CH720000A9 - Process for coating a dried blank. - Google Patents

Abstract

A method for coating a dried blank (61) of a fiber-based product made of pulp is disclosed, comprising the steps of - providing the dried blank (61), - applying a powder coating to an inner side (63) of the dried blank (61), - curing the powder coating by exposure to infrared radiation so that a coherent film is formed. The infrared radiation penetrates a wall of the dried blank (61) during the irradiation.

Description

[0001] The present invention relates to a method for coating a dried blank of a fiber-based pulp product.

[0002] Various containers for holding liquids are known from the prior art. For example, glass bottles or plastic bottles for holding drinks have become known. Containers made from fiber-based material have also already been proposed.

[0003] A fiber-based container was proposed in WO 2012/139590 A1. To produce this container, so-called pulp is introduced into a mold and pressed against a corresponding wall in this mold using a flexible balloon and compressed accordingly.

[0004] Recently, at the same time or after this compression step, the still wet blank is dried by applying energy so that a dried blank is available for further processing.

[0005] Pulp is a mixture of fibers and water, in particular natural fibers such as hemp fibers, cellulose fibers or flax fibers or a mixture thereof. The pulp may contain additives, such as those known from PCT/EP2019/076839, which, for example, improve the hardening of the compressed pulp or influence the subsequent appearance or generally change the properties of the pulp or the subsequent container.

[0006] With these containers, there is a risk that they will become softened by liquid stored in the container and, for example, become leaky or that substances from the container will diffuse into the liquid.

[0007] It has been proposed to provide such fiber-based containers with an inner layer of plastic, in particular to arrange a plastic bottle within the fiber-based container, which can take on corresponding barrier functions. The fiber-based container here therefore merely provides a shell for a thin-walled plastic container. Such a combination is known from WO 2018/167192 A1.

[0008] Typically, a fiber-based container is provided into which a plastic lining is introduced. This typically takes place in the form that in a first step the fiber-based container is provided and in a second step a preform is introduced into this container. This preform is then inflated within the fiber-based container until it touches an inner contour of the fiber-based container or is in contact with the inner contour of the fiber-based container. This inflation process typically takes place in a blow mold whose cavity corresponds to the outer contour of the fiber-based container. Since, in contrast to conventional blow molding processes, two separate elements have to be handled, such a process is much more complicated because the preform has to be positioned and held relative to the fiber-based container and this in turn has to be positioned and held within the blow mold.

[0009] An exemplary process is described, for example, in EP 3 375 593 A1. EP 3 375 593 A1 describes a preform for producing a container which has extensions in its neck area which can be brought into engagement with a corresponding fiber-based container with the aim of the container and the preform remaining connected to one another. However, it has been found that this type of connection is prone to errors, since the inner contour or the inner surface of the container is subject to varying degrees of deviation depending on the specific properties of the pulp from which the container is formed. This is particularly disadvantageous in the neck area, since certain tolerance limits must be observed for a clean pressing between the preform or its extensions and the fiber-based container, but this is not always possible due to the different process parameters and the possibly changing nature of the pulp. In addition, this type of production still requires a relatively large amount of plastic.

[0010] It is an object of the invention to remedy one or more disadvantages of the prior art. In particular, a method is to be provided which makes it possible to design a fiber-based container in a liquid-tight manner and in particular reduces the use of plastics.

[0011] This object is achieved by the method described in the independent claim. Further advantageous embodiments emerge from the dependent patent claims.

[0012] A method according to the invention for coating a dried blank of a fiber-based product made of pulp, in particular a container or a fiber-based closure element for a container, comprises the steps of: - providing the dried blank, - applying a powder coating to an inner side of the dried blank, - curing the powder coating by exposure to infrared radiation so that a coherent film is formed.

[0013] In order to apply infrared radiation to the powder coating, the latter penetrates a wall of the dried blank during irradiation, particularly in the area of the interior of the container before it hits the powder coating.

[0014] This allows the infrared source to be arranged outside the dried blank, but the powder coating inside the dried blank can be exposed to the infrared radiation.

[0015] The penetration of the dried blank makes it possible in particular to bring the powder coating to a higher temperature level than the blank.

[0016] An additional or alternative aspect of the invention also relates to a method in which the blank and the powder coating are heated, in particular differently. The powder coating is heated to a higher temperature than the blank, in particular by infrared radiation that penetrates a wall of the blank. In particular, the blank is heated essentially only by the heat radiation from the powder coating.

[0017] Preferably, the dried blank is provided with a prefabricated opening before the powder coating is applied.

[0018] By providing a prefabricated opening, a corresponding sealing plane or sealing surface can be provided, which can interact with a corresponding closure, such as a lid. In addition, by providing a prefabricated opening, a fibrous or inaccurate closure of the blank can be removed at the same time.

[0019] It is known that certain inaccuracies can arise during production of fiber-based containers. Due to the material properties of the pulp, an upper, closing edge of the opening is subject to greater tolerances and is regularly fibrous. This is particularly disadvantageous because it forms an interface to container closures, for example, and this interface must be dimensionally accurate.

[0020] Preferably, during the production of the container, the dried blank is produced in such a way that a protrusion is provided in the region of an opening at the top, which can be cut off after the blank has dried. By cutting off this protrusion, a correspondingly imprecise or fibrous region of the dried blank can be removed.

[0021] It can be provided that the powder coating is additionally applied to the finished opening of the dried blank. This also allows the finished opening to be sealed.

[0022] In addition, a powder coating can be applied to an outside of a neck area of the dried blank after assembly. The dried blank remains uncoated on its outside at least in some areas.

[0023] By coating the outside of the neck area, it can also be sealed and/or made more durable during use.

[0024] The uncoated area on the outside of the blank makes it possible to recycle the product. The uncoated area provides a surface on which the product can be broken open or softened using water. The product can therefore be recycled more easily.

[0025] A meltable polymer can be applied as a powder coating. Polymers have advantageous properties and are easy to process.

[0026] Preferably, a polyester such as polyethylene terephthalate (PET), polyethylene furanoate (PEF), polyethylene isosorbide terephthalate (PEIT), polylactide (PLA), polybutylene succinate (PBS), poly-ε-caprolactone (PCL) or polyhydroxyalkanoate (PHA), in particular polyhydroxybutyrate (PHB) is used as the meltable polymer. These polyesters are preferably bio-based. Ethylene-vinyl alcohol copolymer (EVOH) has also proven to be suitable.

[0027] To apply the powder coating, it can be electrostatically charged. In particular, it can be provided that a conductive form that encases the dried blank is charged with opposite polarity to the powder. The powder then deposits itself in particular on the inside of the blank and remains there.

[0028] An infrared source for generating the infrared radiation is preferably arranged outside the dried blank.

[0029] This allows the infrared radiation to be directed at the dried blank in a targeted manner, at a specific distance and at a specific angle, and in particular at the powder coating at a corresponding distance and angle. The wavelength of the infrared radiation is preferably between 1 µm and 10 pm, in particular between 2 µm and 7 µm. These wavelengths can be easily absorbed by polymers. The absorption capacity is particularly high in the range from 2 µm to 7 µm.

[0030] These, in turn, are wavelength ranges that are not or only slightly absorbed by the wall of the dried blank. In other words, these wavelengths make it possible to melt the powder coating without radiating unnecessary energy into the dried blank.

[0031] To be exposed to the infrared radiation, the dried blank can be conveyed through an oven. The present oven is a device within which a large number of infrared sources are arranged. These can, for example, be arranged along a congruent contour of the dried blank and at a distance from it.

[0032] It may also be provided to install reflectors in the oven which redirect and/or reflect emitted infrared radiation.

[0033] It can also be provided to arrange different infrared sources along the length of the furnace. For example, infrared sources can be arranged at the entrance to the furnace that emit radiation with a wavelength of 6 µm to quickly heat up the powder coating. Infrared sources can be arranged downstream that emit radiation with a wavelength of 3 µm to keep the melting rate constant.

[0034] During exposure to infrared radiation, the dried blank can be rotated about its longitudinal axis. The dried blank is thus moved linearly through the oven, during which it rotates about its longitudinal axis. Accordingly, each area of a surface of the blank is guided past corresponding infrared sources, so that the dried blank and thus the powder coating is exposed to infrared radiation evenly from all sides.

[0035] To prevent the dried blank from overheating, it can be cooled by a cooling air stream during exposure to infrared radiation. Overheating of the dried blank is avoided. Overheating of the dried blank would cause the fibers to degenerate and the strength to decrease.

[0036] The infrared sources can, for example, be arranged at different heights along the conveying direction of the blank, so that they each act on different areas of the blank.

[0037] Alternatively or additionally, it may be provided to provide infrared sources of different power or to control the infrared sources, for example, based on a respective product profile, so that their power is adjusted depending on the product or the duration of the product.

[0038] Preferably, the dried blank is transparent in the wavelength range from 1 µm to 10 µm. This can be achieved by the chemical composition of the blank having functional groups that cannot be excited in this wavelength range in question.

[0039] Due to the transparency in this wavelength range, the corresponding infrared radiation can penetrate a wall of the dried blank without heating it up unnecessarily.

[0040] Accordingly, the powder coating preferably absorbs infrared radiation in the wavelength range from 1 µm to 10 µm. This means that the coating can be heated up quickly.

[0041] The applied powder coating is thus subjected to energy and the powder is converted into a melt. The melting process creates a homogeneous film that provides a corresponding seal. This film preferably extends from the inside over the prepared opening to an outside area of the neck and forms a continuous seal in this area.

[0042] It can be provided that the dried blank is tested for leaks. This can prevent defective products from being sold.

[0043] After sealing, i.e. after melting the powder coating and subsequent cooling, a product can be filled into the dried blank, provided the dried blank is designed as a container. The dried blank can then be closed with a closure. This creates a closed body for transporting and protecting liquid products. In particular, by sealing the packaging edge, a reliable seal can be created in this area with, for example, a corresponding sealing cone or a sealing plane, for example made of a sealing material such as a liner.

[0044] The method according to the invention is explained using schematic figures. It shows: Figure 1: a powder coating process; Figure 2: the drying step of the powder coating process; Figure 3: a leak test; Figure 4: the sealing process; Figure 5: examples of other typical products that can be produced using the method according to the invention; Figure 6: examples of a typical fiber-based closure that can be produced using the method according to the invention; Figure 7: a representation of first absorption spectra; Figure 8: a representation of second absorption spectra; Figure 9: a representation of third absorption spectra.

[0045] Figure 1 shows a coating step. A dried and already finished blank 61 is fed to a powder coating system (not shown in detail here). In the next step, an electrostatically charged lance 35 is introduced into the finished blank 61. This is located in an oppositely charged casing (also not shown here). Due to the electrostatic charge of the applied powder, it adheres to the inside 63 of the finished blank 61.

[0046] The now coated finished blank 61 is, as shown in Figure 2, transferred to an oven and exposed to infrared radiation. This causes the powder coating to melt, so that a continuous, homogeneous film is created. The blank 61 is thus sealed.

[0047] Subsequently, the blank 61 can be tested for leaks using a corresponding testing device 500, as shown in Figure 3.

[0048] The blank 61 can then be closed with a lid 300, as shown in Figure 4.

[0049] Figure 5 shows, by way of example, further typical fiber-based products that can be produced using the method described here. For example, a container 100 in the form of a bottle is shown. This also has a thread on the bottle neck and essentially corresponds to a fiber-based product produced from a blank 61 according to Figures 1 to 4. The container 100' is in the form of a bowl, the container 100" in the form of a cup.

[0050] Figure 6 shows an example of a typical fiber-based closure 300 that can be manufactured using the method described here.

[0051] Figures 7 to 9 each show different comparisons of different absorption spectra of different materials.

[0052] In Figures 7 to 9, the dark solid line shows the emitted infrared power as a cumulative distribution function of an infrared source with a 3 µm peak and the light solid line shows the emitted infrared power as a cumulative distribution function of an infrared source with a 6 µm peak.

[0053] Figures 7 to 9 also show the absorption capacity or the absorption spectrum of the dried blank. The corresponding line is shown in fine dots. It can be seen that the blank has increased absorption in the range around approx. 10 µm and from the range from approx. 14 µm.

[0054] In Figure 7, the dashed line shows the absorption capacity or the absorption spectrum of a powder coating made of EVOH. As can be seen, EVOH has increased absorption in the range of approx. 3 µm, approx. 3.5 µm and in the range between 7 µm and 8 µm. It is therefore clear that radiation with these wavelengths is absorbed significantly better by EVOH than by the blank. The powder coating is therefore heated up more quickly, until it melts, without the blank being heated up as much.

[0055] In Figure 8, the dashed line shows the absorption capacity or the absorption spectrum of a powder coating made of PHB. As can be seen, PHB has an increased absorption in the range of approx. 6 µm and in the range between 8 µm and 10 µm. It is therefore clear that radiation with these wavelengths is absorbed significantly better by PBH than by the blank. The powder coating is therefore heated up more quickly, until it melts, without the blank being heated up as much.

[0056] In Figure 9, the dashed line shows the absorption capacity or the absorption spectrum of a powder coating made of PET-C, i.e. crystalline PET. As can be seen, PET-C has an increased absorption in the range of approx. 6 µm and approx. 8 µm. It is therefore clear that radiation with these wavelengths is absorbed significantly better by PET-C than by the blank. The powder coating is therefore heated up more quickly, until it melts, without the blank being heated up as much.

Claims (15)

1. Method for coating a dried blank (61) of a fiber-based product made of pulp (40), in particular a container (100, 100', 100'') or a fiber-based closure element (300) for a container (100, 100', 100''), comprising the steps - providing the dried blank (61), - applying a powder coating to an inner side (63) of the dried blank (61) - curing the powder coating by exposure to infrared radiation (81) so that a coherent film is formed, characterized in that the infrared radiation (81) penetrates a wall of the dried blank (61) during irradiation. 2. Method according to claim 1, characterized in that the dried blank (61) is provided with a ready-made opening before the application of the powder coating. 3. Method according to claim 3, characterized in that the powder coating is additionally applied to the finished opening of the dried blank (61). 4. Method according to one of claims 2 to 3, characterized in that a powder coating is additionally applied to an outer side (64) of a neck region of the dried blank (61) after the assembly, the dried blank (61) remaining uncoated on its outer side at least in regions. 5. Method according to one of claims 1 to 4, characterized in that a meltable polymer is applied as a powder coating, in particular polyester such as polyethylene terephthalate (PET), polyethylene furanoate (PEF), polyethylene isosorbide terephthalate (PEIT), polylactide (PLA), polybutylene succinate (PBS), poly-ε-caprolactone (PCL) or polyhydroxyalkanoate (PHA), in particular polyhydroxybutyrate (PHB), wherein these polyesters are preferably bio-based, or ethylene-vinyl alcohol copolymer (EVOH). 6. Method according to one of claims 1 to 5, characterized in that the powder coating is electrostatically charged to apply it. 7. Method according to one of claims 1 to 6, characterized in that an infrared source (80) for generating the infrared radiation (81) is arranged outside the dried blank (61). 8. Method according to one of claims 1 to 7, characterized in that the wavelength of the infrared radiation (81) is between 1 µm and 10 µm, preferably between 2 µm and 7 µm. 9. Method according to one of claims 1 to 8, characterized in that the dried blank (61) is conveyed through a furnace (85) for exposure to the infrared radiation (81). 10. Method according to one of claims 1 to 9, characterized in that the dried blank (61) is rotated about its longitudinal axis during exposure to the infrared radiation (81). 11. Method according to one of claims 1 to 10, characterized in that the dried blank (61) is cooled by a cooling air stream during exposure to the infrared radiation (81). 12. Method according to one of claims 1 to 11, characterized in that the dried blank (61) is transparent in the wavelength range from 1 µm to 10 µm. 13. Method according to one of claims 1 to 12, characterized in that the powder coating absorbs the infrared radiation (81) in the wavelength range from 1 µm to 10 µm. 14. Method according to one of claims 1 to 13, characterized in that the dried blank (61) is tested for its tightness. 15. Method according to one of claims 1 to 14, characterized in that the dried blank (61) is a container (100), wherein a product is filled into the dried blank (61) and the dried blank is subsequently closed with a closure.