MG kraft paper

Abstract

There is provided method of producing a machine-glazed (MG) kraft paper in a paper machine, wherein the paper is having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of 40-65 g / m2 according to ISO 536:2020, wherein - the paper machine comprises a press section and a drying section; - the press section as well as the drying section comprises cylinders; - the paper machine comprises a first open draw between a first and a second cylinder; - the first cylinder is arranged upstream of the second cylinder; and - the second cylinder rotates with a relative speed being 0.5-2.5 % faster than the first cylinder.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of machine glazed (MG) papers, and in particular to MG kraft paper for packaging of sharp-edged products.BACKGROUND

[0002] Traditionally, many items have been packed in plastic packaging due to price and material properties. However, due to environmental concerns, there is a strive to replace the plastics with paper-based materials.

[0003] One such type of packaging is packaging of sharp-edged objects, such as foods, e.g. pasta and rice, or non-foods, e.g. plastic pieces.

[0004] One problem is thus to provide a paper which can be used for packaging of sharp-edged products.SUMMARY

[0005] The present disclosure aims to provide a paper-based material that can replace plastics in packaging of sharp-edged products. The inventors have realized that such paper benefits from a high sufficient puncture resistance and that the stretchability in machine direction (MD) is linked to the puncture resistance. Thereby, the paper should have a sufficiently high stretchability in MD. Moreover, the paper should have folding properties, i.e. being flexible and operable in packing lines. 1. Accordingly, as a first aspect of the present disclosure there is provided a method of producing a machine-glazed (MG) kraft paper in a paper machine, wherein the paper is having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of 40-65 g / m 2< according to ISO 536:2020, wherein the paper machine comprises a press section and a drying section; the press section as well as the drying section comprises cylinders; the paper machine comprises a first open draw between a first and a second cylinder; the first cylinder is arranged upstream of the second cylinder; and the second cylinder rotates with a relative speed being 0.5-2.5 % faster than the first cylinder.

[0006] According to a second aspect of the present disclosure, there is provided an MG kraft paper having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of the paper according to ISO 536:2020 is 40-65 g / m 2< .

[0007] According to a third aspect of the present disclosure, there is provided use of the MG kraft paper according the second aspect of the present disclosure in packaging.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a schematic illustration of an embodiment, wherein a portion 100 of a drying section of a paper machine is shown. Fig. 2 is a schematic illustration of an embodiment, wherein a drying section 200 of a paper machine is shown. DETAILED DESCRIPTION

[0009] As a first aspect of the present disclosure, there is provided a method of producing a machine-glazed (MG) kraft paper in a paper machine, wherein the paper is having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of 40-65 g / m 2< according to ISO 536:2020, , wherein the paper machine comprises a press section and a drying section; the press section as well as the drying section comprises cylinders; the paper machine comprises a first open draw between a first and a second cylinder; the first cylinder is arranged upstream of the second cylinder; and the second cylinder rotates with a relative speed being 0.5-2.5 % faster than the first cylinder.

[0010] A full-scale paper machine comprises a machine chest, an approach flow system, a headbox, a wire section, a press section, a dryer section and a reeler. In the wire section a paper web is formed from the headbox on a forming wire. In the press section the formed web is mechanically dewatered under pressure. In the dryer section, the paper is dried. A full-scale paper machine operates at a wire speed of several hundreds of meters per minute, such as at least 400 m / min.

[0011] For an MG paper, the dryer section includes a Yankee cylinder. The Yankee cylinder is a polished metal cylinder sometimes referred to as a MG cylinder used for drying the paper web in the MG papermaking machine. An MG paper has glazed side and a non-glazed side. The glazed side is the side that faced the Yankee cylinder. The contact with the polished metal surface during drying makes the glazed side smoother than the non-glazed side. The smooth, glazed side, is typically printed. The glazed side may be coated with a thin layer of starch (≤ 1 g / m 2< ) for curl prevention. A lacquer may be provided on the optional print, e.g. to modify gloss, friction and / or release properties.

[0012] The press section as well as the drying section comprises cylinders. In case the web between cylinders is unsupported, i.e. without any lifting support from a cylinder, the web is thereby in an open draw. That is, an unsupported web is called an open draw. To pull the web downstream in the paper machine the cylinder downstream of an open draw pulls the web. This is done by that the rotational speed of the cylinder downstream of an open draw is faster than the rotational speed of the cylinder upstream of the open draw.

[0013] As understood by the skilled person, the term downstream refers to a process step further down in the process, i.e. closer to the end of the process, whereas the term upstream refers to a process step further up in the process, i.e. closer to the start of the process. Accordingly, upstream of the press section is the wire section, and downstream of the press section is the dryer section.

[0014] The paper machine comprises a first open draw between a first and a second cylinder and in that first open draw the second cylinder arranged downstream of the first cylinder rotates with a speed being higher than that of the first cylinder. For the avoidance of doubt, the first cylinder as well as the second cylinder are cylinders referred to when defining that the press section as well as the drying section of the paper machine comprises cylinders, i.e. the first cylinder as well as the second cylinder are arranged in the press section and / or the drying section of the paper machine.

[0015] The inventors have realized that if the difference in speed is in accordance with the present disclosure, an MG kraft paper is provided with improved resistance from breaking by sharp-edged objects. Moreover, such paper is also flexible due to its grammage. The inventors have realized that by running the cylinders with such difference in rotational speed the strain at break in MD is improved and that in turn the strain at break correlates with puncture resistance of the MG kraft paper.

[0016] Preferably, the first open draw is the first open draw of the paper machine, i.e. that there are no open draws arranged upstream of the first open draw in the paper machine. Such arrangement is beneficial since the web still contains a significant of moisture if it has not passed any open draws prior to the first open draw. The inventors have realized that the effect of obtaining an increased strain at break is more pronounced if the web contains a lower dry content in the first open draw. Since the effect of increased stretch is reduced by increased dry content of the web it is thereby the first open draw of the paper machine that makes the most important contribution.

[0017] The first open draw is provided in the press section or the drying section. Typically, the first open draw is provided between the first cylinder of the drying section and the second cylinder of the drying section, wherein the second cylinder of the drying section is arranged downstream of the first cylinder of the drying section. The first cylinder of the drying section can also be referred to as a pre-drying cylinder. Alternatively, the first open draw is provided in the press section. The beneficial effect of that the first open draw is the first open draw of the paper machine is provided independently of if the first open draw is in the press section or in the drying section.

[0018] Typically, the dry content of the web in the first open draw is 30-45%, in particular if the first open draw is provided in the drying section.

[0019] The drying section is not limited to a certain type of wire. There can also be a single wire drying section or double wire drying section.

[0020] In the first open draw, the second cylinder rotates with a relative speed typically being 1.0-2.5 %, such as 1.0-2.0%, such as 1.2-2.0 %, faster than the first cylinder.

[0021] To even further improve the effect of increased stretch in MD and thereby resistance of breaking from sharp-edged objects by the MG kraft paper the paper machine may further comprise a second open draw. In such case; the second open draw is arranged downstream of the first open draw; the second open draw is between a third cylinder and a fourth cylinder; the third cylinder is arranged upstream of the fourth cylinder; and the fourth cylinder rotates with a relative speed being 0.05-0.5 % faster than the third cylinder.

[0022] The paper machine may further comprise a third open draw arranged downstream of the second open draw, wherein: the third open draw is arranged downstream of the second open draw; the third open draw is between a fifth cylinder and a sixth cylinder; the fifth cylinder is arranged upstream of the sixth cylinder; and the sixth cylinder rotates with a relative speed being 0.05-0.5 % faster than the fifth cylinder.

[0023] The first and / or second cylinder as well as second / and or third cylinder as well as fourth and / or fifth cylinder may form part of a group of cylinders, wherein all cylinders within the group rotates with the same rotational speed. By a group of cylinders is meant at least two cylinders. A typical upper limit is eight cylinders. In such case, the difference in rotational speed according to the present disclosure is between the last cylinder of the group upstream and the group downstream of respective open draw in the paper machine. Preferably, the first open draw is the first open draw in the paper machine and thereby it is preferred that the first cylinder is a single cylinder that is not provided in a group of cylinders. In embodiments, the sixth cylinder is the Yankee cylinder.

[0024] The MG kraft paper is typically calendered downstream of the press section. By calendering the paper, a smoother surface better adapted for printing is provided. One advantage is that such paper is having both an improved stretch at break and a smooth surface with beneficial printing properties.

[0025] The MG kraft paper typically has one or more of the following properties: a stretchability according to ISO 1924-3:2011 in MD of 1.8-2.5 %, such as 1.9-2.5 %; a grammage according to ISO 536:2020 of 40-60 g / m 2< , such as 43-60 g / m 2< , such as 43-57 g / m 2< ; a Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper of 400 ml / min or lower, such as 300 ml / min or lower, such as 200 ml / min or lower, such as 80 ml / min or lower; tensile energy absorption (TEA) in MD according to ISO 1924-3:2011 of at least 60 J / m 2< , such as at least 65 J / m 2< ; burst strength according to ISO 2758:2014 of at least 200 kPa, such as at least 220 kPa; puncture work according to EN-14477:2004 of at least 0.80 mJ, such as at least 0.86 mJ; puncture force according to EN-14477:2004 of at least 3.7 N.

[0026] Fig. 1 shows an exemplary embodiment of the present disclosure. In Fig. 1 a portion 100 of a drying section of a paper machine is shown. There are four cylinders shown 101-104. Between cylinder 101 and 102 there is a first open draw 105. Between cylinder 102 and 103 there is a second open draw 106. Between cylinder 103 and 104 there is a third open draw 107. A paper web 108 runs through the drying section. The rotational speed of cylinder 102 is higher than the rotational speed of cylinder 101 with a percentage according to the present disclosure. The rotational speed of cylinder 103 is higher than the rotational speed of cylinder 102. The rotational speed of cylinder 104 is higher than the rotational speed of cylinder 103. Thereby, the web 108 is pulled downstream in the portion 100 of the drying section.

[0027] Fig. 2 shows another exemplary embodiment of the present disclosure. In Fig. 2 a drying section 200 of a paper machine is shown. The drying section 200 contains a pre-drying cylinder 201; a first group of drying cylinders 201-201‴; a second group of drying cylinders 203-203" and a Yankee cylinder 204. Between cylinder 201 and 202 there is a first open draw 205. The rotational speed of cylinders 201-201‴ is the same. Between cylinder 202‴ and 203 there is a second open draw 206 where there is a difference in rotational speed of the cylinders at the ends of the open draw. Between cylinder 203" and 204 there is a third open draw 207 where there is a difference in rotational speed of the cylinders at the ends of the open draw. A paper web 208 runs through the drying section. The rotational speed of cylinder 202 is higher than the rotational speed of cylinder 201 with a percentage according to the present disclosure. The rotational speed of cylinder 203 is higher than the rotational speed of cylinder 202‴. The rotational speed of cylinder 204 is higher than the rotational speed of cylinder 203". Thereby, the web 208 is pulled downstream in the drying section 200 of the paper machine.

[0028] As a second aspect of the present disclosure there is provided an MG kraft paper having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of the paper according to ISO 536:2020 is 40-65 g / m 2< .

[0029] The MG kraft paper is typically having a stretchability according to ISO 1924-3:2011 in MD of 1.8-2.5 %, such as 1.9-2.5 %. The inventors have realized that an improvement in strain at break in MD is beneficial for the resistance to breakage of sharp-edged objects.

[0030] The stretch at break in the cross direction (CD) according to ISO 1924-3:2011 is typically at least 3.4 %, such as 3.4-6 %, such as 3.5-5.5%.

[0031] The grammage of the paper according to ISO 536:2020 is typically 40-60 g / m 2< , such as 43-60 g / m 2< , such as 43-57 g / m 2< . Such grammage is beneficial for flexibility.

[0032] The Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is typically 400 ml / min or lower, such as 300 ml / min or lower, such as 200 ml / min or lower. A fine surface, i.e. a surface of low roughness, is beneficial for printing properties. Thereby, a paper having both improved stretch at break and a smooth surface with beneficial printing properties is provided. The paper may have been calendered and an even smoother paper with still maintained stretch at break is thereby provided. Typically, a Bendtsen roughness of 80 ml / min or lower is in such case provided.

[0033] The tensile energy absorption (TEA) in MD of the paper according to ISO 1924-3:2011 is typically at least 60 J / m 2< , such as at least 65 J / m 2< . Such TEA in MD is beneficial for resistance to breakage of the paper from sharp-edged objects.

[0034] TEA index in MD of the paper according to ISO 1924-3:2011 is typically at least 12.5 J / g, such as at least 13.0 J / g.

[0035] Burst strength of the paper according to ISO 2758:2014 is typically at least 200 kPa, such as at least 220 kPa. Burst index according to ISO 2758:2014 is typically at least 4.5 kN / g.

[0036] Puncture work of the paper according to EN-14477:2004 is typically at least 0.80 mJ, such as at least 0.86 mJ. Puncture work index according to EN-14477:2004 is typically at least 17.0 Jm 2< / g, such as at least 17.5 Jm 2< / g. Puncture force of the paper according to EN-14477:2004 is typically at least 3.7 N. Preferably, a testing speed of 10 mm / min is used when performing EN-14477:2004. Typically, the above listed properties in terms of puncture work, puncture work index and puncture force are provided on at least one side of the paper.

[0037] TEA, burst strength and puncture resistance are all mechanical properties relevant for resistance to breakage by sharp-edged objects. Burst strength is a measurement for determining resistance to breakage when grabbing a package made from paper with the hand of a consumer as burst strength measures resistance to puncture with a "softer" test probe as compared with e.g. puncture resistance that is a more pointy and thereby "harder" test probe, and thereby more similar to sharp-edged objects.

[0038] TEA in CD of the paper according to ISO 1924-3:2011 is typically at least 70 J / m 2< .

[0039] TEA index in CD of the paper according to ISO 1924-3:2011 is typically at least 14.0 J / g, such as at least 14.5 J / g. Such TEA will further ensure the durability of the paper.

[0040] The density measured according to ISO 534:2011 of the MG kraft paper is typically 750-900 kg / m 3< .

[0041] Tensile strength in MD according to ISO 1924-3:2011 is typically at least 4.5 kN / m. Tensile strength in CD according to ISO 1924-3:2011 is typically at least 2.5 kN / m.

[0042] The air permeance measured according to ISO 5636-5:2013 of the MG kraft paper is typically at least 25 s.

[0043] The examples and embodiments discussed above in connection to the first aspect apply to the second aspect mutatis mutandis.

[0044] As a third aspect of the present disclosure, there is provided use of the MG kraft paper according the second aspect of the present disclosure in packaging.

[0045] When the paper is used for packaging, the packaging is typically produced via form fill sealing (FFS) of the paper, such as vertical form fill sealing (VFFS) or horizontal form fill sealing (HFFS), or via sealing of a self-opening satchel (SOS) bag of the paper.

[0046] Examples of packaging formed via form fill sealing (FFS) are a gusseted packaging or a pillow packaging, having a longitudinal seal and each end portion sealed by a fin seal. FFS is either produced as vertical form fill sealing (VFFS) or horizontal form fill sealing (HFFS).

[0047] A filled gusseted paper packaging has a longitudinal seal adhering two overlapping ends of the paper material to each other to form a lap seal. Further, the packaging has a top end sealed by a fin seal and a bottom end sealed by a fin seal.

[0048] A filled pillow paper package has a longitudinal seal adhering two overlapping ends of the paper material to each other to form a lap seal. Further, the packaging has a top end sealed by a fin seal and a bottom end sealed by a fin seal.

[0049] A self-opening satchel (SOS) bag is typically delivered in a folded form a bag machine. To fill it, it is opened fully and, lastly filled and sealed. An SOS bag, when fully opened, is free-standing. Products packed in these bags comprise basic foodstuffs, for example flour, sugar, cereals, tea, coffee, biscuits and confectionery, but also food mixes for both retail and catering requirements. Numerous other industries are also served, including cat litter, small pet-food packs, agrochemicals and horticulture.

[0050] The examples and embodiments discussed above in connection to the first and second aspect apply to the third aspect mutatis mutandis.EXAMPLES Production of MG kraft papers

[0051] MG kraft papers were produced from webs in a paper machine. The paper machine comprised a forming section with a wire, a press section, and a drying section comprising a Yankee cylinder. In the drying section there were in total three open draws having a difference in rotational speed of the cylinders on each side of the open draw. In each of the open draws the cylinder arranged downstream pulls the web, i.e. the speed with which the cylinder arranged downstream rotates of each open draw is higher than the rotational speed of the cylinder arranged upstream of each open draw.

[0052] The first open draw, being the first open draw of the paper machine, was between the first cylinder of the drying section, also referred to as a pre-drying cylinder, and a second cylinder downstream of the first cylinder. The second cylinder formed part of a group of cylinders each rotating with the same rotational speed. Between the last cylinder downstream in the group where the second cylinder was the first cylinder and a third cylinder forming part of a second group of cylinders, the second open draw having a difference in rotational speed of the cylinders on each side of the open draw was provided. The third cylinder formed part of a group of cylinders each rotating with the same rotational speed. The third open draw was provided between the last cylinder downstream in the group where the third cylinder was the first cylinder and the Yankee cylinder.

[0053] Two types of MG kraft papers were produced; a comparative example (CE) wherein the relative difference in rotational speed is higher in the three open draws and an inventive example wherein the relative difference in rotational speed is lower in the three open draws. On both types of MG kraft paper, the stretch at break in machine direction (MD) was evaluated using an L&W Autoline equipment (ABB). The positive relative difference in rotational speed means that the cylinder downstream rotates faster than the cylinder upstream of each open draw.

[0054] The dry content in the first open draw was not measured, but it was estimated to be in the range of 30-45 % by dry weight as the web had not been dewatered upstream of the first open draw in the drying section. The papers were not calendered.

[0055] In Table 1 below the results are presented. Relative rotational speed between cylinder downstream and upstream of each open draw (%) Paper Stretch MD (%) First open draw Second open draw Third open draw CE1.43.060.320.34CE1.53.060.320.34CE1.53.050.340.33IE1.91.590.160.22IE1.91.610.140.22IE1.91.610.140.21IE1.91.600.150.22IE1.91.610.150.21IE1.91.590.160.22CE1.31.900.320.34

[0056] The produced papers were then further evaluated in a testing lab. The results are presented in Table 2 below. That the stretch in MD was observed to be slightly higher in the testing lab is believed to be due to the difference in equipment; in the standard ISO 1924-3:2011 a tensile tester is used instead of the L&W Autoline equipment. Nevertheless, that the stretchability in MD is improved is clear. Table 2. Properties of an inventive example (IE) of the MG kraft paper produced as well as the properties of the comparative example (CE) of an MG kraft paper.PropertyUnitStandard methodValue IEValue CEGrammageg / m 2< ISO 536:20205050Tensile Strength MDkN / mISO 1924-3:20115.05.2Tensile Strength CDkN / mISO 1924-3:20113.02.9Stretch at break MD%ISO 1924-3:20112.11.7Stretch at break CD%ISO 1924-3:20113.83.8TEA MDJ / m 2< ISO 1924-3:201168.357.9TEA CDJ / m 2< ISO 1924-3:201177.677.6TEA Index MDJ / gISO 1924-3:201113.711.6TEA Index CDJ / gISO 1924-3:201115.515.5Bendtsen Roughness glazed sideml / minISO 8791-2:2013135126Bendtsen Roughness non-glazed sideml / minISO 8791-2:201310311038Burst strengthkPaISO 2758:2014229218Burst indexkN / gISO 2758:20144.64.4Puncture work a< mJEN-14477: 20040.940.79Puncture work index a< Jm 2< / gEN-14477: 200418.715.7Puncture force a< NEN-14477: 20043.83.5 a< Measured using a test speed of 10 mm / min.

[0057] As seen in the results in Table 2, as a consequence of the decrease in difference in rotational speed of the cylinders in the end of an open draw, the stretch at break increases in MD. Moreover, not only stretch at break, but also other properties, such as burst strength, puncture work and TEA in MD. Hence, the inventors have realized that by the particular adjustment of the open draws of the paper machine, a paper with properties desirable for packing of sharp-edged objects is provided.

[0058] Moreover, also the surface roughness is suitably low, which is beneficial for printing even though the papers had not been calendered. Accordingly, a paper both with stretching properties as well as printing properties is provided.

Claims

1. A method of producing a machine-glazed (MG) kraft paper in a paper machine, wherein the paper is having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of 40-65 g / m2 according to ISO 536:2020, wherein - the paper machine comprises a press section and a drying section; - the press section as well as the drying section comprises cylinders; - the paper machine comprises a first open draw between a first and a second cylinder; - the first cylinder is arranged upstream of the second cylinder; and - the second cylinder rotates with a relative speed being 0.5-2.5 % faster than the first cylinder.

2. The method of claim 1, wherein the second cylinder rotates with a relative speed being 1.0-2.5 %, such as 1.0-2.0%, such as 1.2-2.0 %, faster than the first cylinder.

3. The method of claim 1 or 2, wherein the paper machine further comprises a second open draw, wherein: - the second open draw is arranged downstream of the first open draw; - the second open draw is between a third cylinder and a fourth cylinder; - the third cylinder is arranged upstream of the fourth cylinder; and - the fourth cylinder rotates with a relative speed being 0.05-0.5 % faster than the third cylinder.

4. The method of claim 3, wherein the paper machine further comprises a third open draw arranged downstream of the second open draw, wherein: - the third open draw is arranged downstream of the second open draw; - the third open draw is between a fifth cylinder and a sixth cylinder; - the fifth cylinder is arranged upstream of the sixth cylinder; and - the sixth cylinder rotates with a relative speed being 0.05-0.5 % faster than the fifth cylinder.

5. The method of any one of the preceding claims, wherein the dry content of the web in the first open draw is 30-45%.

6. The method of any one of the preceding claims, wherein there are no open draws arranged upstream of the first open draw in the paper machine.

7. The method of any one of the preceding claims, wherein the paper is calendered downstream of the press section.

8. An MG kraft paper having a stretchability according to ISO 1924-3:2011 of 1.8-2.8 % in the machine direction (MD) and a grammage of the paper according to ISO 536:2020 is 40-65 g / m2.

9. The MG kraft paper of claim 8, wherein the stretchability according to ISO 1924-3:2011 in MD of the paper is 1.8-2.5 %, such as 1.9-2.5 %.

10. The MG kraft paper of claim 8 or 9, wherein the grammage of the paper according to ISO 536:2020 is 40-60 g / m2, such as 43-60 g / m2, such as 43-57 g / m2.

11. The MG kraft paper of any one of the claims 8-10, wherein Bendtsen roughness according to ISO 8791-2:2013 of at least one side of the paper is 400 ml / min or lower, such as 300 ml / min or lower, such as 200 ml / min or lower, such as 80 ml / min or lower.

12. The MG kraft paper of any one of the claims 8-11, wherein tensile energy absorption (TEA) in MD of the paper according to ISO 1924-3:2011 is at least 60 J / m2, such as at least 65 J / m2.

13. The MG kraft paper of any one of the claims 8-12, wherein burst strength of the paper according to ISO 2758:2014 is at least 200 kPa, such as at least 220 kPa.

14. The MG kraft paper of any one of the claims 8-13, wherein puncture work of the paper according to EN-14477:2004 is at least 0.80 mJ, such as at least 0.86 mJ.

15. Use of the MG kraft paper according to any one of the claims 8-14 in packaging.

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