METHOD, SYSTEM, AND COATED SHEET OF PAPER-BASED PACKAGING MATERIAL

Abstract

 Disclosed herein are a method and system for locally coating a sheet of paper-based packaging material, as well as a coated sheet obtainable by the method. The sheet is transferred in a transfer direction (T). At a first printing stage (200-1), a first coating layer is printed onto the sheet (50). A first transfer roller (211) transfers a first coating material to a first printing plate (212) configured to apply the first coating material onto the sheet (50) for thereby locally coating a first area. Downstream, at one or more further printing stages (200-2...200-N), further coating layers are printed onto the sheet (50). Each further printing stage (200-2...200-N) comprises a further transfer roller (211) and a further printing plate (212) for locally coating a further area. When applied on the sheet, the first and further coating layers form a single closed film on the sheet that provides a barrier coating.

Description

[0001] The invention relates to a method for locally coating a sheet of paper-based packaging material, in particular a sheet of corrugated fiberboard or cardboard. The invention also relates to a locally coated sheet of paper-based packaging material obtainable by the method. The invention further relates to a system for coating a sheet of paper-based packaging material.

[0002] Corrugated fiberboard, e.g. corrugated cardboard, is a type of packaging material comprising one or more fluted corrugated sheets between flat linerboards. Corrugated fiberboard can e.g. be used for making corrugated packaging. A limitation of common corrugated material is the difficulty in applying graphic print, e.g. for informative and marketing purposes, or coatings for protecting the corrugated fiberboard against other materials, such as water. Inks and coatings comprise fluids with similar chemical compositions and may have similar rheological behavior. Their composition is usually adapted to the application process (rheology, curing, etc.). The difference is in their functionality. Inks are intended to locally bring color to a surface for texts or graphical applications. The color is locally applied, as accurately as required/possible. The application process is often optimized for transferring "spots" of ink of minimal size.

[0003] Coatings typically fully cover a larger surface. They may bring gloss, color or other (haptonomic) properties to the surface, but most coatings are used to protect a surface. For example protection against scratches, radiation (UV-light), bacterial contamination, dirt, penetration by fluids, gasses, etc. The latter are also called barrier-coatings. In order to provide sufficient barrier, the coating layer must cover the full surface (closed layer, no pin-holes) and may need a certain layer thickness across the surface of the sheet.

[0004] The corrugated medium sheet and the linerboard(s) are generally made of a fibrous (e.g. paperboard) material. Such materials are relatively porous and have a rough surface, which causes any coating fluids applied on the surface to be absorbed by the sheet ("sink-in").

[0005] Moreover, in box making packaging manufacturing plants, along with the print being produced on the surface, the corrugated board is usually creased to provide controlled bending of the board. Such creases may be formed across the applied coating and the coating may be locally damaged. For this reason it is difficult to maintain a closed, covering layer of barrier coating over such creases and bends.

[0006] The required coating barrier level is determined by the material that the coating should protect against. Compared to water, for example, fats and oils may require a lower barrier level, while water vapor, odors and gasses may require a higher barrier level.

[0007] The water absorption capacity of cardboard is usually defined by the Cobb value. By means of a standardized Cobb test, the amount of water that is taken up by a defined area of cardboard through one-sided contact with water, within a certain amount of time, can be determined. Depending on the material, contact times of 60, 180 and 1800 seconds are common. The result of the test is expressed by an indication of the applied contact time (e.g. Cobb1800 for a contact time of 1800 seconds), followed by the determined water absorbency value in grams of water absorbed per square meter of the sheet.

[0008] For packaging material that is exposed to fluid contact, it makes sense to use board grades with a low Cobb value. The lower the Cobb value (low water absorbency), the more stable the package remains even in case of water contact or contact with other fluids.

[0009] Besides corrugated cardboard, it may also be difficult to provide a barrier coating on other types of paper-based packaging material, e.g. for holding foodstuffs, such as sheets of solid board or folding carton or even paper sheets. To protect such packaging material against leakage or ingress of water, water vapor, solvents, oils, fatty acids, and other substances, it is known to provide the material with a physical barrier, e.g. by laminating plastic or aluminum onto the paper-based sheet. However, known coating processes apply a uniform coating layer across the sheet surface, which may not provide an adequate level of performance or protection in some parts of the sheet, e.g. where the sheet is to be bent or folded, and which may result in excessive amounts of coating material applied in other parts.

SUMMARY

[0010] It is an object of the present disclosure to provide an improved method and system for locally coating paper-based packaging material, in particular to enable non-uniform barrier coatings, e.g. locally adapted to the geometry and/or functional requirements of the packaging. Due to the relatively rough and porous surface, as well as the structure of paper-based packaging material, it is a challenge to provide a sheet, e.g. of paper, paperboard, or corrugated cardboard, with a (locally) required barrier coating level in a reliable and efficient manner. For this purpose, the invention provides an improved method for coating such sheets. In particular, the method can be applied in a discontinuous process for coating individual, separate sheets of paper-based material, such as corrugated fibreboard where the risk of crushing the fluting is relatively high. The method can however be applied in any paper-based post-printed packaging, such as solid board or folding carton or even paper sheets.

[0011] The method comprises: (i) transferring the sheet in a transfer direction; (ii) printing, at a first printing stage provided along the transfer direction, a first coating layer onto the sheet, wherein the first printing stage comprises a first transfer roller configured to transfer a first coating material to a first printing plate circumferentially mounted on a first plate roller, wherein the first printing plate is configured to apply the first coating material onto the sheet for thereby locally coating a first area; and (iii) printing, at one or more further printing stages provided downstream the first printing stage, one or more further coating layers onto the sheet, the or each further printing stage comprising a further transfer roller configured to transfer a further coating material to a further printing plate circumferentially mounted on a further plate roller, wherein the further printing plate is configured to apply the further coating material onto the sheet for thereby locally coating the same (first) and/or a further area. When applied on the sheet, the first and one or more further coating layers form a single closed film on the sheet, which film provides a barrier coating.

[0012] In other words, a multi-stage flexography (flexo) printing process is applied for printing multiple layers of coating material (e.g. coating fluid) onto the sheet of paper-based packaging material. Accordingly, the sheet can be provided with a reliable multi-layer barrier coating in a fast and efficient fashion. For example, the first coating material may comprise a sealing substance (e.g. primer) arranged for sealing pores of the paper material, to bridge performance between the sheet itself and the further coating layers that are subsequently printed thereon. At least one of the one or more further coating materials preferably comprises a hydrophobic substance arranged for repelling water. As a result, the barrier coating may provide the sheet with a water absorbency of 40 g/m2 or less according to Cobb 1800.

[0013] Thus, in contrast to a conventional web coating process where a coating fluid is applied on the full surface of a paper web, the method disclosed herein proposes to apply a flexo printing process in which a sheet of paper-based packaging material is transferred along multiple, serially arranged, printing stages where a layer of coating material is locally applied only where it is needed. This minimizes the coating fluid consumption and allows the properties of the surface of the sheet to be locally different.

[0014] The coating that is required in packaging may not be necessary on the parts of the sheet that are cut off and recycled. Also, the hydrophobic barrier coating may be in conflict with local packaging requirements such as local gluing or printing with waterbased glue or ink. For that reason it is beneficial to apply the coating only on the locations in the packaging where it provides the required functionality and not apply the coating where it hinders the functionality or is not necessary. For that reason conventional coating processes may not be suitable for some applications.

[0015] Coated packaging may be closed via a heat sealing process in which the coating is locally connected to a closing surface. Such a heat sealing process may require a functionality from the coating different from a barrier function. Usually this issue is solved by creating a coating chemistry (compound) that can provide both heat sealability as well as protective barrier, but such a chemistry is a compromise and therefore suboptimal.

[0016] The present invention allows local, selective application of coating material, e.g. to leave open spots on the sheet where a (water-based) glue is to be applied when forming a tray or box, or to not waste coating material on areas of the sheet that need a thinner layer of coating with respect to other areas of the sheet, and to avoid coating areas of the sheet that are to be cutoff or do not need any coating. The invention also allows selective application of different coating material on the sheet, e.g. coating materials of different chemistries or colours, for example to optimize the coating in some areas for heat sealing while optimizing the coating in other areas for other functionalities, such as fluid sealing. The first and further coating layers may be applied in patterns that are complementary or adjacent to each other, or in patterns that overlap partially or fully. However, according to the invention, the first and/or further layer(s) may also cover the entire surface of the sheet when appropriate, e.g. for applying a layer of primer material or finishing material. In other words, each respective coating layer can be applied with any desired size, shape and pattern to coat corresponding first and further area(s) of the sheet, by using the process described herein. In this way, coating can be applied in a versatile manner, adapted to the size and shape of the area to be coated, e.g. covering the sheet entirely or only a specific part thereof, to provide a dedicated coating thickness and functionality where it is needed on the sheet, while minimizing consumption of coating material.

[0017] In order to obtain sufficient layer thickness the coating material is transferred onto the sheet's surface via at least a transfer roller that determines the amount of coating material and a printing plate (or cliché) that determines at which location the coating material is deposited.

[0018] In some embodiments, the locally applied first and further areas at least partially overlap for thereby providing the barrier coating with local differences in layer thickness. For example, the layer thickness may be locally increased, for example but not limited to at or around parts of the sheet that are to be creased or scored, e.g. for folding the sheet into a box. Accordingly, water absorbency remains low in those parts. At the same time, other parts of the sheet can be provided with a relatively thin coating layer, e.g. to reduce the amount of coating material applied on the sheet while still providing a required barrier level. Accordingly, the coating can be applied with local layer thicknesses that are just sufficient to provide a desired local protection. In contrast, in conventional coating methods the sheet is provided with a uniform coating layer thickness so that the level of protection is equal across the sheet.

[0019] In some applications, different functions may be required from the coating. For example, when folded into a box, parts of the sheet located on a top side of the box may require a different barrier level and/or a different property with respect to parts of the sheet located on a bottom side of the box. For this reason, the first and further coating materials can have different material compositions, e.g. different with respect to each other, for thereby providing a coating with local differences in coating properties. Rather than mixing substances, thereby creating a compromise, and/or using thicker layers, thinner layers of optimised material composition can be used locally.

[0020] Preferably, to facilitate transfer of the materials in a flexo printing process as described herein, each of the first and further coating materials comprise a water based fluid with relatively low viscosity, for example a viscosity lower than about 300 mPas (millipascal-seconds), more preferably between 150 and 250 mPas. Such a low viscosity allows the fluid to flow and form a pinhole-free closed film on the surface of the sheet.

[0021] In order to reach a predefined barrier performance the coating layer is to have a certain thickness. For that reason, each of the first and further coating materials preferably comprises solid particles at a weight percentage of at least 45% prior to application on the sheet.

[0022] In order to form a closed film on the sheet's surface the sheet preferably has a low surface roughness and low porosity. For example, the sheet can have, but is not limited to a surface roughness lower than 1500 ml/min as measured according to ISO 8791, preferably lower than 500 ml/min. Even better results can be achieved when the sheet is made of a clay coated paper with a surface roughness lower than 10 ml/min.

[0023] To prevent a water based coating fluid from being absorbed by the paper-based material (sink-in) the sheet is preferably hydrophobic. Prior to the steps of printing at the first and further printing stages, the sheet may be pre-treated with a hydrophobic sizing agent, for thereby providing the sheet with a water absorbency level lower than 30 g/m² according to Cobb60, preferably lower than 27 g/m².

[0024] The inventors have found that very good results can be obtained with sheets that are hydrophobic by nature (such as Kraftliner), or bulk- or surface- sized with hydrophobic sizing, rather than with a strength improving sizing. In general, sizing improves the surface strength, printability, and water resistance of the paper or material to which it is applied. Surface sizing solutions or agents can be used for water-resistance (e.g. hydrophobic sizing) to prevent the coating being absorbed by the sheet. Surface sizing solutions mainly include modified starches and sometimes other hydrocolloids, such as gelatine, or surface sizing agents such as acrylic co-polymers. Surface sizing agents may comprise amphiphilic molecules, having both hydrophilic (water-loving) and hydrophobic (water-repelling) ends. The sizing agent adheres to substrate fibers and forms a film, with the hydrophilic tail facing the fiber and the hydrophobic tail facing outwards, resulting in a smooth finish that tends to be water-repellent.

[0025] Other aspects of the present invention relate to a selectively coated sheet of paper-based packaging material, e.g. corrugated fiberboard, obtainable by the method described herein. The coated sheet preferably has a water absorbency level lower than 40 g/m² according to Cobb 1800, e.g. 35 g/m², 30 g/m², or lower.

[0026] Yet other aspects of the present invention relate to a system for locally coating a sheet of paper-based packaging material. The system comprises a transfer arrangement configured to transfer the sheet in a transfer direction, a first printing stage provided along the transfer direction and at least one further printing stage arranged downstream the first printing stage.

[0027] The first printing stage is arranged for printing a first coating layer onto the sheet. The first printing stage comprises a first transfer roller configured to transfer first coating material to a first printing plate circumferentially mounted on a first plate roller. The first printing plate is configured to apply the first coating material onto the sheet for thereby locally coating a first area.

[0028] The or each further printing stage is arranged for printing a respective further coating layer onto the sheet, and comprises a further transfer roller configured to transfer a further coating material to a further printing plate circumferentially mounted on a further plate roller. The further printing plate is configured to apply the further coating material onto the sheet for thereby locally coating the same and/or a further area.

[0029] When applied on the sheet, the first and further coating layers at least together form a single closed film on the sheet, which film provides a barrier coating.

[0030] To optimize the transfer rollers for maximum material transfer, at least one of the first and further transfer rollers is preferably an anilox roller, arranged for transferring the amount of first or further coating material to the respective first or further printing plate. Besides anilox rollers, other transfer rollers or systems may also be suitable for the method and system disclosed herein, as long as they can be used in the respective printing stages.

[0031] Preferably, each cell of the cell structure has a hexagonal shape that is elongated, e.g. in the transfer direction, to reduce the total aggregate length of cell walls in relation to the material transfer volume of the cell structure. Instead of hexagonal, the cells in the (anilox) transfer rollers may have other shapes. For example, the cell structure may comprise sinusoidal, wavy channels, or corrugated channels. Preferably, the cell structure has a volume of at least 12 cm³/m², preferably 14 cm³/m² or higher, to maximize the transfer of coating material. For the same reason, each cell of the cell structure preferably has a cell opening larger than 60 microns in each direction, and a cell depth smaller than 20 microns.

[0032] The first and further printing plates preferably have a dimpled outer surface. For example, the plate's outer surface may be covered with a number of dimples, e.g. depressions or recesses. Preferably, the dimples in conjunction reduce the area of the plate's outer surface that can be in contact with the sheet by 5-20%. Each dimple may have a diameter in a range of 250-750 micrometer. For example, the outer surface may be provided with a large number of small dimples (e.g. 120 lpi) or a smaller number of larger dimples (e.g. 70 lpi). The main function of the dimpled outer surface is to disrupt the surface tension of the printing plate. For this reason, the depth of the dimples is less important than their number, or the total area that is covered by the dimples.

[0033] The dimpled outer surface allows to hold a relatively large amount of coating material to be deposited on the sheet, while providing a relatively soft contact between the printing plate and the sheet, e.g. to "kiss print" the coating material on the sheet. In other words, the dimpled surface allows the pressure of the printing plate on the sheet of paper-based packaging material to be minimized. For the same reason, each of the first and further printing plates may be at least partially made of a flexible material, such as rubber or silicone. Preferably, the shoreA hardness of the printing plate is about 40, or lower, e.g. lower than 30, preferably between 20 and 30 to also ensure proper transfer of the coating material and/or prevent deformation of the paper. Accordingly, increased amounts of coating material can be transferred by the printing plate (cliché) without pressing the coating material (e.g. fluid) into the pores of the paper.

[0034] After the last stage, the coating may need to dry before the sheets are palletized or stacked. For this reason, the system may further comprise a dryer device, e.g. a hot-air dryer or an infrared (IR) dryer, arranged for drying the first and the or each further coating layers downstream the first printing stage, e.g. at the end of the printing process. For example, a dryer device may be provided between printing stations to dry a coating layer applied on the sheet before applying a subsequent further coating layer, e.g. in case of multiple layers of primer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The invention will be further elucidated in the figures:

FIG. 1 schematically illustrates an embodiment of a method and system for coating a sheet of paper-based packaging material;

FIGs. 2A and B illustrate embodiments of a printing stage of the method and system described herein;

FIG. 3 illustrates an embodiment of a transfer roller of the printing stage described herein;

FIG. 4 illustrates an embodiment of a printing plate of the printing stage described herein;

FIG. 5 provides a graph of exemplary trial runs of an embodiment of the system and method described herein;

FIG. 6 provides a graph of other exemplary trial runs of an embodiment of the system and method described herein.

FIGs. 7A and B illustrate an embodiment of a sheet of paper-based packaging material obtainable by the method described herein;

FIG. 8 illustrates another or further embodiment of a coated sheet of paper-based packaging material obtainable by the method described herein;

FIGs. 9A and B respectively show a sheet coated according to the disclosure, and a tray formed from such a coated sheet.

DETAILED DESCRIPTION

[0036] The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

[0037] FIG. 1 schematically illustrates an embodiment of a system for coating a sheet of paper-based packaging material, e.g. corrugated fiberboard or cardboard, solid board, folding carton or even paper sheets. FIG. 1 also represents an exemplary embodiment of a method for coating such a sheet. Corrugated fiberboard or corrugated cardboard is a type of packaging material comprising a fluted corrugated sheet and one or two flat linerboards. It is usually made on "flute lamination machines" or "corrugators" and can e.g. be used for making corrugated boxes. The corrugated medium sheet and the linerboard(s) are sometimes made of Kraft containerboard ("Kraftliner"), a paperboard material usually over 0.25 millimeters thick.

[0038] A limitation of common corrugated material is that the prefabricated corrugated sheets are relatively thick and spongy, compared to the thin and incompressible nature of solid fiber paper such as paperboard. Corrugated sheets may absorb a relatively large amount of fluids, such as water or oil, as well as gases, such as (water) vapor and odors. In some conditions, e.g. high humidity, this may limit the performance of the corrugated sheet. For this reason, it may be desired to provide the corrugated sheet with a barrier coating that effectively reduces the (water) absorbency of the sheet. Coating of corrugated board material is particularly difficult when provided as individual sheets. The coating process is thus discontinuous, and uncontrolled web-tension may occur in the sheet. The corrugations create a risk of uneven surface and a risk of crushing the fluting when applying the coating. For these reasons, it is difficult to apply a barrier coating on the sheet in a fast and reliable manner.

[0039] The system 100 and method disclosed herein are based, by way of example, on a modified flexography (flexo) printing process. As illustrated in FIG. 1, a sheet 50 of paper-based packaging material is transferred in a transfer direction T, e.g. by a transfer arrangement 110. A first printing stage 200-1 and three further printing stages 200-2, 200-3, 200-4 are serially arranged along the transfer direction T and configured to successively print coating layers onto the sheet 50. While being transferred, the sheet 50 is passed consecutively through the printing stages. Instead of four printing stages as illustrated in FIG. 1, two, three, five, six, or more printing stages 200-1...200-N can be provided along the transfer direction T. Optionally, a dryer device (not illustrated) such as a hot air blower or an IR lamp can be arranged downstream the last printing stage 200-N and/or between adjacent printing stations 200, for drying coating, such as at the end of the printing stage the multilayer coating, e.g. before the sheet 50 is stacked or palletized.

[0040] It will be realized that transferring a sheet should be understood in this disclosure as at least encompassing movement of a sheet relative to printing equipment, movement of printing equipment relative to a sheet or both relative to each other.

[0041] The first printing stage 200-1 is arranged for printing a first coating layer onto the sheet 50, e.g. a primer layer. A first transfer roller 211, here shown, by way of example only, as an anilox roller is configured to transfer an amount of first coating material, e.g. primer material for sealing pores of the sheet, to a first printing plate 212 circumferentially mounted on a first plate roller 213, configured to apply the amount of first coating material onto the sheet 50. Accordingly, a first area of the sheet is coated by the first printing stage 200-1.

[0042] Each of the further printing stages 200-2...200-N is arranged for printing a respective further coating layer onto the sheet 50. The further coating materials may have different material compositions. In this way, a barrier coating with local differences in coating properties can be applied on the sheet 50. Preferably, the further coating layers include at least one or more layers comprising a hydrophobic substance for repelling water, to reduce the water absorbency of the sheet. Optionally, an ink for coloring the sheet or for printing graphics or text may be included in a layer, and/or a lacquer for increasing the durability of the sheet.

[0043] Each layer is applied on the sheet by a further transfer roller 211, such as an anilox roller that is configured to transfer an amount of coating material to a further printing plate 212 configured to apply the further coating material onto the sheet 50. For example, subsequently printed coating layers may overlap at least partially, to provide a multi-layer coating on the sheet 50. When applied on the sheet, the first and further coating layers form a single closed film on the sheet that provides a barrier coating. In other words, the barrier coating covers the surface of the sheet in a hole-free fashion. Accordingly, the coating may form a water barrier to provide the sheet with a water absorbency between 25-35 g/m² or lower, when measured in a Cobb1800 test (contact duration of 1800 seconds).

[0044] The first and further coated areas of the sheet may overlap at least partially, or locally, for thereby providing a barrier coating with local differences in layer thickness. In this way, areas of the sheet can be provided with thicker coating than other areas of the sheet. For example areas that are to be creased or folded can be provided with a relative thick coating, to ensure that the water absorbency is not inadvertently increased after shaping the sheet, e.g. into a box shape.

[0045] FIG. 2A illustrates an exemplary embodiment of a first or further printing stage 210. The printing stage 210 comprises a transfer roller, here shown as an anilox roller 211 arranged for collecting coating material 30 from a coating material chamber 214. As the anilox roller 211 is rotated, the coating material 30 is carried along the outer surface of the anilox roller 211, which e.g. is provided with pockets for collecting the coating material 30. Optionally the coating material chamber 214 can be provided with a doctor blade system 215 for scraping excess coating material off the anilox roller 211.

[0046] The coating material 30 is transferred from the anilox roller 211 to the outer surface of a printing plate 212 circumferentially mounted to a plate cylinder 213, which is rotated in a direction opposite to the rotation direction of the anilox roller. As illustrated, the printing plate 212 has an embossed outer surface, e.g. comprising dimples, protrusions, or ribs, defining essentially a pattern of coating to be applied to the sheet.

[0047] The printing plate 212 transfers the coating material 30 to the sheet, which is transferred along the transfer direction T, to locally form a layer of coating. In FIG. 2 by way of example the sheet 50 is shown as fed over counter roller 216. It will however be clear that a sheet 50 can also be fed between the printing plate 212 and such counter roller 216 or another support in a flat or at least substantially flat orientation, e.g. as illustrated in FIG. 2B.

[0048] FIG. 3 illustrates an embodiment of a transfer roller, especially an anilox roller 211, e.g. as provided in the first and further printing stages. The anilox roller 211 is provided with a hexagonally shaped cell structure 218, in particular an elongated hexagonally shaped cell structure. The cell structure 218 comprises hexagonally shaped pockets, in each of which pockets a volume of coating material can be collected and transferred to a respective first or further printing plate. Preferably the cell structure 218 has a volume of at least 12 cm³/m², preferably 14 cm³/m² or higher. In other words, the total aggregate volume of all hexagonally shaped pockets of the cell structure 218 is at least 12 cubic centimeters per meter squared of the circumferential area of the anilox roller, preferably more than 14 cubic centimeter per meter squared.

[0049] The roller 211 e.g. has a 70- or 75-degree extended or elongated hexagonal cell structure, which offers better ink release due to the cell's shape compared to an equilateral hexagonal cell structure. The elongated engraving allows for more volume in a particular lines per inch (lpi) configuration, because the cell structure has a reduced number of cell walls around the roller's circumference, leaving more room to carry ink. As illustrated in FIG. 3, the degree of extension is measured as the angle A between corresponding (e.g. similar) vertices of adjacent cells. Each cell 218A can for example have a length, extending in the circumferential direction of the roller 211, and a width, parallel to the rotational axis of the roller 212. Alternatively, any other configuration that allows deposition of a maximum amount of fluid may be used, such as a continuous sinusoidal channel shape of the GTT2.0 Anilox as supplied by APEX.

[0050] FIG. 4 illustrates an embodiment of a printing plate 212, comprising a dimpled outer surface 292, see left hand side of FIG 4. For comparison, the right hand side of FIG. 4 illustrates an alternative embodiment of a printing plate 212, comprising a uniform, non-dimpled outer surface 291. Compared to a conventional (flexo) printing process, the printing plates (clichés) used in the present invention are adapted for transferring an increased amount of coating material, e.g. fluid. Besides selecting a relatively soft plate material such as rubber, the printing plate 212 may be "rastered", e.g. comprising a pattern of depressions to break the surface tension of the printing plate 212. As illustrated on the left hand side of FIG. 4, the plate's outer surface 291 may be covered with a number dimples 292 that together cover 5-20% of the outer surface. By means of example not considered to be limiting for the present invention, several trials were performed using a Gopfert Ovation flexo post printer. In the trials, coating material was transferred in amounts of fluid not used in traditional flexo printing processes to enable transfer of increased amounts of coating material to the sheet surface. In other words the limits of flexo printing were extended, for example by using the process for coating disclosed herein and by using specific hydrophobic lining paper, primer and coating material, and process parameters.

[0051] The printer was equipped with six printing stages, each with a coating material chamber provided with a doctor blade system. The anilox rolls were equipped with 75 degrees Eflo elongated hexagonal cells, 130 lines per centimeter (Ipcm), with a volume of 14.0 cm³/m². In general, the 75 degree inclined (tapering) side walls of the Eflo cells allow transfer of the full cell volume, thereby providing better coverage on liners. Alternatively, for example an APEX GTT2.0 anilox roll was used to transfer large amounts of coating material.

[0052] The printing plates (or clichés) were XSYS Nyoflex FSC394D plates with 90% grid 120 lines per inch (Ipi) and a shoreA hardness of 28, thereby providing a relatively soft contact surface compared to conventional printing plates. The settings were chosen in such a way that the soft cliché just touched the sheet and transferred the coating material without crushing the flute ("kiss printing"). The profile on the printing plates was formed primarily by stubs having a semi spherical or at least rounded top surface with relatively deep throughs in between them for holding coating prior to and during transfer of the coating to the sheet. The rounded top surfaces are considered to protect the sheet further from damage during applying the coating or coatings.

[0053] In the trials conducted the corrugated board was lined with a brown Kraftliner 135 from SK Pitea with a surface roughness Bendtsen (ISO8791) = 842 ml/min, and a hydrophobic surface of Cobb60 = 28 g/m². The equipment was optimized to apply coating materials with viscosities of 20-30 seconds DIN Cup4 at a line speed of 2000-6000 sheets, especially for applying a first coating. To allow the coating to dry trials were done at low speeds. At maximum coating thickness and speed an IR-drier was used to dry the coating.

[0054] As illustrated in FIG. 5, amongst others, coated sheets with 3 or 4 layers were found to have between 5-10 g/m² dry coating. An oil spray test showed that the coatings were pinhole-free. Their water barriers were measured in Cobb1800 tests and showed values between 25-35 g/m².

[0055] The trials were repeated in combination with a graphic print. In the second printing stage an ink was applied over which 3 or 4 layers of coating material were applied. The printed and coated sheets had the same properties as abovementioned.

[0056] The trials were also repeated on corrugated sheets with clay coated TCH140 liner with an extremely low surface roughness of 12 ml/min Bendtsen. Here, a Cobb1800 value of 13 g/m² was found, even at a low coating deposit of 3.2 g/m², see FIG 6. Based on these results, values lower than 10 g/m² are possible with some minor tweaking.

[0057] FIGs. 7A-B and FIG. 8 schematically illustrate an embodiment of a coated sheet 51 that can be obtained by the system and method described herein. Preferably, prior to coating the sheet 50, the sheet 50 has a surface roughness lower than 1500 ml/min as measured according to ISO 8791, preferably lower than 500 ml/min, to facilitate formation of a closed film during the multi-stage coating process. For example, the sheet 50 is made of a clay coated paper with a surface roughness lower than 10 ml/min. In corrugated board the said surface roughness is measured at the outer layer to be coated.

[0058] Also, the coating process is improved when, prior to the steps of printing at the first and further printing stages 200, the sheet 50 is treated with a hydrophobic sizing agent, or when the sheet is already hydrophobic, e.g. by nature. In other words, the sheet may be inherently hydrophobic, i.e. water repelling. Examples of sheet materials that are hydrophobic or grease repellent. by nature include Kraftliner, asphalt paper, or parchment paper (Glassine or Pergamyn).

[0059] As illustrated in FIG. 7A, the sheet 50 can be provided with a barrier coating with local differences in layer thickness, e.g. by partially or locally overlapping the coating layer 30 by a second coating layer 31, using a second printing stage, or even further coating layers (not shown in fig. 6). Beneficially, the layer thickness can thus be locally increased, e.g. around parts of the sheet 50 that are to be creased or scored for folding the sheet into a box, as illustrated in FIG 7B. In this way even after creasing, scoring and folding, water absorbency remains low in those parts, while other parts of the sheet can be provided with a relatively thin coating layer, e.g. to minimize consumption of coating material.

[0060] In FIG. 8, another exemplary embodiment of the coated sheet 51 is illustrated. Here, a barrier coating with local differences in coating properties is provided, by applying first coating materials 30 and further coating materials 31 with different material compositions onto the sheet 50, for example next to each other and/or partially overlapping.

[0061] FIG. 9A shows a sheet 50, 51, coated according to the disclosure, for use for forming a tray 300 as shown in FIG. 9B. In FIG. 9A on the right hand side a part of a blank is shown, made of corrugated card board, for forming said tray 300, in substantially flat position, and at the left hand side the tray is shown in folded position. The blank comprises a bottom forming portion 301, two side wall forming portions 302, connected to the bottom forming portion 301 through first creased folding lines 303. The blank further comprises two end wall forming portions 304 connected to the bottom forming portion 301 by second creased folding lines 305. Corner forming flaps 306 are provided connecting the side wall forming portions 302 and end wall forming portions 304, connected by third creased folding lines 307. Each corner forming flap 306 is provided with a fourth folding line 308, creased for folding the corner forming flaps 306 into two portions which can be folded against each other. The fourth folding lines 308 extend from corners 309 of the bottom forming portion 301, at the intersection of first, second, third folding lines 303, 305 and 307, upward to a rim portion 310. Along the side wall forming portions 302 and the end wall forming portions 304 sealing flaps 311 are provided, connected to the side wall portions 302 and end wall forming portions 304 through fifth folding lines 312, creased into the blank.

[0062] When folded into the tray, the side wall forming portions 302 and end wall portions 304 are folded upward to an inclined position relative to the bottom forming portion 301 along the first and second folding lines. At the same time the corner forming flaps 306 are folded inward and can for example be adhered to the side wall forming portions 302, as shown schematically in FIG. 9A left hand side. The sealing flaps 311 are folded outward, for forming a sealing edge 313.

[0063] In a blank and tray 300 as disclosed all panels and wall forming portions have been coated with at least one coating layer, preferably at least two coating layers, with a method of the disclosure. As can be seen in FIG. 9A different areas of the flaps and wall forming portions have been coated with different coatings, indicated by different hatchings or such patterns. For example, the bottom forming portion, the side wall forming and end wall forming portions and part of the corner forming flaps have been coated at least with a relatively highly moisture such as water resistant coating, indicated by a dotted pattern. The remaining parts of the corner forming flaps have been coated with a coating which can be glued easily to the inside of for example a side wall forming portion. For example but not limited to using hot melt glue, pressure glue or heat sealing. The sealing flaps 311 are provided with a coating to which a plastic film 315 can be heat sealed.

[0064] Areas in which folding lines are to be formed may be provided with an additional layer of coating. By way of example around two corners 309 of the bottom forming portion a circle 314 is drawn, indicating an area which can be provided with an extra layer of coating, as for example shown in FIG. 7A, for improving moisture resistance in such area even after creasing the board. It shall be clear that similar areas can be provided along some or all of the folding lines.

[0065] Coatings used in the method and products according to the disclosure can be clear coatings or opaque coatings, and coatings used can be colored. Combinations thereof can be used, for example a clear coating for covering an underlying coating of a chosen color or provided with printing, as described before.

[0066] Transfer rollers 211 used in the present disclosure may be anilox rollers or other types of rollers, preferably comprising a rastered surface structure for transferring relatively large amounts of coating. For example the structure may be formed by cells, such as the hexagonal cells as for example shown in fig. 3, or by ribs or the like extending over said surface, such as but not limited to a pattern of wavy ribs, such as for example sinusoidal ribs, for example parallel to each other and having a main direction parallel to a longitudinal axis of the roller or at an angle to that axis.

[0067] It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which may be considered within the scope of the appended claims. Also kinematic inversions are considered inherent to the invention disclosed herein. In the claims, any reference signs shall not be construed as limiting the claim.

[0068] The terms 'comprising' and 'including' when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus expression as 'including' or 'comprising' as used herein does not exclude the presence of other elements, additional structure or additional acts or steps in addition to those listed. Furthermore, the words 'a' and 'an' shall not be construed as limited to 'only one', but instead are used to mean `at least one', and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may additionally be included in the structure of the invention without departing from its scope.

[0069] Expressions such as: "means for ..." should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. To the extent that structure, material, or acts are considered to be essential they are inexpressively indicated as such. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the scope of the invention, as determined by the claims.

Claims

1. A method for coating a sheet of paper-based packaging material, comprising:

transferring the sheet in a transfer direction;

printing, at a first printing stage provided along the transfer direction, a first coating layer onto the sheet, wherein the first printing stage comprises a first transfer roller configured to transfer a first coating material to a first printing plate circumferentially mounted on a first plate roller, wherein the first printing plate is configured to apply the first coating material onto the sheet for thereby locally coating a first area;

printing, at one or more further printing stage provided downstream the first printing stage, one or more further coating layers onto the sheet, the or each further printing stage comprising a further transfer roller configured to transfer a further coating material to a further printing plate circumferentially mounted on a further plate roller, wherein the further printing plate is configured to apply the further coating material onto the sheet for thereby locally coating a further area;

wherein, when applied on the sheet, the first and further coating layer or layers form a single closed film on the sheet that provides a barrier coating.

2. The method according to claim 1, wherein the first and further areas at least partially overlap for thereby providing a barrier coating with local differences in layer thickness.

3. The method according to any preceding claim, wherein the first and further coating materials have different material compositions for thereby providing a barrier coating with local differences in coating properties.

4. The method according to any preceding claim, wherein each of the first and further coating materials comprise a water based fluid with a viscosity lower than about 300 mPas, more preferably between 150 and 250 mPas.

5. The method according to any preceding claim, wherein each of the first and further coating materials comprise solid particles at a weight percentage of at least 45% prior to application on the sheet.

6. The method according to any preceding claim, wherein the first coating material comprises a sealing substance arranged for sealing pores of the paper material, and wherein the further coating materials comprises a hydrophobic substance arranged for repelling water.

7. The method according to any preceding claim, wherein the sheet has a surface roughness lower than 1500 ml/min as measured according to ISO 8791, preferably lower than 500 ml/min.

8. The method according to any preceding claim, wherein the sheet is made of a clay coated paper with a surface roughness lower than 10 ml/min.

9. The method according to any preceding claim, wherein, prior to the steps of printing at the first and further printing stages, the sheet is treated with a hydrophobic sizing agent, for thereby providing the sheet with a water absorbency level lower than 30 g/m2 according to Cobb60, preferably lower than 27 g/m2.

10. The method according to any preceding claim, wherein the sheet is hydrophobic by nature.

11. The method according to any preceding claim, wherein, prior to the steps of printing at the first and further printing stages, the method comprises the step of bulk sizing the sheet by hydrophobic sizing.

12. A coated sheet of paper-based packaging material obtainable by the method according to any preceding claim, wherein the coated sheet has a water absorbency level lower than 40 g/m2 according to Cobb1800.

13. A system for coating a sheet of paper-based packaging material, comprising:

a transfer arrangement configured to transfer the sheet in a transfer direction;

a first printing stage provided along the transfer direction and arranged for printing a first coating layer onto the sheet, the first printing stage comprising a first transfer roller configured to transfer a first coating material to a first printing plate circumferentially mounted on a first plate roller, wherein the first printing plate is configured to apply the first coating material onto the sheet for thereby locally coating a first area;

at least one further printing stage arranged downstream the first printing stage, wherein the or each further printing stage is arranged for printing a respective further coating layer onto the sheet, the or each further printing stage comprising a further transfer roller configured to transfer a further coating material to a further printing plate circumferentially mounted on a further plate roller, wherein the further printing plate is configured to apply the further coating material onto the sheet for thereby locally coating a further area;

wherein, when applied on the sheet, the first and further coating layers form a single closed film on the sheet that provides a barrier coating.

14. The system according to claim 13, wherein at least one of the first and further transfer rollers is an anilox roller, preferably provided with a hexagonally shaped cell structure for transferring the amount of first or further coating material to the respective first or further printing plate.

15. The system according to claim 14, wherein the cell structure has a volume of at least 12 cm3/m2, preferably 14 cm3/m2 or higher.

16. The system according to any of the preceding claims 13-15, wherein the first and or further printing plates have a dimpled outer surface.

17. The system according to any of the preceding claims 13-16, further comprising a dryer device arranged for drying the first and or further coating layers downstream the first printing stage.

Drawing