KRAFT PAPER

Description

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

Packaging materials are used in various sections of modern industries. In the case of impact-sensitive products, for example, it is necessary to provide them with a cushioning agent to enable increased product protection during transport. In the state of the art, plastic air bubble film is often used for this purpose. The air bubble film is at least a two-layer plastic film, with gas-filled nubs formed between the layers, allowing for a cushioning effect.

Alternatives to plastic packaging materials are required, in particular due to ecological and regulatory aspects. Paper-based packaging materials are one of the options. However, no paper-based packaging material is known in the state of the art that is at least similar to the structure and functionality of plastic bubble wrap.

One object of the present invention may therefore optionally be providing a paper material suitable for producing a cushioning packaging material.

A further object of the present invention may optionally be providing a paper material which may be subjected to a forming process in order to produce a three-dimensional structure, in particular bubbles, nubs and/or hollow spaces.

Another object may optionally be providing a paper material that may be subjected to a forming process in order to achieve a structure that is at least similar in its cushioning effect to conventional air bubble film.

In the context of the present invention, it was surprisingly found that at least one of these objects may optionally be solved by a kraft paper having specific properties.

A kraft paper according to the context 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 produced from wood, in particular by a sulphate process. A preferred raw material for the production of kraft pulp for use in the present invention is softwood, in particular coniferous wood, for example pine, spruce, fir, larch, but also other known coniferous woods. A mixture of one or more coniferous wood species can also be used.

In particular, the kraft pulp used to produce the kraft paper is unbleached or of a natural brown colour. Optionally, the kraft pulp may also be partially or completely bleached.

Optionally, the pulp used may also contain a proportion of short-fibre pulp, which is obtained in particular from hardwood. Optionally, up to 15 wt. %, in particular up to 10 wt. %, of short fibre pulp may be included relative to the total amount of pulp.

The kraft paper of the present invention may have an isotropic expansion behaviour combined with a high stiffness. In the context of the present invention, it was found that such kraft paper has particularly advantageous properties for use as a packaging material or as a substitute for plastic air bubble film.

In particular, the isotropic expansion behaviour may facilitate the formation of particularly uniform nubs or structures when forming the kraft paper. The high stiffness may facilitate maintaining the integrity of the formed nubs or structures and still allow for a sufficient cushioning effect.

The advantageous properties may be achieved in particular by selecting certain parameters during the production of the paper. Optionally, a lower high consistency milling leads to a higher stiffness of the kraft paper in the transverse direction. Optionally, a lower differential speed between the pre-drying and post-drying sections leads to a higher stiffness of the kraft paper in the machine direction.

A paper according to the invention may be produced by subjecting kraft pulp to a milling step. The milling step comprises high consistency milling and optionally low consistency milling. Optionally, high consistency milling improves stretchability in the transverse direction. Optionally, high consistency milling also influences the stiffness of the paper.

Additives may be added to the milled 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 generally known manner.

The resulting paper web may be pre-dried in a pre-drying step. The paper web may then be subjected to creping, in particular microcreping, which optionally improves the stretch properties. The creping process may be carried out in a Clupak unit, in which the paper web is guided between an elastomer cloth and a roller in a known manner.

The creped, in particular microcreped, paper web may then be subjected to a post-drying step.

Optionally, the paper web is fed at a lower speed in the post-drying step than in the pre-drying step. This differential speed may increase the stretch properties of the kraft paper, in particular in the machine direction.

Optionally, the grammage of the kraft paper is at least 90 g/m², in particular between 95 and 140 g/m², preferably about 100 g/m² or about 130 g/m². Optionally, the tensile strength of the kraft paper in the machine direction is between 6 and 9 kN/m. Optionally, the tensile strength of the kraft paper in the transverse direction is between 5 and 8 kN/m. Optionally, the tensile strength index of the kraft paper in the machine direction is between 60 and 90 Nm/g. Optionally, the tensile strength index of the kraft paper in the transverse direction is between 50 and 80 Nm/g.

Optionally, the tensile energy absorption of the kraft paper in the machine direction is between 370 and 430 J/m². Optionally, the tensile energy absorption of the kraft paper in the transverse direction is between 370 and 430 J/m².

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

Optionally, the tearing resistance index of the kraft paper in the machine direction is between 13 and 15 mN·g/m². Optionally, the tearing 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 Nm⁶ kg⁻³, in particular between 65 and 85 Nm⁶ kg⁻³. Optionally, the bending stiffness index of the kraft paper in the transverse direction is between 50 and 80 Nm⁶ kg⁻³, in particular between 55 and 75 Nm⁶ kg⁻³. It was found that a certain degree of stiffness is advantageous in order to obtain a good cushioning effect from hollow structures.

Optionally, the grammage of the kraft paper is determined in accordance with the ISO 536:2019 standard. Optionally, the tensile strength of the kraft paper is determined in accordance with the ISO 1924-3:2005 standard. Optionally, the tensile strength index of the kraft paper is determined in accordance with the ISO 1924-3:2005 standard. Optionally, the elongation at break of the kraft paper is determined in accordance with the ISO 1924-3:2005 standard. Optionally, the tensile energy absorption of the kraft paper is determined in accordance with the ISO 1924-3:2005 standard. Optionally, the bending stiffness of the kraft paper is determined in accordance with the ISO 2493-1:2010 standard. Optionally, air resistance of the kraft paper is determined in accordance with the ISO 5636-5:2013 standard. Optionally, the tearing resistance index of the kraft paper is determined in accordance with the ISO 1974:2012 standard.

The resulting kraft paper may be subjected to a forming step to obtain a packaging material. In particular, the kraft paper has a nub structure after it's forming or forming the packaging material. In the context of the present invention, the term “nub structure” refers in particular to a uniform or non-uniform sequence of bump-shaped elevations. Optionally, the individual nubs of the nub structure are hollow spaces open on one side, for example open spherical segments or cylinders open on one side, although other geometric shapes are also possible.

In particular, the forming of the kraft paper does not substantially change 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 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 50mN.

In the scope of the present invention, indicated differences in elongation at break refer in particular to absolute values. For example, if a paper has an elongation at break in the transverse direction of 8% and an elongation at break in the machine direction of 9%, the difference between the elongation at break in the transverse direction and the elongation at break in the machine direction is 1%.

Optionally, it is provided that the bending stiffness index in the machine direction is between 60 and 90 Nm⁶ kg⁻³. Optionally, it is provided that the bending stiffness index in the transverse direction is at least 50 and 80 Nm⁶ 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%.

Optionally, it is provided that it has a grammage of at least 90 g/m², in particular between 95 g/m² and 140 g/m².

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

Optionally, it is provided that the kraft paper contains kraft pulp formed entirely from fresh fibre, in particular entirely from fresh fibre from softwood.

Optionally, it is provided that the kraft paper includes kraft pulp which has a kappa number according to ISO 302:2015 between 30 and 50, in particular between 38 and 47.

Optionally, it is provided that the kraft paper includes cationic bulk starch in a content of between 1.2 wt. % and 1.6 wt. % relative to the dry weight of the kraft paper.

Optionally, it is provided that the kraft paper includes a sizing agent, in particular at least one alkenyl succinic anhydride, in a content of between 0.06 wt. % and 0.09 wt.-% relative to the dry weight of the kraft paper.

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

Optionally, it is provided that the kraft paper has a residual moisture content of between 6 wt. % and 11 wt. %, relative to the total weight of the kraft paper.

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

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

Optionally, the invention relates to a method for producing a packaging material according to the invention.

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

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

Optionally, the method comprises the step of: Milling of kraft pulp.

Optionally, the method comprises the step of: Mixing the milled kraft pulp with one or more additives

Optionally, the method comprises the step of: applying the milled kraft pulp, optionally mixed with one or more additives, to the wire of a paper machine and pressing to obtain a paper web,

Optionally, the method comprises the step of: Pre-drying of the resulting paper web.

Optionally, the method comprises the step of: Creping of the resulting pre-dried paper web, in particular in a Clupak unit.

Optionally, the method comprises the step of: post-drying of the resulting creped paper web.

Optionally, it is provided that the step of milling comprises high consistency milling in which the stock consistency is between 30% and 35%.

Optionally, 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. Optionally, there is a percentage difference in the 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. Optionally, the percentage speed difference is between 6% and 10%, in particular between 7.5% and 8.5%.

Optionally, it is provided that the ratio between stock consistency during high consistency milling 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 differ by less than 2.5%, in particular by less than 1.5%.

Optionally, it is provided that the post-drying is carried out by convection drying, in particular at a temperature of at least 170° C.

Optionally, it is provided that the milling capacity during high consistency milling is between 150 kWh/t and 320 kWh/t, in particular between 220 kWh/t and 270 kWh/t.

Optionally, a low consistency milling step is provided after the high consistency milling step, in which the stock consistency is between 4% and 6%.

Optionally, it is provided that the milling capacity during low consistency milling is between 90 kWh/t and 160 kWh/t, in particular between 110 kWh/t and 140 kWh/t.

Optionally, it is provided that the pH value is set to 6.2 to 7.2, in particular 6.6 to 7.0, when mixing the kraft pulp with one or more additives.

Optionally, it is provided that the pre-drying step is carried out by contact drying.

Optionally, the present invention also relates to a kraft paper obtained by a method according to the invention, in particular by a method having the above-mentioned features, or obtainable by such a method.

Further features of the invention become apparent from the claims, as well as the description of the exemplary embodiments.

In the following, the present invention will be discussed in detail with reference to an exemplary embodiment.

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

The pulp is first treated in a high consistency milling process with a stock consistency of about 32% and a milling capacity of about 240 kWh/t. The high-consistency refined pulp was then subjected to low consistency milling with a stock consistency of between about 4.0 and 6.0% and a milling capacity of about 120 kWh/t. In this exemplary embodiment, a kraft paper may be obtained that has an air resistance of about 22 s Gurley.

In the approach flow system of the paper machine, additives are added: A pH value of about 6.8 is set using sulphuric acid and aluminium sulphate. Cationic bulk starch is added in a quantity of about 14 kg/t paper (dry weight). Alkenyl succinic anhydride is used as a sizing agent in a quantity of about 0.7 kg/t kraft paper (dry weight). Fillers were not added.

A conventional steam chest, which applies water vapour to the paper under pressure and temperature, is used to steam the paper. A high-pressure hood was also used.

The paper machine is provided with a Fourdrinier wire section as a fourdrinier wire and a 3-nip press section, which can be used to produce a paper web from the starting material.

The resulting paper web is dried in several steps in a dryer section.

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

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

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

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

The resulting kraft paper is used to produce packaging material. For this purpose, a piece of kraft paper of a defined size is subjected to a deep-drawing process so that a nub structure is obtained. In this exemplary embodiment, the nub structure consists of uniformly arranged hemispherical protrusions of the kraft paper. The deep-drawing process does not substantially change the thickness of the paper. This is of particular importance in connection with the present invention, since the stiffness and the thickness are cubically related, i.e. the stiffness is a function of the thickness to the third power. The produced nubs provide a cushioning effect under mechanical stress, similar to a plastic air bubble film.

The combination of bending stiffness and isotropic stretch properties makes kraft paper particularly suitable for the production of this type of packaging material.