Kraft paper

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

[0001] Kraft paper

[0002] The present invention relates to a kraft paper, a packaging material and a method having the features of the independent patent claims.

[0003] Packaging materials are used in various areas of modern industry. For example, shock-sensitive products require cushioning to provide increased product protection during transport. Bubble wrap made of plastic is often used for this purpose. This consists of at least two layers of plastic film, with gas-filled nubs formed between the layers to provide a cushioning effect.

[0004] Particularly due to ecological and regulatory considerations, alternatives to plastic packaging materials are needed. One possible solution is paper-based packaging materials. However, no paper-based packaging material is known in the state of the art that even comes close to the structure and functionality of plastic bubble wrap.

[0005] An object of the present invention can therefore be seen as providing a paper material suitable for producing a cushioning packaging material. A further object of the present invention can be seen as providing a paper material that can be subjected to a forming process to produce a three-dimensional structure, in particular bubbles, nubs, and / or hollow bodies.

[0006] 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 that is at least similar in its cushioning effect to conventional bubble wrap.

[0007] Within the scope of the present invention, it was surprisingly found that at least one of these objects can optionally be achieved by a kraft paper with certain properties.

[0008] Kraft paper, in the sense of the present invention, refers in particular to a type of paper with particularly high strength. Optionally, the pulp used to produce the kraft paper is kraft pulp produced from wood, in particular by means of a sulfate 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.

[0009] In particular, the kraft pulp used to produce kraft paper is unbleached or natural brown. However, the kraft pulp may also be partially or fully bleached.

[0010] Optionally, the pulp used may also contain a proportion of short-fiber pulp, which is 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 contained. The kraft paper of the present invention can exhibit isotropic stretching behavior combined with high rigidity. Within the scope of the present invention, it has been found that such kraft paper has particularly advantageous properties for use as packaging material or as a replacement for plastic bubble wrap.

[0011] In particular, the isotropic stretching behavior can contribute to the formation of particularly uniform nubs or structures during the forming process of the kraft paper. The high stiffness can help maintain the integrity of the formed nubs or structures while still providing sufficient cushioning.

[0012] The advantageous properties can be achieved, in particular, by selecting certain parameters during paper production. A lower high-consistency refining may result in greater stiffness of the kraft paper in the cross direction. A lower differential speed between the pre-dryer and post-dryer sections may result in greater stiffness of the kraft paper in the machine direction.

[0013] A paper according to the invention can be produced by subjecting kraft pulp to a refining step. The refining step comprises high-consistency refining and, optionally, low-consistency refining. High-consistency refining may improve transverse extensibility. High-consistency refining may also influence the stiffness of the paper.

[0014] Additives such as starch, sizing agents, and / or pH regulators can be added to the ground kraft pulp. In particular, no fillers are added to the kraft pulp.

[0015] The kraft pulp can then be processed into a paper web in a conventional manner in a paper machine. The resulting paper web can be pre-dried in a pre-drying step. The paper web can then be subjected to creping, particularly micro-creping, which may improve its stretch properties. Creping can be performed in a Clupak unit, in which the paper web is guided between an elastomer blanket and a roller in a conventional manner.

[0016] The paper web creped in this way, in particular micro-creped, can then be subjected to a post-drying step.

[0017] If necessary, the paper web is conveyed at a lower speed during the post-drying step than during the pre-drying step. This differential speed can increase the stretch properties of the kraft paper, particularly in its machine direction.

[0018] If necessary, the grammage of the kraft paper is at least 90 g / m 2 , especially between 95 and 140 g / m 2 , preferably about 100 g / m 2 or about 130 g / m 2 . The tensile strength of the kraft paper in the machine direction may be between 6 and 9 kN / m. The tensile strength of the kraft paper in the transverse direction may be between 5 and 8 kN / m. The tensile strength index of the kraft paper in the machine direction may be between 60 and 90 Nm / g. The tensile strength index of the kraft paper in the transverse direction may be between 50 and 80 Nm / g.

[0019] If necessary, the tensile strength of the kraft paper in the machine direction is between 370 and 430 J / m 2 If necessary, the tensile breaking energy of the kraft paper in the transverse direction is between 370 and 430 J / m 2 .

[0020] In some cases, the air resistance of the kraft paper may be more than 16 s Gurley, in particular more than 18 s Gurley, especially more than 20 s Gurley.

[0021] Where appropriate, the tear resistance index of the kraft paper in the machine direction is between 13 and 15 mN g / m 2 . The tear resistance index of the kraft paper in the transverse direction may be between 16 and 20 mN g / m 2 . If necessary, the bending stiffness index of the kraft paper in the machine direction is between 60 and 90 Nm 6 kg' 3 , especially between 65 and 85 Nm 6 kg' 3 . If necessary, the flexural stiffness index of the kraft paper in the transverse direction is between 50 and 80 Nm 6 kg' 3 , especially between 55 and 75 Nm 6 kg -3 It has been found that a certain degree of stiffness is beneficial to obtain a good cushioning effect of hollow structures.

[0022] Where applicable, the grammage of the kraft paper is determined according to ISO 536:2019. Where applicable, the tensile strength of the kraft paper is determined according to ISO 1924-3:2005. Where applicable, the tensile strength index of the kraft paper is determined according to ISO 1924-3:2005. Where applicable, the elongation at break of the kraft paper is determined according to ISO 1924-3:2005. Where applicable, the tensile work at break of the kraft paper is determined according to ISO 1924-3:2005.

[0023] Where applicable, the flexural rigidity of the kraft paper is determined according to ISO 2493-1:2010. Where applicable, the air resistance of the kraft paper is determined according to ISO 5636-5:2013. Where applicable, the tear resistance index of the kraft paper is determined according to ISO 1974:2012.

[0024] The resulting kraft paper can be subjected to a forming step to obtain a packaging material. In particular, the kraft paper after forming or the packaging material has a dimpled structure. In the context of the present invention, the term "dimpled structure" refers in particular to a uniform or irregular sequence of hump-shaped elevations. The individual dimples of the dimpled structure may be hollow bodies open on one side, for example, open spherical segments or cylinders open on one side, although other geometric shapes are also possible.

[0025] In particular, the forming of the kraft paper does not result in any significant change in the thickness of the kraft paper.

[0026] 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%, 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 wherein the flexural rigidity in the machine direction and the flexural rigidity in the transverse direction are at least 50 mN.

[0027] In the context of the present invention, stated differences in elongation at break refer, in particular, to absolute values. For example, if a paper has a transverse elongation at break of 8% and a machine direction elongation at break of 9%, the difference between the transverse elongation at break and the machine direction elongation at break is 1%.

[0028] Where appropriate, the bending stiffness index in the machine direction is between 60 and 90 Nm 6 kg-3 Where appropriate, the bending stiffness index in the transverse direction shall be at least 50 and 80 Nm 6 kg -3 amounts.

[0029] 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%.

[0030] Where appropriate, it is intended that there is a grammage of at least 90 g / m 2 , especially between 95 g / m 2 and 140 g / m 2 , has.

[0031] Where appropriate, it is intended to have an air resistance of at least 16 s Gurley.

[0032] Where appropriate, the kraft paper is intended to contain kraft pulp made entirely from virgin fibre, in particular entirely from virgin fibre from softwood.

[0033] Where appropriate, the kraft paper shall contain kraft pulp having a kappa number according to ISO 302:2015 between 30 and 50, in particular between 38 and 47. Where appropriate, the kraft paper shall contain cationic bulk starch in a content of between 1.2% and 1.6% by weight, based on the dry weight of the kraft paper.

[0034] Optionally, 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 wt.% and 0.09 wt.% based on the dry weight of the kraft paper.

[0035] Where appropriate, the kraft paper is intended to be filler-free.

[0036] Where appropriate, the kraft paper is intended to have a residual moisture content of between 6% and 11% by weight, based on the total weight of the kraft paper.

[0037] Optionally, the invention relates to a packaging material comprising or consisting of a kraft paper according to the invention.

[0038] Where appropriate, the kraft paper may have a dimpled structure produced by forming, in particular deep drawing, embossing or pressing, the kraft paper.

[0039] Optionally, the invention relates to a process for producing a packaging material according to the invention

[0040] Where appropriate, it is intended that a dimpled structure is produced in the kraft paper by forming the kraft paper, in particular by deep drawing, embossing or pressing.

[0041] Optionally, the invention relates to a process for producing kraft paper, in particular a kraft paper according to the invention.

[0042] Optionally, the process comprises the step of refining kraft pulp. Optionally, the process comprises the step of mixing the refined kraft pulp with one or more additives

[0043] Optionally, the process comprises the step of: applying the ground kraft pulp, optionally mixed with one or more auxiliaries, to the screen of a paper machine and pressing to obtain a paper web,

[0044] If necessary, the process comprises the step of pre-drying the resulting paper web.

[0045] Optionally, the process comprises the step of creping the obtained pre-dried paper web, in particular in a Clupak unit.

[0046] If appropriate, the process comprises the step of post-drying the resulting creped paper web.

[0047] Where appropriate, the grinding step shall include high consistency grinding, with a pulp consistency of between 30% and 35%.

[0048] Where appropriate, the speed of the paper web during the pre-drying step is between 6% and 10%, in particular between 7.5% and 8.5%, higher than during the post-drying step. Where appropriate, there is a percentage speed difference of the paper web between the pre-drying and post-drying steps, with the speed of the paper web being higher during the pre-drying step than during the post-drying step. Where appropriate, the percentage speed difference is between 6% and 10%, in particular between 7.5% and 8.5%.

[0049] Where appropriate, the ratio between stock consistency during high consistency refining 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%.

[0050] Where appropriate, the final drying is carried out by means of convection drying, in particular at a temperature of at least 170°C.

[0051] Where appropriate, the grinding capacity for high-consistency grinding is intended to be between 150 kWh / t and 320 kWh / t, in particular between 220 kWh / t and 270 kWh / t.

[0052] Where appropriate, the grinding step following the high-consistency grinding may be followed by a low-consistency grinding, in which the pulp consistency is between 4% and 6%.

[0053] Where appropriate, the grinding capacity for low-consistency grinding is intended to be between 90 kWh / t and 160 kWh / t, in particular between 110 kWh / t and 140 kWh / t.

[0054] Where appropriate, 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 auxiliaries.

[0055] Where appropriate, the pre-drying step is carried out by contract drying.

[0056] Optionally, the present invention also relates to a kraft paper obtained by a process according to the invention, in particular by a process having the above-mentioned features, or obtainable by such a process. Further features of the invention emerge from the patent claims and the description of the exemplary embodiments.

[0057] The present invention will be discussed in detail below using an exemplary embodiment.

[0058] In this embodiment, an unbleached kraft pulp consisting of 100% fresh softwood fiber (namely a mixture of spruce and pine) with a kappa number of about 42 is used as starting material, which is processed in a paper machine.

[0059] The pulp is first treated with a high-consistency refining process at a consistency of approximately 32% and a refining capacity of approximately 240 kWh / t. The high-consistency refining process is then subjected to a low-consistency refining process at a consistency between approximately 4.0 and 6.0% and a refining capacity of approximately 120 kWh / t. In this exemplary embodiment, this process produces a kraft paper with an air resistance of approximately 22 s Gurley.

[0060] In the approach flow system of the paper machine, additives are added: a pH of approximately 6.8 is adjusted using sulfuric acid and aluminum sulfate. Cationic pulp starch is added at a rate of approximately 14 kg / t of paper (dry weight). Alkenyl succinic anhydride is used as a sizing agent at a rate of approximately 0.7 kg / t of kraft paper (dry weight). No fillers were added.

[0061] A conventional steam box, which applies steam to the paper at both pressure and temperature, is used to steam the paper. A high-pressure hood was also used.

[0062] The paper machine is equipped with a Fourdrinier wire section and a 3-nip press section, which can produce a paper web from the starting material. The resulting paper web is dried in a drying section in several steps.

[0063] After pressing in the paper machine, the still moist paper web is subjected to a pre-drying step, which takes place in the slalom or in a 2-row drying section by means of contact drying.

[0064] The pre-dried paper web is creped in a Clupak system, treated with a differential speed of about 8% and finally dried to a final residual moisture content of about 9%.

[0065] In the post-dryer section, the paper web is additionally subjected to conventional drying using hot air at temperatures exceeding 170°C. This achieves a particularly uniform transverse stretch profile across the entire paper web width. This is advantageous for achieving the most isotropic stretch characteristics possible across the entire paper web width, which in turn enables particularly uniform deformation behavior.

[0066] The kraft paper produced in this way has the paper parameters listed below in Table 1.

[0067] Table 1

[0068]

[0069] *MD = machine direction; CD = cross direction

[0070] A packaging material is produced from the resulting kraft paper. For this purpose, a piece of kraft paper of a defined size is subjected to a deep-drawing process to create a dimpled structure. In this exemplary embodiment, the dimpled structure consists of uniformly arranged hemispherical bulges in the kraft paper. The deep-drawing process essentially does not change the thickness of the paper. This is of particular importance in the context of the present invention, since stiffness and thickness are cubically related, meaning stiffness is a cube-power function of thickness. The resulting dimples provide a cushioning effect under mechanical stress, similar to plastic bubble wrap.

[0071] The combination of flexural rigidity and isotropic stretch properties makes kraft paper particularly suitable for the production of this type of packaging material.

Claims

Patent claims 1. Kraft paper with an elongation at break in the machine direction A MD of at least 7.5% and with a transverse elongation at break A CD 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 wherein the bending stiffness in the machine direction S Ö(MD) and the bending stiffness in the transverse direction S Ö(CD) must be 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 is, and that the bending stiffness index in the transverse direction is between 50 and 80 Nm 6 kg -3 amounts.

3. Kraft paper according to claim 1 or 2, characterized in that the elongation at break in machine direction A MD and / or the elongation at break in the transverse direction A CDbetween 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 basis weight of at least 90 g / m² 2 , especially between 95 g / m² 2 and 140 g / m² 2 , exhibits.

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 any 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, in particular between 38 and 47.

7. Kraft paper according to any one of claims 1 to 6, characterized in that the kraft paper contains cationic bulk strength in a content between 1.2 wt.% 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 alkenyl succinic anhydride, in a content between 0.06 wt.% and 0.09 wt.% based on the dry weight of the kraft paper. Kraft paper according to any one of claims 1 to 7, characterized in that the kraft paper is filler-free. Kraft paper according to any one of claims 1 to 8, characterized in that the kraft paper has a residual moisture content between 6 wt.% and 11 wt.% based on the total weight of the kraft paper. Packaging material comprising or consisting of kraft paper according to any one of claims 1 to 9, wherein the kraft paper has a dimpled 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 any one of claims 1 to 9, wherein a dimpled structure is produced in the kraft paper by forming, in particular by deep drawing, embossing, or pressing. A method for producing kraft paper, in particular kraft paper according to any one of claims 1 to 9, comprising the following steps: a. milling kraft pulp, b. optionally mixing the milled kraft pulp with one or more additives, c. 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, d. 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 a high-consistency milling process, wherein the material density during the high-consistency milling process is between 30% and 35%, and - wherein 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%, wherein the speed of the paper web in step (d) is higher than in step (f), characterized in that the ratio between the stock density during high-consistency refining and the percentage speed difference is adjusted 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%, and that the final drying in step (f) is carried out by means of convection drying, in particular at a temperature of at least 170°C. Method according to claim 12, characterized in that the refining capacity during high-consistency refining is between 150 kWh / t and 320 kWh / t, in particular between 220 kWh / t and 270 kWh / t.A process according to claim 12 or 13, characterized in that step (a) following high-consistency grinding comprises low-consistency grinding, in which the material density is between 4% and 6%. A process according to claim 14, characterized in that the grinding capacity during low-consistency grinding is between 90 kWh / t and 160 kWh / t, in particular between 110 kWh / t and 140 kWh / t. A process according to any 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. A process according to any one of claims 12 to 16, characterized in that the pre-drying in step (d) is carried out by contract drying.

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