EP1818451B1 - Packaging paper and paper sack made therefrom as well as method of producing the packaging paper. - Google Patents

Abstract

Wrapping paper comprises discrete islands surrounded by a network structure, where the islands have a lower fiber content and thus a lower density than the network structure. Independent claims are also included for: (1) paper bag for transporting bulk goods, with walls made of wrapping paper as above; (2) producing wrapping paper by dewatering pulp on a sheet former wire comprising a permeable support with a pattern of discrete islands of covering material surrounded by a network structure, where the islands have a lower permeability than the network structure.

Description

The invention relates to a kraft paper, in particular kraft or sack paper. It further relates to a paper bag using the aforesaid wrapping paper and a method for producing wrapping paper.

Wrapping paper is understood to mean wrapping papers which are subdivided into different groups according to the requirements imposed on them, depending on the strength, density, describability and / or printability as well as the material composition. Such packaging papers have a basis weight of at least 25 g / m ² . If packing papers are used for pack sacks which are used in the industrial sector, for example for the transport of bulk goods (industrial sacks), the basis weight is at least 30 g / m ² and reaches up to 200 g / m ² .

Wrapping papers, like other types of paper, are made in papermaking machines by passing a fiber pulp containing paper fibers over a headbox onto which the Surface evenly permeable sheet forming screen is applied, which is formed as a wire and runs in the sheet forming section of the paper machine. On the sheet forming side of the sheet forming screen, this causes a paper web formation while dewatering the paper web through the sheet forming screen. In the subsequent press section of the paper machine, the paper web is further dehydrated mechanically by pressing. Thereafter, a thermal dewatering takes place in the dryer section of the paper machine.

The known packaging papers have a density and thickness substantially equal over the surface. They must have a tensile strength and tensile elongation at break and tensile breaking load capacity to ensure that they do not break or break during transport. If such packaging papers or paper bags made from them are used for the transport of bulk material, the packaging paper must be permeable to air so that the air in the paper bag can also escape through the packaging paper when filling the bulk material. This property determines the speed of the filling process. On the one hand, high strength on the one hand and good air permeability on the other hand are contradictory requirements, because a packing paper with good air permeability has a low strength, while a packing paper with high strength has a low air permeability. It is therefore necessary to look for a suitable compromise, but in the first place the strength required for transport is decisive. If high strength is required, a low air permeability and thus a correspondingly low filling speed must be accepted.

In the WO 03/010055 A1 is a bag for the packaging of heavyweight material, such as cement, sugar, flour, feed, etc., disclosed whose walls are formed in three or four layers, the outer layer of a kraft paper or a modified center paper, the liner of one or two Paper webs consist of central paper, dual-center paper or modified center paper and the inner layer of only one paper web of dual-center paper, modified center paper or kraft paper. The dual center paper is produced from two medium paper layers, between which a network of reinforcing fibers is inserted. The net is intended to increase the strength of the bag.

Although such a bag has due to the multi-layered design of the walls of a very high strength. However, this strength comes at the expense of air permeability, i. H. Air can only escape via the filling opening of the bag.

The invention has for its object to develop a packaging paper and a paper bag produced therefrom, or that for a given strength has a higher porosity or air permeability than the known Packing papers or paper sacks of the same strength. Another object is to provide a method for producing such packaging paper.

The first part of the object is achieved in that the wrapping paper has at least partially a network structure with enclosed by the network structure, discrete surface islands, which are depleted compared to the network structure of paper fibers and their density is accordingly lower than the density of the network structure. The basic idea of the invention is therefore to distribute the paper fibers no longer uniformly over the surface of the packaging paper, but to concentrate them where a network structure is provided, in this network structure and thereby enclosed by the network structure surface islands, the paper fibers in favor of Net structure are depleted and thus have a lower density and thus a high porosity or air permeability. The wrapping paper is thus characterized for a given strength by a much better air permeability. A paper bag made from it, which is intended for the transport of bulk material, can thus be considerably faster fill, which reduces the process costs accordingly.

In an embodiment of the invention, it is provided that the density in the surface islands is at least 10% lower than the density in the network structure, so that the increased air permeability is clearly noticeable. Preferably, the density in the surface islands should be at most 700 kg / m ³ , but at least 350 kg / m ³ . In the network structure, however, the density should be at least 700 kg / m ³ , but not more than 1050 kg / m ³ .

For the formation of a fixed network structure, it is advantageous if the diameter of circular-shaped surface islands and / or of the surface islands each enclosing the smallest possible enveloping circles is at most 2.5 times the average length-weighted fiber length of the paper fibers from which the packaging paper is made. Preferably, the diameter to fiber length ratio as defined above should be less than 1.0, but at least 0.1.

For most applications, it is useful if the network structure extends over the entire surface of the packaging paper. However, this does not exclude that only a portion of the packaging paper has the network structure. The network structure can also be distributed over a plurality of discrete subregions, the subregions preferably arranged in a regular, recurring pattern. By expanding and distributing these subregions, the air permeability can be adjusted as desired according to the respective requirements.

For the strength of the packaging paper, it is essential that the individual webs, which make up the network structure, have a certain minimum cross-section. Therefore, the smallest free space between two surface islands should not be less than 0.7 mm.

The network structure itself should form a regularly recurring pattern, preferably even be completely regular. Again, it is not excluded that an irregular network structure is useful in certain applications. It is part of the basic idea of the invention that the formation of the network structure and the surface islands is arbitrarily adaptable. Conveniently, however, the surface islands are the same size and have the same shape. They may be circular, oval, rectangular or polygonal, for example.

An expedient variant of the network structure results when the network structure forms two families of parallel network lines, with one group of parallel network lines running at an angle, expediently perpendicular to the other family of parallel network lines. This results in a grid-like network structure with, for example, rectangular, in particular square Plane islands enclosed by the net lines. In this case, the network lines may have at least one share, better of both at an angle to each other flocks of network lines the same distance from each other to obtain a regular network structure.

As an alternative to the grid-like network structure, it is possible to form the network structure such that the surface islands each form adjacent rows and columns of surface islands running side by side and perpendicular to the rows, the surface islands of adjacent rows or columns each being separated by half the center distance of two neighboring centers Plane are offset. It is advantageous if the surface islands are arranged so that they lie on mutually parallel diagonals. The diagonals may include an angle of 15 ° to 75 °, conveniently 45 ° to 70 °, with the rows of surface islands, with an angle of 60 ° being particularly advantageous because it allows a dense packing of the surface islands. Preferably, the rows and / or columns have the same distance from one another.

The structuring according to the invention of at least a part of the packaging paper by local compaction (network structure) and local depletion (surface islands) of paper fibers should, if possible, have little, even better, no influence on the thickness of the packaging paper, ie the thickness should be as uniform as possible over the surface.

This promotes especially the printability and promotes rigidity on the one hand and porosity on the other. The thickness of the packing paper should be adapted to the respective requirements. This is a minimum thickness of 0.05 mm in question. The upper limit is determined by the respective requirements and may, for example, be 0.2 mm.

The invention also provides a paper bag for the transport of bulk material, wherein the walls of the paper bag made of kraft paper of the type described above.

The second part of the object is achieved by a method in which in the wet end of a paper machine a fiber pulp having paper fibers is applied via a headbox to a circulating forming wire with a sheet forming side and a machine side and dewatered through the forming wire. According to the invention, a forming wire with a permeable carrier is used, which at least in some areas has a pattern of discrete surface islands whose permeability is lower than that of the areas surrounding the surface islands forming a network structure. The cover material should not protrude beyond the sheet forming side of the carrier. Preferably, the cover material should be completely embedded in the carrier. In addition, it is expedient that the surface islands have no permeability.

Through the use of such a sheet forming screen in a paper machine produces a wrapping paper of the type described above. By draining the fiber pulp or the then forming paper web in the area of the surface islands arise due to the cover material present in the carrier flow conditions that lead to a compression of the paper fibers in the The web structure forming surface areas as well as an alignment of the paper fibers tangentially around the surface islands leads, so that the network structure is the wrapping paper high strength.

In order to produce a desired network structure in the paper with the method according to the invention, the sheet forming screen used in this case with the network structure identical thereto is used for the areas of low permeability. With regard to the formation of the network structure in detail, therefore, the description of the result of this method, namely the packaging paper, can be referred to. It is expedient to take into account the shrinkage behavior of the paper web produced for the wrapping paper in the dimensioning of the distances of the surface islands on the forming wire, in such a way that this distance is increased by the degree of shrinkage in the longitudinal and / or transverse direction, the Wrapping paper suffers during its manufacture.

In a further embodiment of the invention, it is provided that the fiber pulp used has a consistency of 0.1 to 5 g / l when running on the sheet forming screen. This relatively low fiber content favors the above-described fiber orientation in the region of the surface islands formed by the covering material. The fiber pulp can be applied to the sheet forming screen in one layer or else by means of a multi-layer headbox, or at least from a single headbox nozzle.

In order to favor the desired tangential orientation of the paper fibers around the surface islands, the paper fibers should be flexibilized before application to the forming screen by high consistency painting, suitably at a dry content of 20 to 38%, a pH of 8 to 12, and an energy input from 20 to 500 kWh / t. The paper fibers should, for example, be subjected to an intensive kneading or crimping process in a high-consistency refiner. This makes it possible to use as fibers predominantly, if not exclusively long-fiber pulps, which have a length of 1.5 mm to 7 mm.

To give the network structure a good strength, the formation of sufficient fiber bonds is advantageous. For this purpose, the paper fibers should be ground fibrillating and / or collapsing at dry contents of 2.5 to 7%, preferably in a low consistency refiner. Instead, or in combination therewith, the paper fibers should be ground fibrillating and / or collapsing, for example, in a medium consistency refiner at dry contents of 7 to 20%. This enhances the binding ability of the fibers.

According to a further feature of the invention, it is advisable to shake the sheet forming screen so that alignment of the paper fibers in a certain direction is largely avoided. Such alignment results from the head box and the circulation of the forming wire in its direction of travel. By appropriate shaking this alignment can be destroyed or destroyed. This makes it possible for the paper fibers to lay around the surface islands tangentially both in the longitudinal and in the transverse direction and thus to give the paper web thus produced a substantially equal strength in both directions. Such a wrapping paper is particularly suitable for the production of paper sacks, since such bags should have similar strength properties both in the longitudinal and in the transverse direction. Shaking is preferably normal to the direction of travel of the forming wire by appropriate action on the breast roll around which the forming wire runs, preferably at a frequency of up to 1000 Hz and with a shaking stroke of up to 100 mm.

• Figur 1 einen Längsschnitt durch ein Blattbildungssieb;
• Figur 2 eine Draufsicht auf einen Ausschnitt eines Packpapiers, hergestellt mit dem Blattbildungssieb gemäß Figur 1; und
• Figur 3 eine Draufsicht auf einen Ausschnitt eines anderen Packpapiers.

In the drawing, the invention is illustrated by means of exemplary embodiments. Show it:

• FIG. 1 a longitudinal section through a sheet forming screen;
• FIG. 2 a plan view of a section of a packaging paper, made with the forming wire according to FIG. 1 ; and
• FIG. 3 a plan view of a section of another packaging paper.

This in FIG. 1 Partial forming sheet 1 has a carrier 2 in the form of a fabric with longitudinal threads 3, 4 and an upper layer of transverse threads - by way of example denoted by 5 - and a lower layer of transverse threads - exemplified by 6. The longitudinal threads 3, 4 bind in each case in the lower layer only a transverse thread 6, then float between the two layers over three transverse threads 6 and then tie in the upper layer five transverse threads 5 alternately top and bottom, before they again between the layers float over three transverse threads 5, 6.

In the carrier 2 are stored - each spaced - in plan view circular Abdeckinseln - exemplified denoted by 7. They close the top side, ie flush with the sheet forming side, so there are not on the sheet forming side before. On the lower side, ie on the machine side, they go up to approximately the lower layer of the transverse threads 6. The cover islands 7 are made of a plastic material, as described above, and are impermeable. Around the cover islands 7 are free surface areas - designated by way of example with 8 - over which takes place when using the sheet forming wire 1 in the paper machine dewatering of the paper web.

FIG. 2 shows a section of a packaging paper 31 produced with the sheet forming screen 1. The wrapping paper 31 has a network structure 32 which substantially corresponds to the free surface areas 8 of the forming wire 1. Because of the cover islands 7 of the sheet forming screen 1, in the production of the wrapping paper 1 around the surface areas 8, dewatering flows result, which lead to a concentration of paper fibers in this area and thus to the formation of a fiber-rich network structure 32 in the wrapping paper 31. The network structure 31 encloses surface islands-designated by way of example by 33-whose arrangement, shape and size of the arrangement, shape and size of the cover islands 7 of the sheet-forming screen 1 correspond.

The surface islands 33 form in one direction adjacent rows of islands - exemplified by 34 - and in the direction perpendicular island columns - exemplified with 35 -, with two adjacent island rows 34 - like the Abdeckinseln 7 in the forming wire 1 - each by half Center offset are offset. The same applies to the island columns 35. The distance between two adjacent island rows 34 and two adjacent island columns 35 is identical.

Also, the distances of the surface islands 33 with each other are the same, so that a regular pattern on the surface of the packaging paper 31 results.

Due to the above-described distribution, the surface islands 33 are each adjacent or on diagonals - for example, designated 36 - which an angle α with a straight line - by way of example denoted by 37 - include, which runs parallel to the island lines 34. In the present case, the angle α is about 45 °. In a particularly preferred embodiment of the angle α has a value of 60 °, which can be effected by increasing the distances of the island lines 34.

Due to the flow conditions described above, the surface islands 33 are depleted of paper fibers, i. There, the density of the packaging paper 31 is reduced compared to the density in the region of the network structure 32. When filling a paper sack made of this wrapping paper 31, therefore, the displaced air can escape over the surface islands 33 in an efficient manner, so that the paper sack can be filled quickly. The strength of the wrapping paper 31 is substantially ensured by the net structure 32 in which the paper fibers are concentrated.

The size ratios between surface islands 33 and network structure 32 can of course be adapted according to the respective requirements. A bigger one Area fraction of the network structure 32 provides higher strength, but with loss of permeability. The same applies vice versa when increasing the proportion of the surface islands 33. Moreover, it is also not mandatory that the surface islands 33 have round shape. Other shapes, such as polygonal, honeycomb or rectangular shapes are possible.

FIG. 3 shows a variant of the packing paper 31 according to FIG. 3 , The wrapping paper 41 has a grid-like network structure 42 with parallel network lines - for example denoted by 43 - in one direction and also parallel network lines - for example denoted by 44 - perpendicular to the network lines 43. The network lines 43, 44 have identical distances to each other, so that the network structure 42 square islands - exemplarily with 45 - include. The surface islands 45 are also depleted of paper fibers in favor of the network structure 42, ie in the network structure 42, the paper fibers are concentrated at the expense of the surface islands 45. The strength of the packaging paper 41 is therefore ensured essentially by the network structure 42, while the surface islands 45 provide good air permeability and thus favor the filling of a paper bag produced from the packaging paper 41.

The production of the packaging paper 41 is carried out with a correspondingly adapted sheet forming screen. This sheet forming screen then deviates from the forming wire 1 according to FIG. 1 lattice-like free surface areas which are produced by the fact that each spaced square Abdeckinseln in the arrangement, as they have been deposited as surface islands 45 in the wrapping paper 41, applied and stored.

It is understood that the network structure according to the wrapping paper 41 can also be designed differently. For example, the distances of the network lines extending in one direction can be selected to be greater than the distances between the network lines extending perpendicularly therefrom, so that rectangular islands of surface of high permeability arise. Of course, the width of the network lines in relation to the extension of the surface islands can be changed in the same direction, so that smaller surface islands arise. Such a wrapping paper would then have greater strength, with the permeability being controlled by the size and number of surface islands. In addition, the surface islands may also have other shapes, such as a round shape. It would then be a wrapping paper, which differs from the wrapping paper 31 according to FIG. 2 differs in that the individual surface islands adjacent island rows or island columns would not be offset from each other.

Claims (44)

1. Packaging paper (31, 41), particularly kraft or sack paper, characterised in that at least in some areas the packaging paper (31, 41) has a network structure (32, 42) with discrete spots on the surface (33, 45) enclosed by the network structure (32, 42), which are low in paper fibres compared with the network structure (32, 42) and the density of which is therefore lower than the density of the network structure (32, 42).
2. Packaging paper according to claim 1, characterised in that the density in the spots on the surface (33, 45) is at least 10% lower than the density in the network structure (32, 42).
3. Packaging paper according to claim 1 or 2, characterised in that the density in the spots on the surface (33, 45) is at most 700 kg/m³.
4. Packaging paper according to one of claims 1 to 3, characterised in that the density in the spots on the surface (33, 45) is at least 350 kg/m³.
5. Packaging paper according to one of claims 1 to 4, characterised in that the density in the network structure (32, 42) is at least 700 kg/m³.
6. Packaging paper according to one of claims 1 to 5, characterised in that the density in the network structure (32, 42) is at most 1050 kg/m³.
7. Packaging paper according to one of claims 1 to 6, characterised in that the diameter of circular spots on the surface (33) and/or the envelope surrounding the spots on the surface (45) as closely as possible is at most 2.5 times the average length evaluated fibre length of the paper fibres, from which the packaging paper (31, 41) is produced.
8. Packaging paper according to claim 7, characterised in that the diameter of circular spots on the surface (33) and/or of the envelope surrounding the spots on the surface (45) as closely as possible is 0.1 times the average length evaluated fibre length of the paper fibres, from which the packaging paper (31, 41) is produced.
9. Packaging paper according to one of claims 1 to 8, characterised in that the network structure (32, 42) extends over the whole surface of the packaging paper (31, 41).
10. Packaging paper according to one of claims 1 to 9, characterised in that only a part area or only part areas of the packaging paper have the network structure.
11. Packaging paper according to claim 10, characterised in that the part areas are arranged in a regular pattern.
12. Packaging paper according to one of claims 1 to 11, characterised in that the smallest free distance between two spots on the surface (33, 45) is not less than 0.7 mm.
13. Packaging paper according to one of claims 1 to 12, characterised in that the network structure (32, 42) forms a regularly repeating pattern.
14. Packaging paper according to one of claims 1 to 13, characterised in that the spots on the surface (33, 45) are the same shape and size.
15. Packaging paper according to one of claims 1 to 14, characterised in that the spots on the surface (33, 45) are circular, oval, rectangular or quadrangular.
16. Packaging paper according to one of claims 1 to 15, characterised in that the network structure (32, 42) has a regular form.
17. Packaging paper according to one of claims 1 to 16, characterised in that the network structure (42) forms two groups of parallel network lines (43, 44), in which one group of parallel network lines (43) runs at an angle to the other group of parallel network lines (44).
18. Packaging paper according to claim 17, characterised in that the angle is 90°.
19. Packaging paper according to claim 17 or 18, characterised in that the network lines (43, 44) of at least one group are at the same distance from each other.
20. Packaging paper according to one of claims 1 to 16, characterised in that the network structure (32) is made so that the spots on the surface (33) form rows (34) running next to each other and columns (35) of spots on the surface (33) running next to each other vertical to the rows (34), in which the spots on the surface (33) of adjacent rows (34) or columns (35) are offset by half the distance between two adjacent spots on the surface (33).
21. Packaging paper according to claim 20, characterised in that the spots on the surface (33) are on diagonals (36) parallel to each other.
22. Packaging paper according to claim 21, characterised in that the diagonals (36) enclose an angle of 15° to 75° with the rows of spots on the surface (34).
23. Packaging paper according to one of claims 1 to 22, characterised in that the thickness of the packaging paper (21, 41) in the area of the network structure (32, 42) and also in the area of the spots on the surface (33, 45) is essentially the same.
24. Packaging paper according to one of claims 1 to 23, characterised in that the thickness of the packaging paper (31, 41) is at least 0.05 mm.
25. Packaging paper according to one of claims 1 to 24, characterised in that the thickness of the packaging paper (31, 41) is at most 0.2 mm.
26. Paper sack for transporting bulk material, in which the walls of the paper sack are made of packaging paper, characterised in that the packaging paper (31, 41) is made according to one of claims 1 to 25.
27. Method for producing packaging paper (31, 41), in which in the wet part of a paper machine a fibre pulp with paper fibres is applied through a material outlet onto a circulating sheet forming screen (1) with a sheet forming side and a machine side and is dehydrated through the sheet forming screen (1), characterised in that a sheet forming screen (1) is used with a permeable carrier (2), which has a pattern of discrete spots on the surface (7) made of covering material, the permeability of which is less than that of the surface areas surrounding the spots on the surface (7) forming a network structure (8).
28. Method according to claim 27, characterised in that a sheet forming screen (1) is used, in which the distance of the spots on the surface (7) is increased in the longitudinal and/or transverse direction by the degree of contraction, which the packaging paper (31, 41) undergoes in its production.
29. Method according to claim 27 or 28, characterised in that the diameter of circular spots on the surface (33) and/or the envelope surrounding the spots on the surface (45) as closely as possible is at most 2.5 times the average length evaluated fibre length of the paper fibres, from which the packaging paper (31, 41) is produced.
30. Method according to claim 29, characterised in that the diameter of circular spots on the surface (33) and/or the envelope surrounding the spots on the surface (45) as closely as possible is at least 0.1 times the average length evaluated fibre length of the paper fibres, from which the packaging paper (31, 41) is produced.
31. Method according to one of claims 27 to 30, characterised in that the spots on the surface (33, 45) occurring in the packaging paper (31, 41) are circular, oval and/or polygonal.
32. Method according to one of claims 27 to 31, characterised in that the fibre pulp used to apply to the sheet forming screen (1) has a material density of 0.1 to 5 g/l.
33. Method according to one of claims 27 or 32, characterised in that the fibre pulp is applied in several layers, but from a single material outlet nozzle, onto the sheet forming screen (1).
34. Method according to one of claims 27 to 33, characterised in that the paper fibres are made flexible before applying to the sheet forming screen (1) by means of high consistency grinding.
35. Method according to claim 34, characterised in that the paper fibres are made flexible with a dry content of 20% to 38% by high consistency grinding.
36. Method according to claim 34 or 35, characterised in that the paper fibres are made flexible at a pH value of 8 to 12 by high consistency grinding.
37. Method according to one of claims 34 to 36, characterised in that the paper fibres are made flexible with an energy input of 20 to 500 kWh/t by high consistency grinding.
38. Method according to one of claims 34 to 37, characterised in that the paper fibres are subject to a kneading and crimping process in a high consistency refiner by high consistency grinding.
39. Method according to one of claims 27 to 38, characterised in that the paper fibres with a dry content of 2.5 to 7% are ground in a collapsing and/or fibrillating way.
40. Method according to one of claims 27 to 39, characterised in that the paper fibres with a dry content of 7 to 20% are ground in a collapsing and/or fibrillating way.
41. Method according to one of claims 27 to 40, characterised in that the sheet forming screen (1) is shaken.
42. Method according to claim 41, characterised in that shaking is normally done in the running direction of the sheet forming screen (1).
43. Method according to claim 41 or 42, characterised in that shaking is carried out with a frequency of up to 1000 Hz.
44. Method according to one of claims 41 to 43, characterised in that shaking is done with a shaking stroke of up to 100 mm.

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