is achieved.

According to a preferred embodiment of the present invention, the embossments are arranged in such a manner that any straight line drawn across the patterned surface must cross embossments.

According to another embodiment of the present invention the embossments running in a first direction

are each"enclosed"or surrounded by four embossments running essentially transversely thereto. In turn, these latter embossments are"enclosed"by four embossments running in said first direction. This means that a pattern of embossments running alternately in said first and said transverse direction is achieved across the entire embossed area of the kraft paper.

According to a preferred embodiment of the present invention embossments running in the same direction are arranged in parallel, but off-set or staggered, rows of embossments. Accordingly, embossments in a specific row are aligned in a generally straight line.

According to another embodiment of the present invention the length of an individual embossment in a row of embossments is greater than the space between two consecutive embossments in such a row.

According to a further aspect of the present invention, during manufacturing of the kraft paper, a kraft paper web is conveyed in a machine or longitudinal direction. Perpendicular to this direction is the cross direction. Preferably, the web is provided with first embossments and second embossments being oriented obliquely to said machine direction and said cross direction. More preferably, said first and second embossments are oriented +45° and-45°, respectively, relative to the machine direction. Suitably, the embossments are arranged symmetrically.

It has been found that suitable dimensions of the individual embossments are: length 3-10 mm, width 0.5-1.5 mm, height 0.05-0.3 mm, suitably 0.1-0.3 mm.

Preferably, the embossments cover about 30% of the overall paper surface.

The result of the pattern is that it stiffens the paper web in its x-y-plane, that is the plane defined by the machine (longitudinal) direction and the cross direction. The embossing depth in the z-direction and the chosen pattern determines the stiffening effect.

Also, different stiffening effects are attained by proper manipulation of embossments. For instance, one particular direction may be given a somewhat higher stiffness then other directions, by providing embossments that are displaced or off-centred relative to each other.

It would also be possible to apply, at a certain angle, a rotated embossment pattern in order to change stiffening effects.

During the manufacturing process the kraft paper web may be hot or cold. If the kraft paper is embossed immediately after having left the drying section of the paper machine, it is still hot and flexible to work on.

However, it has been found that a good effect is also achieved if the embossing occurs at a later stage, such as in a separate machine off-line or at converting in a sack manufacturing factory. There is naturally an advantage of being able to freely choose the location of the embossment process, rather than being forced to carry out that particular operation at the paper machine.

A kraft paper having a grammage of 70 g/m2 and 0,1 mm high embossments, has been found to provide an increased stiffness of at least 35% compared with a non-embossed kraft paper. However, it has be found that the stiffening effect can be up to 100%, or even higher, if for instance the embossment height or depth is substantially twice the paper thickness, and the kraft paper can have a grammage of up to 150 g/m2.

Preferably, the paper web is provided with the pattern by embossing it in a nip of a pair of rolls. The rolls may be hard or soft, rubber or steel, and may comprise a"male"roll and a"female"roll. These may be in combination with a soft roll of rubber or paper. The desired embossing pattern in the roll or rolls can for instance be accomplished by etching, or in any other suitable manner.

These and other advantages and features of the present invention will become apparent in the following

description of preferred embodiments, with reference to the accompanied drawings.

Brief description of the drawings Figure 1 shows schematically a preferred embossing pattern in accordance with the present invention.

Figure 2 shows an enlarged view of an embossed area of figure 1.

Figure 3 shows schematically an area of an embossment pattern according to a preferred embodiment in which characteristics in a certain direction are prioritised.

Figure 4 shows schematically a preferred arrangement for providing a kraft paper with a pattern of embossments, in accordance with the present invention.

Detailed description of preferred embodiments of the Invention Figure 1 shows schematically a preferred embossing pattern 10 in accordance with the present invention. A kraft paper having a paper surface lying in the x-y-plane is provided with a first and a second set of embossments 12,14. The x-direction and y-direction would in the manufacturing process correspond to the machine direction and the cross direction, respectively. The embossments 12,14 are represented as lines and each embossment is limited by two ends 16,18 that can be characterised as a low x-value end 16 and a high x-value end 18, respectively. Embossments 12 of the first set extend in a first diagonal direction in the x-y-plane (from low x, low y to high x, high y), whereas embossments 14 of the second set extend in a second diagonal direction in said plane (from low x, high y to high x, low y). Thus, the second diagonal direction is perpendicular to the first diagonal direction. As can be seen from the figure, the individual embossments do not contact or cross each

other, but form a pattern 10, which complies with the following condition: a > 0 b > 0 wherein a is the minimum distance in the x-direction between the low x-value end 16 of an embossment 12 from the first set and the high x-value end 18 of an embossment 14 from the second set, and b is the minimum distance in the y-direction between the low x-value end 16 of an embossment 12 from the first set and the low x-value 16 end of an embossment 14 from the second set.

Figure 2 shows an enlarged view of an embossed area of figure 1. As can be seen, the embossments 12,14 consist of elongated ridges with rounded ends. Rounded ends are chosen so as to avoid sharp corners which can damage, or cut off, the paper during the embossing process. Typically the dimensions of the beams are as follows: length 3 mm, width 1 mm, height 0.2 mm. The distance between adjacent beams running in the same direction is 5 mm, whereas the distance between the end of a beam running in one direction and a beam running in the transverse direction is 1 mm. The junction surface between the pattern surface and the surrounding paper surface can be substantially perpendicular to the paper plane. The"perpendicularity"depends on the embossing depth. The embossed pattern will be more distinct if said junction surface is substantially perpendicular.

Figure 3 shows schematically an area of an embossment pattern 30 according to a preferred embodiment in which characteristics in a certain direction are prioritised. In this embodiment the embossments 32,34 still have the basic configuration of the previously shown embodiment, however the ends 36 of embossments 32 of one set are off-centred with respect to embossments 34

of the other set, and vice versa. In other words, the embossment ends 36,38 of the two sets are arranged closer to each other. This particular configuration shown in Figure 3 promotes increased stiffness in the y-direction.

Figure 4 shows schematically a preferred arrangement 40 for providing a kraft paper with a pattern of embossments, in accordance with the present invention. In this arrangement 40, after having been formed, the kraft paper web 42 (shown by the dashed line) is conveyed by conveying rolls 44 to, and through, a drying portion 46 comprising drying rolls. As the web exits the drying portion 46 the dry solids content should be at least 80%.

The web is passed on by a guiding roll 48 to a pair of embossing rolls 50, which is where the kraft paper web is provided with a pattern of embossments. Finally, the web is passed through conventional means and is reeled up on a reeling drum 52.

Although the embossing rolls 50 in the arrangement 40 of Figure 4 are shown as located in proximity to the drying portion 46, the person skilled in the art appreciates that other locations are also possible, such as for instance in a separate machine to which the reeled up web is brought.

Thus, it is to be understood that numerous modifications and variations can be made without departing from the scope of the present invention defined in the accompanied claims.