Method for producing multi-layer machine-made glossy paper containing highly refined cellulose fibers and multi-layer machine-made glossy paper produced

Abstract

The present invention relates to a method for manufacturing a multi-layer machine-made glazed paper comprising highly refined cellulose fibers, the method comprising the steps of forming a first wet web by applying at least one first pulp suspension comprising highly refined cellulose fibers on a first wire; partially dewatering the first wet web to obtain a first partially dewatered web; forming a second wet web by applying at least one second pulp suspension comprising highly refined cellulose fibers on a second wire; partially dewatering the second wet web to obtain a second partially dewatered web; joining the first and second partially dewatered webs to obtain a multi-layer web; optionally dewatering the multi-layer web in a dewatering unit, and glazing the multi-layer web in at least one glazing unit to obtain a multi-layer machine-made glazed paper comprising highly refined cellulose fibers. The present invention also relates to a multi-layer MG paper produced according to the method.

Description

Technical Field

[0001] The present disclosure relates to methods for producing multi-layer glazed (MG) paper comprising highly refined cellulose fibers, in particular multi-layer MG paper comprising microfibrillated cellulose (MFC). Background Art

[0002] Mechanically glazed (MG) papers are papers used in label papers, special printing applications and in different food and hygiene packaging applications. Usually, one surface of the paper is glazed, i.e. treated in a way that increases the gloss of the surface of the paper. Glazing of at least one surface of the paper is done in order to provide the paper with improved gloss and increased surface density without losing too much bulk. The glazed surface improves the barrier properties, especially the improved barrier to grease and oil, and it imparts improved printing properties to the surface. In addition to having good barrier properties, it is important that MG papers also have good mechanical strength to cope with the high demands in different end-use applications.

[0003] Microfibrillated cellulose (MFC) is known to be used as a strength additive or barrier additive in the production of paper or paperboard products. However, MFC has a very high water binding capacity and it is therefore very difficult to reduce the water content of pulps containing microfibrillated cellulose and the dewatering requirements for products containing large amounts of MFC are very high. It is therefore difficult to dewater products containing large amounts of MFC without deteriorating the mechanical or barrier properties of the product.

[0004] During the production of machine-coated paper it is important to improve the runnability of the paper.By adding barrier or strength additives to the paper there is a risk of lifting or blistering of the web during drying and coating.

[0005] There is therefore a need for a new method to produce improved MG paper with good strength and barrier properties in an efficient manner. Summary of the invention

[0006] It is an object of the present disclosure to provide a method of producing machine-finished paper comprising highly refined cellulose fibers, such as microfibrillated cellulose (MFC), which alleviates at least some of the above-mentioned problems associated with prior art methods.

[0007] Another object of the present disclosure is to provide a method for producing machine-made glazed paper comprising highly refined cellulose fibers having improved strength and barrier properties in an efficient manner.

[0008] Another object of the present disclosure is to provide an improved process for manufacturing multi-layer MG paper comprising highly refined cellulose fibers in a paper or board machine type process.

[0009] Another object of the present disclosure is to provide a multi-layer machine-glazed paper which is strong and can be used as a barrier packaging material based on renewable raw materials.

[0010] The above-mentioned objects, as well as other objects that a skilled person will recognize based on the present disclosure, are achieved through various aspects of the present disclosure.

[0011] According to a first aspect described herein, there is provided a method for producing a multi-layer machine-made glazed paper comprising highly refined cellulose fibers, the method comprising the following steps:

[0012] a) forming a first wet web by applying at least one first pulp suspension comprising highly refined cellulose fibers on a first wire;

[0013] b) partially dewatering the first wet web to obtain a first partially dewatered web;

[0014] c) forming a second wet web by applying at least one second pulp suspension comprising highly refined cellulose fibers on a second wire;

[0015] d) partially dewatering the second wet web to obtain a second partially dewatered web;

[0016] e) joining the first and second partially dewatered webs to obtain a multilayer web;

[0017] f) optionally dewatering the multilayer web in a dewatering unit, and

[0018] g) Glazing the multilayer web in at least one glazing unit to obtain a multilayer machine-glazed paper comprising highly refined cellulose fibers.

[0019] The term machine-made glossy paper as used herein generally refers to a paper product having at least one glossy surface. The machine-made glossy paper preferably has a glossy surface of 25-160 g / m 2 Weight within the range.

[0020] The process of the invention allows the manufacture of multi-layer machine-finished papers comprising highly refined cellulose fibers in a paper machine type process. By the present invention it was found that multi-layer MG papers comprising highly refined fibers, preferably microfibrillated cellulose, having improved strength and barrier properties can be produced in a more efficient manner.

[0021] The manufacturing method comprises the separate preparation and partial dewatering of at least two webs, both of which have a lower grammage than the final multi-layer MG paper. The partially dewatered but still wet webs are joined to form a higher grammage multi-layer web, which is then optionally further dewatered and dried to obtain a drier multi-layer web. Joining the webs while they are still wet ensures good adhesion between the layers. In fact, if the composition of the two layers is the same, the resulting multi-layer paper may even be difficult to distinguish from a single-layer paper of corresponding thickness. It has been found that partial dewatering and lamination of the webs in a partially dewatered state substantially eliminates the occurrence of pinholes in the final multi-layer paper while still allowing high production speeds. In the prior art, the dewatering speed is sometimes increased by using large amounts of retention and drainage chemicals at the wet end of the process, resulting in increased flocculation. However, retention and drainage chemicals can also lead to more porous web structures, so it is necessary to minimize the use of such chemicals. The method of the present invention provides an alternative method for increasing the dewatering speed, which is less dependent on the addition of retention and drainage chemicals. The joined multilayer web is subsequently glazed in at least one glazing unit to obtain a multilayer machine-glazed paper comprising highly refined cellulose fibers.

[0022] A paper machine (or paper making machine) is an industrial machine used in the pulp and paper industry to produce paper in large quantities at high speeds. Modern paper making machines are generally based on the principle of a Fourdrinier machine, which uses a moving woven mesh, a "wire", to produce a continuous web by filtering out the fibers held in the pulp suspension and producing a continuously moving wet web of fibers. The wet web is dried in the machine to produce a solid paper web.

[0023] The forming, dewatering and joining steps of the method of the invention are preferably carried out in the forming section of a paper machine, usually called the wet end. The wet web is formed on different wires in the forming section of the paper machine. A preferred type of forming section for the invention comprises 2 or 3 Fourdrinier wire sections, combined with a supporting wire. The wires are preferably endless wires. The wires used in the method of the invention preferably have a relatively high porosity to allow rapid dewatering and high drainage capacity.

[0024] At least one first pulp suspension and at least one second pulp suspension are aqueous suspensions comprising a water-suspended mixture of cellulose-based fibrous material and optional non-fibrous additives. The method of the present invention uses a pulp suspension comprising highly refined cellulose fibers. Refining or beating of cellulose pulp refers to mechanical treatment and modification of cellulose fibers to provide them with desired properties. Highly refined cellulose fibers can be produced from different raw materials, such as softwood pulp or hardwood pulp. Highly refined cellulose fibers are preferably cellulose fibers that have never been dried.

[0025] The term highly refined cellulose fibers as used herein preferably refers to refined cellulose fibers having a Schopper-Riegler (SR) value of 65 or more, preferably 70 or more, preferably above 85, preferably between 75-100 or even more preferably between 85-99, the Schopper-Riegler (SR) value being determined according to standard ISO 5267-1.

[0026] The dry solids content of the first and / or second pulp suspension is typically in the range of 0.1-0.7 wt%, preferably in the range of 0.15-0.5 wt%, more preferably in the range of 0.2-0.4 wt%.

[0027] The first and / or second pulp suspension comprises a mixture of highly refined cellulose fibers, unrefined or slightly refined fibers and optionally other ingredients or additives. In some embodiments, the first and / or second pulp suspension comprises at least 0.1 wt%, preferably at least 2 wt%, more preferably at least 5 wt% or at least 10 wt% of highly refined cellulose fibers, based on the total dry weight of the pulp suspension. Preferably, the first and / or second pulp suspension comprises 0.1-50 wt%, preferably 2-40 wt% or 5-30 wt% or even more preferably 10-25 wt% of highly refined fibers. The highly refined fibers may be produced from bleached pulp to produce a white paper product or from unbleached pulp to produce a brown paper product.

[0028] In some embodiments, the highly refined cellulose fibers of the first and / or second pulp suspension are refined Kraft pulp. Refined Kraft pulp typically contains at least 10% hemicellulose. Thus, in some embodiments, the first and / or second pulp suspension contains hemicellulose in an amount in the range of 10-25% of the amount of highly refined cellulose fibers.

[0029] The first and / or second pulp suspension may also contain additives such as natural starch or starch derivatives, cellulose derivatives such as sodium carboxymethylcellulose, fillers, retention and / or drainage chemicals, flocculation additives, deflocculation additives, dry strength additives, softeners, cross-linking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, defoaming aids, microorganisms and slime control aids, or mixtures thereof. The first and / or second pulp suspension may further contain additives that will improve different properties of the mixture and / or the produced paper, such as latex and / or polyvinyl alcohol (PVOH). The method of the present invention provides an alternative method for increasing the dewatering rate that is less dependent on the addition of retention and drainage chemicals, but can still use smaller amounts of retention and drainage chemicals.

[0030] Highly refined fibers are preferably microfibrillated cellulose (MFC). Microfibrillated cellulose (MFC) in the context of this patent application should be understood to mean nanoscale cellulose particle fibers or fibrils having at least one dimension less than 1000 nm. MFC comprises partially or fully fibrillated cellulose or lignocellulose fibers. The released fibril diameter is less than 100 nm, while the actual fibril diameter or particle size distribution and / or aspect ratio (length / width) depends on the source and manufacturing method. The smallest fibrils are called primary fibrils and have a diameter of about 2-4 nm (see, e.g., Chinga-Carrasco, G., Cellulose fibers, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fiber technology point of view, Nanoscale research letters 2011, 6: 417), although aggregated forms of primary fibrils, also defined as microfibrils (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3.), are common and are the main products obtained when making MFC, for example by using an extended refining process or a pressure drop decomposition process. Depending on the source and the manufacturing process, the length of the fibrils can vary from about 1 micron to more than 10 microns. Crude MFC grades may contain a large amount of fibrillated fibers, i.e., fibrils (cellulose fibers) protruding from the tracheids, and a certain amount of fibrils (cellulose fibers) released from the tracheids.

[0031] MFC has different acronyms, such as cellulose microfibril, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibrils, cellulose nanofibrils, cellulose microfibrils, cellulose fibrils, microfibril cellulose, microfibril aggregates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physicochemical properties, such as its large surface area or the ability to form a gel-like material at low solids (1-5 wt%) when dispersed in water.

[0032] There are multiple methods for making MFC, such as single or multi-pass refining, pre-hydrolysis and then refining or high shear decomposition or fibril release. In order to make MFC manufacturing both energy-saving and sustainable, one or more pretreatment steps are usually required. Therefore, the cellulose fibers of the pulp to be used can be pretreated, such as enzymatic or chemical treatment, to hydrolyze or swell the fibers or reduce the amount of hemicellulose or lignin. Cellulose fibers can be chemically modified before fibrillation so that the cellulose molecules contain (or more) functional groups other than those found in natural cellulose. Such groups especially include carboxymethyl (CMC), aldehyde and / or carboxyl (cellulose obtained by N-oxyl-mediated oxidation, such as "TEMPO"), quaternary ammonium (cationic cellulose) or phosphoryl, etc. After modification or oxidation in one of the above methods, it is easier to decompose the fibers into MFC or nanofibrils.

[0033] Nanofibril cellulose can comprise some hemicelluloses, and its amount depends on plant origin.The mechanical decomposition of pre-treated fiber, for example the cellulose raw material of hydrolysis, pre-swelling or oxidation is carried out with suitable equipment, for example refiner, grinder, homogenizer, gelling machine (colloider), friction grinder, ultrasonic sonicator, fluidizer are microfluidizer, large-scale (macroscopic) fluidizer or fluidizer type homogenizer for example.According to the manufacture method of MFC, product may also contain other chemical substances existing in fines, nanocrystalline cellulose or wood fiber or papermaking process.Product may also comprise the micron-sized fiber particles of not effective fibrillation of various amounts.

[0034] MFC is made of wood cellulose fibers, including hardwood and softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse or other non-wood fiber sources. It is preferably made of pulp, including pulp from virgin fibers, for example, mechanical, chemical and / or thermomechanical pulp. It can also be made from broke or recycled paper.

[0035] In some embodiments, at least some of the MFC is obtained from MFC broke.

[0036] In addition to highly refined cellulose fibers, the first and / or second pulp suspension contains a certain amount of unrefined or slightly refined cellulose fibers. As used herein, the term unrefined or slightly refined fibers preferably refers to cellulose fibers having a Schopper-Riegler (SR) value of less than 30, preferably less than 28, measured according to standard ISO 5267-1. In some embodiments, the first and / or second pulp suspension contains between 50-99.9 wt%, preferably between 60-98 wt%, and more preferably between 70-95 wt% or even more preferably between 75-90 wt% of unrefined or slightly refined cellulose fibers, based on the total dry weight of the pulp suspension. Unrefined or slightly refined cellulose fibers can be obtained, for example, from chemical pulp, such as kraft pulp, mechanical or chemimechanical pulp or other high yield pulps. Unrefined or slightly refined cellulose fibers can be obtained from bleached or unbleached pulp. Unrefined or slightly refined cellulose fibers are preferably pulp from cellulose fibers that have never been dried.

[0037] The composition of the first and second pulp suspensions may be the same or different.

[0038] For example, in some embodiments, one pulp suspension may contain a higher amount of highly refined fibers than another pulp suspension. One possibility is to have a first pulp suspension with less highly refined cellulose fibers and / or a higher amount of unrefined or slightly refined cellulose fibers with a lower SR value to provide faster dewatering, and a second pulp suspension with more highly refined cellulose fibers and / or a lower amount of unrefined or slightly refined cellulose fibers with a higher SR value to provide good barrier properties and high strength. It may be preferred to use a lower amount of highly refined fibers in the suspension to form the web that is in direct contact with the glazing unit. Thus, by varying the composition of the suspension, a multi-layer paper web may be designed such that the web that is not in direct contact with the glazing unit contains a higher amount of highly refined fibers, preferably in an amount of 20-50 wt-%, preferably 25-40 wt-%. In this way, a multi-layer machine-glazed paper with good strength can still be produced in an efficient manner.

[0039] In some embodiments, the first and second pulp suspensions are provided by two different headboxes. This may be advantageous because the headboxes may be operated in slightly different ways, for example with different consistencies, headbox jet angles or jet-to-wire ratios.

[0040] More than one multiple headbox may also be used. In this way, a first suspension may be subjected to a first multiple headbox to form a first web comprising more than one layer, i.e. a first multi-layer wet web, and a second suspension may be subjected to a second multiple headbox to form a second web comprising more than one layer, i.e. a second multi-layer wet paper web. In this way, a multi-layer web is formed comprising more than one wet paper web from the first suspension and more than one wet web layer from the second suspension. It is preferred that the first multi-layer wet paper web comprises layers from more than the first suspension, e.g. also from a third, fourth or additional suspension. It is possible that the second multi-layer wet web comprises layers from more than the second suspension, e.g. also from a fifth, sixth or additional suspension. When more than one suspension is used to form the first and / or second multi-layer wet web, it is preferred that the compositions of the suspensions are different. It is preferred that the suspension forming the inner or middle layer of the multi-layer MG paper comprises a higher amount of highly refined fibers. It is preferred that the suspension used for one or more outer layers comprises a lower amount of highly refined fibers. The suspension for the middle or inner layer preferably contains 15-50 wt% of highly refined fibers, preferably 25-40 wt%, based on the total dry solids content of the suspension. The suspension for one or more outer layers preferably contains 0.1-10 wt% of highly refined fibers, preferably 1-5 wt%, based on the total dry solids content of the suspension.

[0041] The wire mesh used in the process of the present invention preferably has a relatively high porosity to allow rapid dewatering and high drainage capacity.

[0042] In some embodiments, the first and second pulp suspensions have the same composition. This can simplify the process as only one source of pulp suspension is required.

[0043] The basis weight of each of the first and / or second wet web is preferably less than 80 g / m2, based on the total dry weight of the web. 2 And more preferably less than 60g / m 2 It has been found that low grammages allow for rapid partial dewatering of the wet web with little or no pinhole formation. The basis weight of the first and / or second wet web, based on the total dry weight of the web, is preferably at least 5 g / m 2 Thus, in some embodiments, the basis weight of the first and / or second wet webs is in the range of 5-80 g / m2 based on the total dry weight of the webs. 2 In the range of 10-60 g / m 2 within the range.

[0044] After formation, the first and second wet webs are partially dewatered. Dewatering of the webs on the wire screen can be performed using methods and equipment known in the art, examples include but are not limited to table rolls and foils, friction less dewatering, and ultrasonic assisted dewatering. Partial dewatering means that the dry solids content of the wet web is reduced compared to the dry solids content of the pulp suspension, but the dewatered web still contains a large amount of water. In some embodiments, partial dewatering of the wet web means that the dry solids content of the first and second partially dewatered webs is above 1 wt% but below 15 wt%. In some embodiments, partial dewatering of the wet web means that the dry solids content of the first and second partially dewatered webs is above 1 wt% but below 10 wt%. It has been found that the dry solids content of the first and second partially dewatered webs within this range is particularly suitable for joining the first and second wet webs into a multi-layer web. In some embodiments, the dry solids content of the first and second partially dewatered webs prior to the joining step is in the range of 1.5-8 wt%, preferably in the range of 2.5-6 wt%, and more preferably in the range of 3-4.5 wt%.

[0045] Partially dehydrated but still wet webs are joined to form a multilayer web of higher grammage. When the first and second partially dehydrated webs are joined, their dry solids content is preferably above 1 wt% but below 15 wt%, and more preferably above 1 wt% but below 10 wt%. In some embodiments, when the first and second partially dehydrated webs are joined, their dry solids content is in the range of 1.5-8 wt%, preferably in the range of 2.5-6 wt%, and more preferably in the range of 3-4.5 wt%. The partially dehydrated webs are preferably joined by wet lamination. When the pulp suspension is dehydrated on the wire mesh, a visible boundary line will appear where the web has a reflective water layer to where the reflective layer disappears. This boundary line between the reflective web and the non-reflective web is called the waterline. The waterline indicates a certain solid content of the web. The webs are preferably joined after the waterline. Joining the webs while they are still wet ensures good adhesion between the layers. Joining can be achieved by applying one partially dehydrated web on top of another. The joining can be done with non-wire side to non-wire side, or wire side to non-wire side. The joining and further dewatering of the formed multi-layer web can be improved by various additional operations. In some embodiments, the joining also includes pressing the first and second partially dewatered webs together. In some embodiments, the joining also includes applying suction to the joined first and second partially dewatered webs. Applying pressure and / or suction to the formed multi-layer web improves adhesion between the web layers. The wire section of the paper machine can have various dewatering devices, such as blades, table rolls and foil elements, suction boxes, frictionless dewatering, ultrasonic assisted dewatering, couch rolls or dandy rolls.

[0046] The surface of the web facing the wire is referred to as the wire side, and the surface of the web facing away from the wire is referred to as the non-wire side.

[0047] When a web containing highly refined cellulosic fibers, particularly MFC, is dewatered on a wire, it has been found that there will be a difference in fines content between the non-wire side and the wire side. Fines are generally concentrated on the non-wire side, and more fines are washed off the wire side where dewatering occurs. This difference or imbalance in web composition causes curling problems in the finished paper due to humidity changes. Forming multi-ply paper according to the present invention can solve or improve this problem by reducing the imbalance in web composition.

[0048] The joining of the webs may preferably be done non-mesh side to non-mesh side, or non-mesh side to mesh side. Joining the webs non-mesh side to non-mesh side or mesh side to non-mesh side provides an additional advantage, as a larger portion of the fines is concentrated in the middle of the multiple layers of paper. This concentration of fines contributes to the adhesion between the layers and the barrier properties of the paper. The fines may also contribute to the self-healing phenomenon, where the fines are redistributed to fill the voids in the felt on the wet mesh, making the produced paper less porous. Another advantage is that the amount of fines on the surface to be glazed is reduced, which improves the adhesion properties and runnability of the web to the surface of the glazing unit.

[0049] It is preferred to join the web non-wire side to non-wire side because i) the fines will be concentrated in the middle, ii) the paper structure will be symmetrical, reducing curling problems, iii) the high concentration of fines at the contact surface will ensure good bonding between layers, and iv) the more porous outer surface (wire side) allows more efficient dewatering and faster drying in the press section.

[0050] The dry solids content of the multilayer web is generally further increased during the joining step. The increase in dry solids content may be due to the dehydration of the multilayer web on the wire mesh, wherein pressure and / or suction are optionally applied to the web, and may also be due to a drying operation performed during or shortly after joining, such as impingement drying or air or steam drying. The dry solids content of the multilayer web after joining with optional application of pressure and / or suction is generally above 8 wt % but below 28 wt %. In some embodiments, the dry solids content of the multilayer web before further dehydration and optional drying steps is in the range of 8-28 wt %, preferably in the range of 10-20 wt %, and more preferably in the range of 12-18 wt %.

[0051] The basis weight of multi-layer webs and multi-layer MG papers is typically less than 160 g / m2 based on the total dry weight of the web. 2 , preferably less than 140g / m 2 , and more preferably less than 130g / m 2In some embodiments, the multi-layer webs and multi-layer machine-made glossy papers have a basis weight of 25-160 g / m2 based on the total dry weight of the web. 2 In the range of 30-140 g / m 2 In the range of 40-130 g / m 2 within the range.

[0052] The present invention is described herein primarily with reference to embodiments in which the multi-layer MG paper is formed from two web layers comprising highly refined cellulose fibers. However, it should be understood that the multi-layer MG paper may also comprise additional web layers comprising highly refined cellulose fibers. Thus, the formed multi-layer MG paper may also be formed from three or more web layers comprising highly refined cellulose fibers, such as three, four, five, six or seven layers. The formation, composition and structure of each additional layer may be further characterized as described above with reference to the first and second web layers. Thus, in some embodiments, the method of making a multi-layer paper further comprises the following steps:

[0053] c2) forming a third wet web by applying a third pulp suspension comprising highly refined cellulose fibers on a third wire;

[0054] d2) partially dewatering the third wet web to obtain a third partially dewatered web;

[0055] e2) Joining the first, second and third partially dewatered webs to obtain a multilayer web.

[0056] If an MG paper with three or more web layers is produced, it may be preferred that the first and third suspensions have the same composition, preferably containing a lower amount of highly refined fibers, and the second furnish to produce a second layer to be located between the first and third layers contains a higher amount of highly refined fibers. It may be preferred that the first and third pulp suspensions contain 0.1-10 wt-% of highly refined fibers, preferably in an amount of 1-5 wt-%, and the second pulp suspension contains 15-50 wt-%, preferably 25-40 wt-% of highly refined fibers.

[0057] After the multilayer web is dewatered, the paper web may be subjected to further dewatering. Further dewatering typically comprises pressing the web to squeeze out as much water as possible. Further dewatering may, for example, comprise passing the formed multilayer web through the press section of a paper machine, wherein the web is passed between large rolls loaded under high pressure to squeeze out as much water as possible. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the multilayer web after further dewatering is in the range of 15-40 wt%, preferably in the range of 18-35 wt%, and more preferably in the range of 20-30 wt%.

[0058] In the dewatering step f), the multilayer web may optionally be subjected to additional dewatering in a dewatering unit. The dewatering unit is preferably a shoe press, a belt press or a similar extended nip pressing device with a nip length of at least 150 mm. It has been found that the use of a shoe press, a belt press or a similar extended nip pressing device can improve the dewatering of the multilayer web without increasing the risk of wet blistering of the web and damaging the barrier properties of the multilayer MG paper. The extended nip pressing device preferably has a nip length of at least 150 mm, preferably at least 200 mm, preferably 150-350 mm, and even more preferably 200-300 mm. The linear load in the extended nip pressing device is preferably between 250-1500 kN / m, i.e. this is the maximum linear load used in a device such as a shoe press. The linear load used is preferably changed during the processing of the multilayer web. By gradually or stepwise increasing the linear load in the extended nip press, dewatering of the web is improved, i.e. a web with a higher dry solids content can be produced without damaging the barrier properties. The linear load can also be increased in a pulsed manner during the treatment in the nip, i.e. the linear load is increased at least once in at least one pulse during the treatment of the multilayer web in the shoe press. This can be repeated during the treatment in the extended nip press. If more than one extended nip press, such as a shoe press, is used, the same linear load profile can be used in both. However, it is generally preferred to use different linear load profiles to design the linear load profile in a way that the dewatering is improved without deteriorating the barrier properties of the dewatered multilayer web.

[0059] A shoe press refers to an extended nip press comprising a shoe press nip. Any known shoe press may be used. The shoe press nip may be formed by using a shoe and a roll, or by using a large diameter soft roll and a roll. The roll preferably has a synthetic belt, but it may also have a metal belt. The large diameter soft roll may have a diameter of 1.5-2 meters. The position of the shoe relative to the fibrous web may be varied by changing the inclination angle of the shoe press. The inclination angle of at least one shoe press is preferably between 7 and 24 degrees. The inclination angle affects the peak linear load and is a way of adjusting the linear load to improve the dewatering efficiency of the web. The nip time is preferably at least 30 ms. Depending on the nip length and the production speed, the time for which the multilayer web is subjected to pressure in the shoe press varies.

[0060] The dry solids content of the multilayer web after the optional dewatering step is preferably between 25 and 45 wt%.

[0061] A belt press refers to an extended nip pressing device comprising a belt. Any known belt press may be used.

[0062] It may be preferred to use at least two extended nip pressing devices, preferably at least two shoe presses, and the two extended nip pressing devices are located after each other. The multilayer web is then first guided through a first shoe press and then through a second shoe press. In this way, it was found that the dewatering of the web can be improved even further and the production efficiency can be increased, which means that the amount of MFC can be increased. The nip pressure used in the first shoe press is preferably lower than the nip pressure used in the second shoe press. At least two shoe presses are preferably located on different sides of the web. In this way, the web can be dewatered from two directions through the fibrous web. When more than one shoe press is used, it is preferred that the total nip length, i.e. the sum of the nip lengths of each shoe press, is above 350 mm, preferably above 400 mm, and even more preferably above 450 mm. The geometrical design of at least two shoe presses is preferably different, for example one shoe press may have a concave design and one shoe press may have a convex design.

[0063] After the optional dewatering step in the dewatering unit, the multilayer web is guided through a glazing unit, wherein at least one side of the multilayer web is glazed. The glazing unit may be a Yankee cylinder, a glassine calender or an extended nip calender, such as a shoe calender or a belt calender. The glazing unit is preferably a Yankee cylinder. It has been found that the use of a Yankee cylinder as a glazing unit makes it possible to dry and provide a glazed surface to at least one surface of the multilayer web. Yankee cylinders are commonly used for drying tissue paper, which is a very porous material. Walker describes in detail the use of Yankee cylinders and how drying affects paper in the article "High temperature Yankee Hoods Save Energy and Improve Quality, P&P, July 2007". When a product is dried using a Yankee cylinder, the liquid in the product flows through the product towards the Yankee cylinder, i.e. towards the heat and steam formed during drying. The liquid of the product in our case also contains microfibrils, which results in an increased microfibril concentration being achieved on a smooth and glazed surface of the paper.

[0064] The dry solids content of the multilayer web before glazing the multilayer web is preferably in the range of 35-85 wt%, preferably in the range of 45-85 wt%. It is important to adjust the dry solids content of the multilayer web to ensure that the glazing process is as effective as possible. Furthermore, the correct dry solids content of the web will minimize the risk of adhesion problems of the web on the surface of the glazing unit.

[0065] The temperature of the glazing unit is preferably above 100° C., preferably between 110-190° C. The first side of the multilayer web will be in direct contact with the glazing unit, for example with the surface of a Yankee cylinder, an extended nip calender or a glassine calender. In order to control the adhesion of the cellulosic web to the glazing unit, for example a Yankee cylinder, it may be preferred to add an adhesion control additive to the surface of the glazing unit. It has been found that when using microfibrillated cellulose, it is important to control the adhesion to the surface of the glazing unit, because the microfibrillated cellulose in the cellulosic web tends to make the cellulosic web too tense, which causes the web to lift or bubble from the surface of the glazing unit. The adhesion control additive will provide sufficient adhesion of the web to the surface of the glazing unit. Suitable adhesion control additives may be water-soluble or partially water-soluble polymers, for example polyvinyl alcohol (PVOH), polyamidoamine derivatives, polyethyleneimines, polyacrylamides and / or polyacrylamide derivatives. The degree of hydrolysis of the PVOH used is preferably less than 99%, even more preferably less than 98%. Modified polymers may also be used, such as modified PVOH, preferably vinyl, carboxylated, cationized or siliconized PVOH. The adhesion control additive may also contain nanoparticles, such as nanoclay and / or nanocellulose. The adhesion control additive may also contain 0.5-20 wt % of nanoparticles, based on the total dry weight. The amount of adhesion control additive to the surface of the polishing unit is preferably between 0.1-10 gsm dry weight. The adhesion control additive is preferably added to the surface of the polishing unit by spraying. The adhesion control additive is preferably added to the surface of the polishing unit as a solution or as a foam.

[0066] The multilayer web can be calendered in at least one calender after being guided through the glazing unit. Any known calender can be used, such as a machine calender, a multi-nip calender, a soft nip calender, a belt calender. A shoe calender or any other extended nip calender can preferably be used. Calendering can be performed on one or both sides of the machine-glazed paper. The treatment in the calender is preferably done in-line.

[0067] The fibrous web can be treated in a decurling unit after calendering. In this way, the curling tendency of the paper can be reduced even further.

[0068] The produced multilayer machine glazed paper is preferably coated with a coating composition on at least one side. The coating composition preferably comprises starch, carboxymethyl cellulose and / or microfibrillated cellulose. The coating is preferably applied to the glazed surface of the MG paper. The coating composition will further improve the barrier properties of the paper. It was surprisingly found that adding highly refined fibers to the paper improved the coating properties of the paper, i.e. the coverage of the coating on the surface of the paper was significantly improved. One theory is that the increased density of the glazed surface means that the coating "stays" on the surface of the paper, and the amount of coating can be reduced and still complete coating coverage can be achieved on the surface. The coating is preferably applied in an amount of 0.1-5gsm, preferably between 0.2-4gsm and even more preferably between 0.3-3gsm. The coating composition can be applied to the surface of the paper using any known coating technique.

[0069] The multi-layer MG paper preferably has high repulpability.In some embodiments, the multi-layer MG paper exhibits less than 30%, preferably less than 20%, and more preferably less than 10% reject when tested as a Class II material according to the PTS-RH021 / 97 test method.

[0070] According to a second aspect described herein, there is provided a multi-layer MG paper comprising highly refined cellulose, wherein the multi-layer MG paper is obtainable by the method of the present invention.

[0071] The multi-ply machine-made glossy paper preferably contains between 0.1 and 50 wt% of highly refined fibers, based on the total dry solids content, more preferably between 2 and 40 wt%, and even more preferably between 5 and 30 wt%.

[0072] The multi-layer machine-made glossy paper preferably has a gloss of 25-160 g / m 2 In the range of 30-140 g / m 2 In the range of 40-130 g / m 2 Basis weight within the range.

[0073] According to ASTM D-3985, the oxygen transmission rate (OTR) value (23°C, 50% RH) of the multi-layer machine-made glossy paper is preferably 200 cc / m 2 / 24h or less, preferably 150cc / m 2 / 24h or less, and even more preferably at 100cc / m 2 / 24h or less.

[0074] The multi-ply machine-glazed paper preferably has a Gurley Hill value of at least 25,000 s / 100 ml, and more preferably at least 40,000 s / 100 ml, measured according to standard ISO 5636 / 6.

[0075] The multi-layer machine-glazed paper preferably has at least one glazed surface according to ISO 8791-4 with a surface roughness PPS value below 5 μm, preferably below 2 μm (measured before any final coating is added).

[0076] The multi-layer machine-made glossy paper preferably has a thermal conductivity of 1500 J / m, measured according to TAPPI Um-403 on 60 gsm paper. 2 More preferably 1600 J / m 2 Above and most preferably at 1800 J / m 2 The Scott bonding value is above 1.004. Therefore, the produced multilayer MG paper has very high strength.

[0077] Multilayer MG paper will generally show good grease and oil resistance. According to standard ISO16532-2, the grease resistance of paper is evaluated by the KIT test. The test uses a series of castor oil, toluene and heptane mixtures. As the ratio of oil to solvent decreases, viscosity and surface tension also decrease, making continuous mixtures more difficult to withstand. Performance is rated by the highest numbered solution that does not turn the sheet black after 15 seconds. The highest numbered solution (most aggressive) that remains on the surface of the paper without causing failure is reported as a "kit rating" (maximum 12). In some embodiments, the KIT value of the multilayer MG paper is measured according to standard ISO16532-2 as at least 6, preferably at least 8, and even more preferably at least 10.

[0078] The multi-layer machine-made glossy paper of the invention is particularly suitable as a packaging material, in particular as packaging for food or hygiene products.

[0079] Typically, although products, polymers, materials, layers and methods are described in terms of “comprising” various components (components) or steps, the products, polymers, materials, layers and methods may also “consist essentially of” or “consist of” the various components (components) or steps.

[0080] Although the present invention has been described with reference to various exemplary embodiments, it will be appreciated by those skilled in the art that various changes may be made and elements thereof may be replaced with equivalents without departing from the scope of the present invention. In addition, many modifications may be made to adapt specific circumstances or materials to the teachings of the present invention without departing from the basic scope of the present invention. Therefore, the present invention is intended not to be limited to the specific embodiments disclosed as the best mode for implementing the present invention, but the present invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A method for producing a multi-layer machine-made glazed paper comprising highly refined cellulose fibers, the method comprising the following steps: a) forming a first wet web by applying at least one first pulp suspension comprising highly refined cellulose fibers on a first wire; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying at least one second pulp suspension comprising highly refined cellulose fibers on a second wire; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) joining the first and second partially dewatered webs to obtain a multilayer web; f) optionally dewatering the multilayer web in a dewatering unit, and g) subjecting the multilayer web to glazing in at least one glazing unit to obtain a multilayer machine-glazed paper comprising highly refined cellulose fibers, The bonding is done from non-screen side to non-screen side, or screen side to non-screen side. wherein the first and / or second pulp suspension comprises 0.1 to 50 wt % of highly refined cellulose fibers, based on the total dry weight of the pulp suspension, wherein the highly refined pulp in the first and / or second pulp suspension has a Schopper-Riegler (SR) value in the range of 65-99, determined by standard ISO 5267-1, One of the most highly refined cellulose fibers is microfibrillated cellulose (MFC).

2. The method according to claim 1, wherein the highly refined pulp in the first and / or second pulp suspension has a Schopper-Riegler (SR) value in the range of 80-95.

3. The method according to any one of claims 1-2, wherein the first and / or second pulp suspension comprises between 50-99.9 wt% of unrefined or slightly refined cellulose fibers, based on the total dry weight of the pulp suspension.

4. The method according to claim 3, wherein the first and / or second pulp suspension comprises between 60-98 wt% of unrefined or slightly refined cellulose fibers.

5. The method according to claim 4, wherein the first and / or second pulp suspension comprises between 70-95 wt% of unrefined or slightly refined cellulose fibers.

6. The method according to any one of claims 1-2, wherein the first and second pulp suspensions have the same composition.

7. The method according to any one of claims 1-2, wherein the first and second pulp suspensions have different compositions.

8. The method according to any one of claims 1-2, wherein the first and / or second wet web comprises more than one layer.

9. The method according to any one of claims 1-2, wherein the dry solids content of the first and second partially dewatered webs before the joining step is in the range of 1.5-8 wt%.

10. The method of claim 9, wherein the dry solids content of the first and second partially dewatered webs prior to the joining step is in the range of 2.5-6 wt%.

11. The method of claim 10, wherein the dry solids content of the first and second partially dewatered webs prior to the joining step is in the range of 3-4.5 wt%.

12. The method according to any of claims 1-2, wherein the joining is performed by wet lamination of the first and second partially dewatered webs.

13. The method according to any of claims 1-2, wherein the dry solids content of the multilayer web after the optional dewatering step is in the range of 25-45 wt%.

14. The method according to any one of claims 1-2, wherein the dry solids content of the multilayer web before subjecting the multilayer web to glazing is in the range of 35-85 wt%.

15. The method according to claim 14, wherein the dry solids content of the multilayer web before subjecting the multilayer web to glazing is in the range of 45-85 wt%.

16. The method according to any one of claims 1-2, wherein the dewatering step f) is done in a dewatering unit and the dewatering unit is an extended nip press.

17. The method according to claim 16, wherein the dewatering unit is a shoe press or a belt press.

18. The method according to any one of claims 1-2, wherein the glazing unit is a Yankee cylinder, a glass paper calender or an extended nip calender.

19. The method according to claim 18, wherein the glazing unit is a shoe press or a belt press.

20. The method according to any one of claims 1-2, wherein the adhesion control additive is added to the surface of the glazing unit in an amount of 0.1-10 gsm.

21. Multi-ply machine-made glossy paper comprising highly refined cellulose, wherein the multi-ply machine-made glossy paper is obtained by the process according to any one of claims 1 to 20.

22. The multi-ply machine-made glossy paper according to claim 21, wherein the multi-ply machine-made glossy paper comprises between 0.1-50 wt% of highly refined fibers, based on total dry solids content.

23. The multi-layer machine-made glossy paper according to any one of claims 21 to 22, wherein the multi-layer machine-made glossy paper has a gloss of 25 to 160 g / m 2 Basis weight within the range of.

24. The multi-layer machine-made glossy paper according to claim 23, wherein the multi-layer machine-made glossy paper has a gloss of 30-140 g / m 2 Basis weight within the range of.

25. The multi-layer machine-made glossy paper according to claim 24, wherein the multi-layer machine-made glossy paper has a gloss of 40-130 g / m 2 Basis weight within the range of.

26. The multi-layer machine-made glossy paper according to any one of claims 21-22, wherein the multi-layer machine-made glossy paper has a glossiness at 200 cc / m according to ASTM D-3985. 2 The oxygen permeability (OTR) value below 24h is measured at 23℃ and 50% relative humidity.

27. The multi-ply machine-made glossy paper according to any one of claims 21 to 22, wherein the multi-ply machine-made glossy paper has a Gurley Hill value of at least 25000 s / 100 ml measured according to standard ISO 5636 / 6.

28. The multi-ply machine-made glossy paper according to claim 27, wherein the multi-ply machine-made glossy paper has a Gurley Hill value of at least 40000 s / 100 ml measured according to standard ISO 5636 / 6.

29. The multi-layer machine-made glossy paper according to any one of claims 21 to 22, wherein the multi-layer machine-made glossy paper has at least one glossy side with a surface roughness PPS value of less than 5 μm according to ISO 8791-4.

30. The multi-layer machine-made glossy paper according to any one of claims 21 to 22, wherein the multi-layer machine-made glossy paper has at least one glossy side with a surface roughness PPS value of less than 2 μm according to ISO 8791-4.

31. The multi-layer machine-made glossy paper according to any one of claims 21-22, wherein the multi-layer machine-made glossy paper has a thermal conductivity of 1500 J / m2 as measured on 60 gsm paper according to TAPPI UM-403. 2 Scott combined values ​​above.

32. The multi-ply machine-made glossy paper according to any one of claims 21-22, wherein the multi-ply machine-made glossy paper has a KIT value of at least 6 measured according to standard ISO 16532-2.