WO2023212760A1

WO2023212760A1 - Kraft paper

Info

Publication number: WO2023212760A1
Application number: PCT/AT2023/060139
Authority: WO
Inventors: Reinhard Kainz; Franz Jöbstl; Paulus GOESS
Original assignee: Mondi Ag
Priority date: 2022-05-03
Filing date: 2023-04-26
Publication date: 2023-11-09
Also published as: AT526106A1; AT526106B1

Abstract

The invention relates to a kraft paper with an elongation at break in the machine direction of at least 7.5% and an elongation at break in the transverse direction of at least 7.5%, wherein the elongation at break in the machine direction and the elongation at break in the transverse direction differ by less than 2.5%, in particular by less than 1.5%, and the bending stiffness in the machine direction and the bending stiffness in the transverse direction equal at least 50 mN. The invention additionally relates to a packaging material, to a method for producing a packaging material, and to a method for producing a kraft paper.

Description

Kraft paper

The present invention relates to a kraft paper, a packaging material and a method with the features of the independent patent claims.

Packaging materials are used in various areas of modern industry. For example, in the case of shock-sensitive products, it is necessary to provide them with a cushioning material in order to provide increased product protection during transport. In the prior art, plastic bubble wrap is often used for this purpose. This is at least a two-layer plastic film, with gas-filled nubs formed between the layers, which enable a cushioning effect.

Alternatives to plastic packaging materials are required, particularly due to ecological and regulatory aspects. One possibility for this is paper-based packaging materials. However, no paper-based packaging material is known in the prior art that is at least similar to the structure and functionality of plastic bubble wrap.

An object of the present invention may therefore be seen as providing a paper material suitable for producing a cushioning packaging material. A further object of the present invention can optionally be seen in providing a paper material that can be subjected to a forming process in order to produce a three-dimensional structure, in particular bubbles, knobs and/or hollow bodies.

A still further object may be seen in providing a paper material that can be subjected to a forming process in order to achieve a structure which is at least similar in its cushioning effect to conventional bubble wrap.

In the context of the present invention, it was surprisingly found that at least one of these tasks can possibly be solved by a kraft paper with certain properties.

A kraft paper in the sense of the present invention refers in particular to a type of paper with a particularly high strength. Optionally, the pulp used to produce the kraft paper is a kraft pulp that has been produced from wood, in particular by means of the sulfate process. A preferred raw material for the production of kraft pulp for use in the present invention is softwood, in particular softwood, for example pine, spruce, fir, larch, but also other known softwoods. A mixture of one or more types of softwood can also be used.

In particular, the kraft pulp used to produce the kraft paper is unbleached or natural brown. If necessary, the kraft pulp can also be partially or completely bleached.

If necessary, the pulp used can also contain a proportion of short fiber pulp, which was obtained in particular from hardwood. Optionally, up to 15% by weight, in particular up to 10% by weight, of short fiber pulp, based on the total amount of pulp, may be included. The kraft paper of the present invention can have isotropic elongation behavior combined with high rigidity. In the context of the present invention, it was found that such a kraft paper has particularly advantageous properties for use as a packaging material or as a replacement for plastic bubble wrap.

In particular, the isotropic expansion behavior can contribute to enabling the formation of particularly uniform knobs or structures when forming the kraft paper. The high rigidity can help to maintain the integrity of the nubs or structures formed and still enable a sufficient cushioning effect.

The advantageous properties can be achieved in particular by choosing certain parameters when producing the paper. If necessary, a lower high-consistency grinding leads to a higher stiffness of the kraft paper in the transverse direction. If necessary, a lower difference in speed between the pre- and post-drying sections leads to a higher stiffness of the kraft paper in the machine direction.

A paper according to the invention can be produced by subjecting kraft pulp to a refining step. The grinding step includes high-consistency grinding and, if necessary, low-consistency grinding. If necessary, high-consistency grinding improves the extensibility in the transverse direction. The high consistency grinding may also influence the stiffness of the paper.

Auxiliary substances can be added to the ground kraft pulp, for example starch, sizing agents and/or pH regulators. In particular, no filler is added to the kraft pulp.

The kraft pulp can then be processed into a paper web in a paper machine in a manner known per se. The paper web obtained can be pre-dried in a pre-drying step. The paper web can then be subjected to creping, in particular microcreping, which may improve the stretch properties. Creping can be done in a Clupak unit in which the paper web is guided in a known manner between an elastomeric cloth and a roller.

The paper web creped in this way, in particular micro-creped, can then be subjected to a post-drying step.

If necessary, the paper web is guided at a lower speed in the post-drying step than in the pre-drying step. This difference in speed allows the stretching properties of the kraft paper to be increased, particularly in its machine direction.

If necessary, the grammage of the kraft paper is at least 90 g/m ² , in particular between 95 and 140 g/m ² , preferably about 100 g/m ² or about 130 g/m ² . If necessary, the tensile strength of the kraft paper in the machine direction is between 6 and 9 kN/m. If necessary, the tensile strength of the kraft paper in the transverse direction is between 5 and 8 kN/m. Optionally, the machine direction tensile strength index of the kraft paper is between 60 and 90 Nm/g. Optionally, the transverse tensile strength index of the kraft paper is between 50 and 80 Nm/g.

If necessary, the tensile work of rupture of the kraft paper in the machine direction is between 370 and 430 J/m ^2. If necessary, the tensile work of rupture of the kraft paper in the transverse direction is between 370 and 430 J/m ² .

If necessary, the air resistance of the kraft paper is more than 16 s Gurley, in particular more than 18 s Gurley, in particular more than 20 s Gurley.

If necessary, the tear resistance index of the kraft paper in the machine direction is between 13 and 15 mN. g/ ^m2 . If necessary, the tear resistance index of the kraft paper in the transverse direction is between 16 and 20 mN. g/ ^m2 . Optionally, the bending stiffness index of the kraft paper in the machine direction is between 60 and 90 Nm ⁶ kg' ³ , in particular between 65 and 85 Nm ⁶ kg' ³ . If necessary, the bending stiffness index of the kraft paper in the transverse direction is between 50 and 80 Nm ⁶ kg' ³ , in particular between 55 and 75 Nm ⁶ kg ^-3 . It has been found that a certain degree of rigidity is advantageous in order to obtain a good cushioning effect of cavity structures.

Where applicable, the grammage of the kraft paper is determined in accordance with the ISO 536:2019 standard. Where applicable, the tensile strength of the kraft paper is determined in accordance with ISO 1924-3:2005. Where applicable, the tensile strength index of the kraft paper is determined in accordance with ISO 1924-3:2005. If applicable, the elongation at break of the kraft paper is determined in accordance with ISO 1924-3:2005. Where applicable, the tensile strength of the kraft paper is determined in accordance with ISO 1924-3:2005.

If necessary, the bending stiffness of the kraft paper is determined in accordance with the ISO 2493-1:2010 standard. Where applicable, the air resistance of the kraft paper is determined in accordance with the ISO 5636-5:2013 standard. Where applicable, the tear resistance index of the kraft paper is determined in accordance with ISO 1974:2012.

The obtained kraft paper can be subjected to a forming step to obtain a packaging material. In particular, the kraft paper or the packaging material has a knobbed structure after forming. In connection with the present invention, the term “knob structure” refers in particular to a uniform or uneven sequence of bump-shaped elevations. If necessary, the individual knobs of the knob structure are hollow bodies that are open on one side, for example open spherical segments or cylinders that are open on one side, although other geometric shapes are also possible.

In particular, the reshaping of the kraft paper does not result in a significant change in the thickness of the kraft paper.

Optionally, the invention relates to a kraft paper with an elongation at break in the machine direction of at least 7.5% and with an elongation at break in the transverse direction of at least 7.5%, the elongation at break in the machine direction and the Elongation at break in the transverse direction differs by less than 2.5%, in particular by less than 1.5%, and the bending stiffness in the machine direction and the bending stiffness in the transverse direction are at least 50 mN.

In connection with the present invention, stated differences in elongation at break refer in particular to absolute values. For example, if a paper has a cross-direction elongation at break of 8% and a machine-direction elongation at break of 9%, the difference between the cross-direction elongation at break and the machine-direction elongation at break is 1%.

If necessary, it is provided that the bending stiffness index in the machine direction is between 60 and 90 Nm ⁶ kg ^-3 . If necessary, it is provided that the bending stiffness index in the transverse direction is at least 50 and 80 Nm ⁶ kg ^-3 .

Optionally, it is provided that the elongation at break in the machine direction and/or the elongation at break in the transverse direction is between 7.5% and 13%, preferably between 8% and 12%, more preferably between 9% and 11%.

If necessary, it is provided that it has a grammage of at least 90 g/m ² , in particular between 95 g/m ² and 140 g/m ² .

If necessary, it is provided that it has an air resistance of at least 16 s Gurley.

If necessary, it is provided that the kraft paper contains kraft pulp which is formed entirely from virgin fiber, in particular entirely from virgin fiber from softwood.

If necessary, it is provided that the kraft paper contains kraft pulp that has a kappa number according to ISO 302:2015 between 30 and 50, in particular between 38 and 47. If necessary, it is provided that the kraft paper contains cationic starch in a content of between 1.2% by weight and 1.6% by weight, based on the dry weight of the kraft paper.

If necessary, it is provided that the kraft paper contains a sizing agent, in particular at least one alkenylsuccinic anhydride, in a content of between 0.06% by weight and 0.09% by weight, based on the dry weight of the kraft paper.

If necessary, it is provided that the kraft paper is filler-free.

If necessary, it is provided that the kraft paper has a residual moisture content of between 6% by weight and 11% by weight, based on the total weight of kraft paper.

If necessary, the invention relates to a packaging material comprising or consisting of a kraft paper according to the invention.

If necessary, it is provided that the kraft paper has a knobbed structure produced by forming, in particular deep drawing, embossing or pressing, the kraft paper.

If necessary, the invention relates to a method for producing a packaging material according to the invention

If necessary, it is provided that a dimpled structure is produced in the kraft paper by forming, in particular deep drawing, embossing or pressing, the kraft paper.

If necessary, the invention relates to a method for producing kraft paper, in particular kraft paper according to the invention.

Optionally, the method includes the step of: grinding kraft pulp. Optionally, the method includes the step: mixing the ground kraft pulp with one or more auxiliary substances

If necessary, the method includes the step: applying the ground kraft pulp, optionally mixed with one or more auxiliary substances, to the wire of a paper machine and pressing it to obtain a paper web,

If necessary, the process includes the step: pre-drying the paper web obtained.

If necessary, the method includes the step: creping the obtained pre-dried paper web, in particular in a Clupak unit.

If necessary, the process includes the step: post-drying the creped paper web obtained.

If necessary, it is provided that the grinding step includes high-consistency grinding in which the consistency is between 30% and 35%.

If necessary, it is provided that the speed of the paper web in the pre-drying step is between 6% and 10%, in particular between 7.5% and 8.5%, higher than in the post-drying step. There may be a percentage difference in speed of the paper web between the pre-drying and post-drying steps, with the speed of the paper web being greater in the pre-drying step than in the post-drying step. If necessary, the percentage speed difference is between 6% and 10%, in particular between 7.5% and 8.5%.

If necessary, it is provided that the relationship between the consistency of the material in high-consistency grinding and the percentage speed difference is set in such a way that the elongation at break is in the machine direction and the elongation at break is in Transverse direction of the kraft paper differs by less than 2.5%, in particular by less than 1.5%.

If necessary, it is provided that the post-drying takes place by means of convection drying, in particular at a temperature of at least 170 ° C.

If necessary, it is provided that the grinding performance for high-consistency grinding is between 150 kWh/t and 320 kWh/t, in particular between 220 kWh/t and 270 kWh/t.

If necessary, it is provided that in the grinding step following the high-consistency grinding, a low-consistency grinding is provided, in which the consistency is between 4% and 6%.

If necessary, it is provided that the grinding capacity for low-consistency grinding is between 90 kWh/t and 160 kWh/t, in particular between 110 kWh/t and 140 kWh/t.

If necessary, it is provided that the pH value is adjusted to 6.2 to 7.2, in particular 6.6 to 7.0, when mixing the kraft pulp with one or more auxiliary substances.

If necessary, it is provided that the pre-drying step is carried out by contract drying.

If appropriate, the present invention also relates to a kraft paper that was obtained by a process according to the invention, in particular by a process with the above-mentioned features, or is obtainable by such a process. Further features of the invention result from the patent claims and the description of the exemplary embodiments.

The present invention is discussed in detail below using an exemplary embodiment.

In this exemplary embodiment, an unbleached kraft pulp, consisting of 100% fresh fiber from softwood, namely a mixture of spruce and pine wood), with a kappa number of approximately 42 is used as the starting material, which is processed in a paper machine.

The pulp is first treated with high consistency grinding with a consistency of around 32% and a grinding capacity of around 240 kWh/t. The high-consistency pulp was then subjected to low-consistency grinding, with a consistency between about 4.0 and 6.0% and a grinding capacity of about 120 kWh/t. As a result, in this exemplary embodiment, a kraft paper can be obtained which has an air resistance of approximately 22 s Gurley.

Auxiliary materials are added to the constant part of the paper machine: A pH value of around 6.8 is set with sulfuric acid and aluminum sulfate. Cationic bulk starch is added in an amount of around 14 kg/t paper (dry weight). Alkenylsuccinic anhydrides are used as sizing agents in an amount of around 0.7 kg/t of kraft paper (dry weight). No fillers were added.

To steam the paper, a conventional steam blower box is used, which applies water vapor to the paper at pressure and temperature. A high-pressure hood was also used.

The paper machine is equipped with a Fourdrinier wire section as a fourdrinier wire and a 3-nip press section, with which a paper web can be produced from the starting material. The paper web obtained is dried in several steps in a drying section.

After pressing in the paper machine, the still wet paper web is subjected to a pre-drying step, which takes place in a slalom or in a 2-row drying section using contact drying.

The pre-dried paper web is creped in a Clupak system, treated at a differential speed of around 8% and finally dried to a final residual moisture content of around 9%.

In the post-drying section, the paper web is additionally subjected to conventional drying using hot air at >170°C. This results in a particularly uniform transverse expansion profile across the entire width of the paper web. This is advantageous in order to achieve the most isotropic expansion characteristics possible across the entire width of the paper web, which in turn enables particularly uniform deformation behavior.

The kraft paper produced in this way has the paper parameters listed in Table 1 below.

Table 1

*MD = in machine direction; CD = in the transverse direction

A packaging material is produced from the kraft paper obtained. To do this, a piece of kraft paper of a defined size is subjected to a deep-drawing process so that a knobbed structure is obtained. In this exemplary embodiment, the knob structure consists of evenly arranged hemispherical bulges of the kraft paper. The deep-drawing process essentially does not change the thickness of the paper. This is of particular importance in connection with the present invention, since the stiffness and the thickness have a cubic relationship, so the stiffness is a function of the thickness to the third power. The nubs produced provide a cushioning effect under mechanical stress, similar to a plastic bubble wrap.

The combination of bending stiffness and isotropic expansion properties makes kraft paper particularly suitable for the production of such packaging material.

Claims

Patent claims

1 . Kraft paper having an elongation at break in the machine direction A _MD of at least 7.5% and with an elongation at break in the transverse direction A _CD of at least 7.5%, the elongation at break in the machine direction and the elongation at break in the transverse direction being less than 2.5%, in particular by less than 1.5%, and where the bending stiffness in the machine direction S _Ö(MD) and the bending stiffness in the transverse direction S _Ö(CD) are at least 50 mN.

2. Kraft paper according to claim 1, characterized in that the bending stiffness index in the machine direction is between 60 and 90 Nm ⁶ kg' ³ , and that the bending stiffness index in the transverse direction is between 50 and 80 Nm ⁶ kg ^-3 .

3. Kraft paper according to claim 1 or 2, characterized in that the elongation at break in the machine direction A _MD and / or the elongation at break in the transverse direction A _CD between 7.5% and 13%, preferably between 8% and 12%, more preferably between 9% and 11%.

4. Kraft paper according to one of claims 1 to 3, characterized in that it has a grammage of at least 90 g/m ² , in particular between 95 g/m ² and 140 g/m ² .

5. Kraft paper according to one of claims 1 to 4, characterized in that it has an air resistance of at least 16 s Gurley.

6. Kraft paper according to one of claims 1 to 5, characterized in that the kraft paper contains kraft pulp which is formed entirely from virgin fiber, in particular entirely from virgin fiber from softwood, and / or that the kraft paper contains kraft pulp which has a kappa number between 30 and 50, especially between 38 and 47.

7. Kraft paper according to one of claims 1 to 6, characterized in that the kraft paper contains cationic starch in a content of between 1.2% by weight and

Contains 1.6% by weight based on the dry weight of the kraft paper, and/or that the kraft paper contains a sizing agent, in particular at least one alkenylsuccinic anhydride, in a content between 0.06% by weight and 0.09% by weight based on the dry weight of the kraft paper. Kraft paper according to one of claims 1 to 7, characterized in that the kraft paper is filler-free. Kraft paper according to one of claims 1 to 8, characterized in that the kraft paper has a residual moisture content of between 6% by weight and 11% by weight, based on the total weight of kraft paper. Packaging material comprising or consisting of kraft paper according to one of claims 1 to 9, wherein the kraft paper has a knobbed structure produced by forming, in particular deep-drawing, embossing or pressing, the kraft paper. A method for producing a packaging material comprising or consisting of kraft paper according to one of claims 1 to 9, wherein a knobbed structure is produced in the kraft paper by forming, in particular deep-drawing, embossing or pressing, the kraft paper. A process for producing kraft paper, in particular kraft paper according to one of claims 1 to 9, comprising the following steps: a. Grinding kraft pulp, b. optionally mixing the ground kraft pulp with one or more auxiliary substances, c. Applying the ground kraft pulp, optionally mixed with one or more auxiliary substances, to the wire of a paper machine and pressing it to obtain a paper web, i.e. Pre-drying the paper web obtained in step (c), e. creping the pre-dried paper web obtained in step (d), in particular in a Clupak unit, f. post-drying the creped paper web obtained in step (e), - wherein step (a) comprises high-consistency grinding, the consistency of the high-consistency grinding being between 30% and 35%, and

- where the percentage speed difference of the paper web in steps (d) and (f) is between 6% and 10%, in particular between 7.5% and 8.5%, with the speed of the paper web being higher in step (d). is characterized as in step (f), characterized in that the ratio between consistency in high-consistency grinding and percentage speed difference is set such that the elongation at break in the machine direction and the elongation at break in the transverse direction of the kraft paper are less than 2.5%, in particular less than 1.5%, and that the post-drying in step (f) takes place by means of convection drying, in particular at a temperature of at least 170 ° C. Method according to claim 12, characterized in that the grinding performance in high-consistency grinding is between 150 kWh/t and 320 kWh/t, in particular between 220 kWh/t and 270 kWh/t. Method according to one of claims 12 or 13, characterized in that step (a) following the high consistency grinding comprises a low consistency grinding in which the consistency is between 4% and 6%. Method according to claim 14, characterized in that the grinding performance in low-consistency grinding is between 90 kWh/t and 160 kWh/t, in particular between 110 kWh/t and 140 kWh/t. Method according to one of claims 12 to 15, characterized in that the pH value in step (b) is adjusted to 6.2 to 7.2, in particular 6.6 to 7.0. Method according to one of claims 12 to 16, characterized in that the pre-drying in step (d) takes place by contract drying.