Patent Description:
Paperboards are commonly made of one ply or out of two or more plies, wherein the latter is referred to as multi-ply paperboards. Multi-ply paperboards are used in for example packaging, such as in liquid packaging board (LPB).

When a multi-ply paperboard is provided, it is important to prevent that cracks are formed to maintain appearance. Moreover, when the multi-ply paperboard is used in packaging, it is important to hinder contamination of the product contained in the package. A method for making a multi-ply paperboard is for example known from <CIT>.

Multi-ply paperboards, namely paperboards having two or more plies, are commonly used in packaging. In all boards used in packaging, and especially for LPB, it is important that the board do not crack upon folding it into a package.

Accordingly, the present disclosure provides the following listing of itemized embodiments:.

The inventors have realized that the above methods reduce the cracking tendency by producing a board having a top ply in which the tensile strength is about <NUM>-<NUM> times higher in the machine direction (MD) than in the cross direction (CD) and by the formation of a middle ply, in case of at least three plies in the board, and the formation of the back ply, in case of a two-ply board, in which the variations in local grammages is minimised.

As a first aspect of the present disclosure, there is provided a method for making a multi-ply paperboard, wherein the method comprises the following steps:.

By using a jet-to-wire-speed difference in formation of the top ply being greater than the jet-to-wire-speed difference in formation of the middle ply and back ply, the fibres in the top ply will be more oriented in the MD than the fibres in the middle ply and back ply.

As an example, the jet-to-wire speed difference is +<NUM>/min if the jet speed is <NUM>/min and the wire speed is <NUM>/min and -<NUM>/min if the jet speed is <NUM>/min and the wire speed is <NUM>/min. In each case, the jet-to-wire-speed difference is greater than when the jet speed is <NUM>/min and the wire speed is <NUM>/min or when the jet speed is <NUM>/min and the wire speed is <NUM>/min.

The combination of using a jet-to-wire speed difference in the range of +<NUM> to +<NUM>/min or -<NUM> to -<NUM>/min in the formation of the first web and oscillation at certain stroke length of the second web has been found to produce a multi-ply paperboard with minimized crack formation upon folding.

When the second web is oscillated with a stroke length in the range of <NUM>-<NUM>, the fibres in the second web are caused to be more oriented with the cross direction (CD), i.e. the direction transverse to the machine direction. Moreover, local accumulations of flocs of fibres are evened out. As a consequence, the formation is improved. The inventors have discovered that the tensile strength ratio in MD/CD obtained by the jet-to-wire speed difference for the top ply in combination with the obtained formation of the middle ply is beneficial in reduction of cracks when the paperboard is folded.

For these effects, it is not decisive if the jet is faster than the wire, i.e. a positive speed difference, or slower than the wire, i.e. a negative speed difference. The fibre-orienting effect will be obtained in either case. Nevertheless, it is preferred that the speed difference is positive since a lower fibre concentration in the headbox can be used, which is beneficial since it leads to a better formation of the plies.

Typically, the second and third webs are oscillated in the CD. Thereby, the formation is improved both in the middle ply and the back ply, which is even further beneficial for reduction of cracks when the paperboard is folded.

Typically, the second furnish is applied on the second forming wire at a second jet-to-wire speed difference in the range of:.

The third furnish is, typically, applied on the third forming wire at a third jet-to-wire speed difference of, which is in the range of:.

In theory the speed difference should be just above, or below, <NUM>, such as ±<NUM> or ±<NUM>/min to provide good formation. However, due to limitations in equipment it is challenging to maintain such small difference. At a speed difference close to <NUM> there is a risk that the speed difference at some point will be <NUM> due to limitations in the equipment and at such point the formation is impaired. The inventors have now realized that by conducting an oscillations step, the jet-to-wire speed difference can instead be not less than ±<NUM>/min. Such a speed difference together with oscillation provides a more robust system as well as satisfactory sheet formation. By more robust is meant that risk that the speed difference at some point will be <NUM> is avoided or at least minimized.

Moreover, the jet-to-wire speed difference is typically not more than ±<NUM>/min as the formation is not improved above such a speed difference. The third jet speed is, typically, closer to the speed of the third forming wire than the second jet speed to the speed of the second forming wire. Thereby the difference in speed in the formation of the second fibre web is greater than the difference in speed in the formation of the third fibre web.

Formation is a measure of how even the surface is, i.e. that there are as few fibre flocs as possible and that the fibres are evenly distributed in all directions. A low formation is beneficial for crack resistance. Formation is measured according to SCAN-P <NUM>:<NUM>. For the middle ply the formation is preferably <NUM>-<NUM> √g/m, such as <NUM>-<NUM> √g/m. For the back ply the formation is preferably <NUM>-<NUM> √g/m, such as <NUM>-<NUM> √g/m.

Typically, the wire speed of all forming wires is <NUM>-<NUM>/min, such as <NUM>-<NUM>/min. The speed of all forming wires are, typically, the same.

The jet-to-wire speed ratio is the jet speed divided by the wire speed. By using the jet-to-wire speed difference in combination with wire speed, the jet-to-wire speed ratio can be calculated. As it is preferred the speed difference is positive, it is consequently preferred that first jet-to-wire speed ratio is above <NUM>. Typically, the first jet-to-wire speed ratio is <NUM> -<NUM>, such as <NUM>-<NUM>. Typically, the second jet-to-wire speed ratio is <NUM>-<NUM>, such as <NUM>-<NUM>. Typically, the third jet-to-wire speed ratio is above <NUM>, such as <NUM>-<NUM>.

The oscillation(s) in step d) is/are, typically, carried out by (a) breast roll shaker(s). A breast roll shaker shakes the breast roll cross-directionally and breaks up fibre flocs by creating shear forces on the web. Breast roll(s) are typically located downstream and directly after the headbox in the forming section of a paper machine. A breast roll shaker shakes the breast roll.

Preferably, the stroke length in the oscillation in step d) of the second web is <NUM>-<NUM>, such as <NUM>-<NUM>. Typically, if the third web is oscillated the stroke length in the oscillation in step d) of the third web is <NUM>-<NUM>, such as, <NUM>-<NUM>. Typically, the frequency in the oscillation in step d) is <NUM>-<NUM> rpm, such as <NUM>-<NUM> rpm, for the oscillated web(s). That is, the second web is oscillated with such frequency as well as the third web, if oscillated. The inventors have realized that the stroke length has a bigger impact on the formation of the web than the frequency.

MD/CD-tensile strength ratio is an indication of the extent to which the fibres are oriented in the MD (a higher ratio corresponds to more MD-oriented fibres). In the top-ply the MD/CD tensile strength ratio is typically above or equal to <NUM>, such as above or equal to <NUM>, such as <NUM>-<NUM>. In the back ply, the MD/CD-tensile strength ratio is typically above or equal to <NUM>, such as <NUM>-<NUM>. In the middle ply, the MD/CD-tensile strength ratio is typically above or equal to <NUM>, such as <NUM>-<NUM>. Typically, the MD/CD tensile strength ratio is higher for the top ply than for the back ply, and typically the MD/CD tensile strength ratio is higher for the middle ply than for the back ply. If the MD/CD tensile strength ratio is arranged in that order it has been found beneficial for crack resistance when folding. The tensile strength is measured according to ISO <NUM>-<NUM>:<NUM>.

The grammage of the paperboard as well as individual plies is measured according to ISO <NUM>:<NUM>. The grammage of the paperboard is typically <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>. The grammage of the top ply is typically <NUM>-<NUM>/m<NUM>. The grammage of the middle ply is typically <NUM>-<NUM>/m<NUM>. The grammage of the back-ply ply is typically <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>. It is beneficial that the back ply has a grammage of <NUM>-<NUM>/m<NUM> for a paperboard comprising at least <NUM> plies and that the back ply has a grammage of <NUM>-<NUM>/m<NUM> for a paperboard comprising <NUM> plies.

The paperboard is typically coated on the top-ply. A coating may be a printing layer that facilitates printing of the board.

The second furnish typically comprises mechanical pulp, such as chemi-thermomechanical pulp (CTMP). The second furnish also typically comprises broke pulp. The second furnish also typically comprises softwood chemical pulp, such as softwood kraft pulp. The fibres of the second furnish are typically unbleached.

The fibres of the third furnish are typically unbleached.

The fibres of the first furnish are typically bleached. Since the third furnish is used to produce the top ply it is beneficial if they are bleached for printing properties. Alternatively, they are unbleached if such visual appearance is desired.

As a second aspect of the present disclosure, there is provided a method for making a two-ply paperboard, wherein the method comprises the following steps:.

The examples and embodiments discussed above in connection to the first aspect apply to the second aspect mutatis mutandis.

A full-scale board machine provided with two headboxes and two wires was used. In each headbox a furnish was provided that contained fibres. The fibres for the top ply were a mixture of softwood and hardwood bleached kraft fibres and the fibres for the bottom ply were unbleached softwood kraft fibres. Trials were conducted where each furnish was applied to each wire with individual jet speeds. Both wires were running at the same speed and in Trial <NUM> the speed was <NUM>/min. The conditions are shown in Table <NUM> below.

The bottom plies in each trial were oscillated in a direction transverse to the machine direction (MD) by a breast roll shaker. The conditions for the breast roll shaker are shown in Table <NUM> below.

A full-scale board machine provided with three headboxes and three wires was used. In each headbox a furnish was provided that contained fibres. The fibres for the top ply were a mixture of softwood and hardwood bleached kraft fibres. The fibres for the bottom ply were unbleached softwood kraft fibres. In the middle ply a mixture of CTMP, broke and unbleached softwood kraft fibres was used. The grammage of the top ply was about <NUM>/m<NUM>, the grammage of the middle ply was about <NUM>/m<NUM> and the grammage of the back ply was about <NUM>/m<NUM>. Trials were conducted where each furnish was applied to each wire with individual jet speeds. All three wires were running at the same speed in each trial. The conditions are shown in Table <NUM> below.

The bottom and middle plies in each trial were oscillated in a direction transverse to the machine direction (MD) by a breast roll shaker. The conditions for the breast roll shaker are shown in Table <NUM> below.

Pieces of about <NUM> x <NUM> from a multi-ply paperboard was cut and soaked in warm water together with detergent for <NUM> minutes. Thereafter, the plies were separated by hand and dried individually. These plies were used for measuring of formation as they surfaces remain intact from when they were formed. It is not suitable to measure mechanical properties on plies that have been split by this wet-splitting method as the plies will shrink when drying.

Therefore, for the mechanical properties the plies were splitted by a fortuna splitter instead. Fortuna splitter is a precision bandknife splitting machine that is used to separate the plies by cutting.

Tensile strength was measured according to ISO <NUM>-<NUM>:<NUM>.

Formation was measured according to SCAN-P <NUM>:<NUM>.

The MD/CD tensile strength ratio, i.e. indication on how fibres are aligned in MD and CD, respectively, is presented in the tables below. Moreover, the specific formation, i.e. how homogenous in terms of local grammage variations the individual plies were, is shown.

Table <NUM>. The MD/CD tensile strength ratio and specific formation for the two-ply paperboard.

Based on the results from the crack-resistance, it is believed that an improvement, i.e. lowering, of the specific formation for the middle and back plies, is beneficial for crack resistance. Moreover, it is believed that a relatively high MD/CD tensile strength ratio for the whole board and/or in the top ply is beneficial.

Moreover, as seen in trial <NUM> a frequency of <NUM> rpm is sufficient to provide a good formation when the stroke length is <NUM>.

Different defects in the form of cracks appear when the board is folded, for example in production of a package. It is important that such cracks are as small as possible since they may otherwise cause negative effects, e.g. that the barrier is stretched too much, cracks and the content in the package is contaminated.

To evaluate the appearance of defects, packages were produced from the boards and inspection was made for various types of cracks and defects. If the cracks and defects are small, they are within acceptable limits. The results are presented in the table <NUM> below.

Trials <NUM>-<NUM> representing inventive examples all pass the test, or as in the case of trial <NUM> is assumed to pass the test based on its properties. Trial <NUM> is a comparative example that failed. Based on the results, it is concluded that it is advantageous to combine a stroke length of above <NUM> for the middle ply (in case of a <NUM>-ply board) and for the back ply (in case of a <NUM>-ply board) with a jet-to-wire speed difference being more than +<NUM>/min for the top ply. Moreover, it is concluded that it is beneficial if the jet-to-wire speed difference is not being too close to o for the furnish producing the middle ply or, in case that it is also oscillated, the furnish producing the back-ply.

It is further concluded that, for a <NUM>-layered board, a MD/CD tensile strength ratio below <NUM> is beneficial. Contributing to the MD/CD tensile strength ratio of the whole board is the MD/CD tensile strength ratio of the individual plies. It is concluded, that for a <NUM>-layered board, it is advantageous if the MD/CD-tensile strength ratio is <NUM>-<NUM> for the back ply, and <NUM>-<NUM> for the middle ply.

It is further concluded that, for a <NUM>-layered board, the specific formation advantageously is <NUM>-<NUM> √g/m for the back ply, <NUM>-<NUM> √g/m for the middle ply and <NUM>-<NUM> √g/m for the whole board.

Claim 1:
A method for making a multi-ply paperboard, wherein the method comprises the following steps:
a) forming a first web from a first furnish comprising fibres on a first forming wire wherein the first furnish is applied on the first forming wire using a first jet speed at a first jet-to-wire speed difference, wherein the first jet-to-wire speed difference is in the range of:
+<NUM> to +<NUM>/min, such as +<NUM> to +<NUM>/min; or
-<NUM> to -<NUM>/min, such as -<NUM> to -<NUM>/min;
b) forming a second web from a second furnish comprising fibres on a second forming wire using a second jet speed, wherein the second jet speed is closer to the speed of the second forming wire than the first jet speed to the speed of the first forming wire;
c) forming a third web from a third furnish comprising fibres on a third forming wire using a third jet speed, wherein the third jet speed is closer to the speed of the third forming wire than the first jet speed to the speed of the first forming wire;
d) causing the second web to oscillate in a direction transverse to the machine direction (MD), wherein the stroke length in the oscillation of the second web is <NUM>-<NUM>; and
e) combining the first, second and third webs into the multi-ply paperboard, wherein the first web forms a top-ply, the second web forms a middle ply and the third web forms a back-ply in the multi-ply paperboard.