Method for coating a molded fiber-based product, a coated molded fiber-based product prepared by said method, and use thereof as a foodstuff container

Abstract

The present invention relates to a method of producing a coated, molded fiber-based product which involves filling of at least one depression of the molded fiber-based product with a liquid water-based coating composition and removing excess coating composition to obtain a wet coating film, followed by curing the wet coating film. The present invention further relates to a coated, molded fiber-based product prepared by said method and its use as a foodstuff container.

Description

METHOD FOR COATING A MOLDED FIBER-BASED PRODUCT, A COATED MOLDED FIBER-BASED PRODUCT PREPARED BY SAID METHOD, AND USE THEREOF AS A FOODSTUFF CONTAINERFIELD OF THE INVENTION

[0001] The present invention relates to a method of producing a coated, molded fiber-based product which involves filling of at least one depression of the molded fiber-based product with a liquid water-based coating composition and removing excess coating composition to obtain a wet coating film, followed by curing the wet coating film. The present invention further relates to a coated, molded fiber-based product prepared by said method and its use as a foodstuff container.BACKGROUND OF THE INVENTION

[0002] Plastics are widely used as food packaging materials since they are generally chemically inert and lightweight, can be formed in any shape, and offer a number of beneficial properties in terms of appearance and chemical properties such as heat resistance and barrier properties. However, the widespread use of plastics contributes to waste and pollution issues and is harmful to human and marine health. A particular concern in this regard are disposable or single-use plastic articles like single-use food containers, which are not biodegradable, cannot be recycled, and have an impact on human and marine life. Therefore, great efforts are made to develop single-use articles that are no longer based on plastics but are made from recyclable and more eco-friendly materials.

[0003] In the food packaging industry, natural fibers have been used to produce articles for single-use that are biodegradable. In particular, cellulose fibers can be used that are derived from various raw materials such as wood, bamboo, bagasse or even wheat straw. The cellulose fibers are dispersed in water, then formed and dried. However, the problem with cellulose fibers is that they swell when brough into contact with food due to the water andgrease contained in the food and, thus, cellulose said fibers as such do not possess sufficient barrier properties. Therefore, a key requirement in the field of fiber-based food packaging is to generate desired barrier properties against different foodstuffs. Known approaches for creating adequate barrier properties include the following.

[0004] Firstly, a film lamination can be applied to articles for single-use in order to provide a film against moisture or grease from food or beverages to be filled in the articles for single-use. In most cases, these laminations are made from plastics or aluminized plastics and, thus, are not biodegradable and cannot, or only with difficulty, be recycled.

[0005] Secondly, it is also known to directly incorporate additives in the fiber matrix. While these additives may render the fiber mixtures water-resistant, they are resin-based, AKD (alkyl ketene dimer)-based, wax-based, rosin-based, paraffin-based, or PFAS (per- and polyfluoroalkyl substances)-based and are therefore not biodegradeable.

[0006] Thirdly, a coating may be applied to the surface of articles for single-use. These coatings are provided as solutions, emulsions, dispersions or suspensions of at least one solvent and at least one substance capable of forming a solid layer on the surface of the article for single-use after removing the solvent base. However, the substances used for forming the coatings, are often not biodegradable. In addition, if 3D surfaces are coated, the resulting coatings may not be completely closed resulting in inadequate barrier properties.

[0007] Generally, the above-mentioned conventional food barrier options, e.g. being based on PFAS, do not suit the recent urge of sustainability, although they may providing excellent grease resistance. Therefore, coated articles for single-use that are not only made from natural materials such as cellulose fibers but also have a coating that meets the needs of sustainability are needed in the art.

[0008] To address this need, water-based biodegradable coatings have been developed, which can be used for coating natural and biodegradable fibers-based products, thereby offering the possibility of obtaining sustainable products. Water-based coatings can also be used in a much lower amount on the fiber substrate, which contributes to sustainability. Additionally, it is proven that fiber-based products with water-based coatings have much better re-pulping properties in the recycling process compared to plastic laminated fiber-based products. This is because the coating amount is much lower and the coating layer fractionatesmuch easier during the repulping process which, for example, prevents filtering sieves from being blocked and thus facilitates recycling processes.

[0009] As already mentioned, application of coatings to 3D surfaces is still a challenging field. While the application of coatings on a 2D surface such as paper or board fiber materials can be carried out by known technologies such as different printing technologies, curtain coating, air knife coating etc., the application of a coating on an already 3D molded fiber product is much more challenging because the common application techniques for 2D surfaces cannot be simply transferred to a 3D surface.

[0010] Generally, the application of coatings to 3D surfaces is done by spray-coating. However, this technology has several drawbacks. For example, if the 3D article has steep areas, e.g., sidewalls, it is difficult to achieve a full coverage of these parts of the article. Also, it is a challenge to apply the coating uniformly since the applied coating composition tends to flow down due to gravity. This leads to varying thickness and compromised aesthetics.

[0011] Spray-coating is also problematic in that the spraying process results in the deposition of small droplets on the surface, the surface is hardly closed on a microscopic level and small openings remain in the coating. In other words, it is difficult to achieve a completely closed shell of the coating on the surface of the coated article, in particular on the surface of a fiber-based article for single-use. The small openings are referred to as "pinholes" in the art and are one of the biggest challenges in the process of coating disposable articles suitable for contacting foodstuffs because grease and moisture can migrate through these openings into the coated article, e.g., a fiber-based article.

[0012] In view of the above, there is a need for a process of producing a coated, molded fiber-based product, which is not only eco-friendly, i.e., is sustainable, biodegradable and / or compostable, but has also exhibits the desired physical and chemical properties for contacting foodstuffs, in particularly good barrier properties.OBJECT OF THE INVENTION

[0013] The objective of the present invention is to provide a molded fiber-based product which has an improved environmental impact profile while at the same time providing excellent properties for contacting foodstuffs.SUMMARY OF THE INVENTION

[0014] The above object is solved by a method for coating a molded fiber-based product, in particular a cellulose-based product, by applying a liquid water-based coating composition to one or more surfaces of the product each defining a depression for contacting foodstuffs, removing excess coating composition, and curing the resulting wet coating film. The resulting coated, molded fiber-based product advantageously has a smooth, uniform coating with minimized pin holes and shows the required properties for contacting foodstuffs. It can be prepared in a simple, cost-efficient and sustainable manner.

[0015] In a first aspect, the present invention provides a method of producing a coated, molded fiber-based product, the method comprising the steps of:(i) providing a molded fiber-based product having one or more surfaces each defining a depression for contacting foodstuffs,(ii) filling the depression of the molded fiber-based product with a liquid water-based coating composition to allow the coating composition to contact the surface defining the depression at least partially,(iii) removing excess liquid water-based coating composition to obtain a wet coating film on the at least partially contacted surface,(iv) curing the wet coating film to produce the coated, molded fiber-based product.

[0016] If desired or needed, steps (ii) to (iv) may be repeated once or multiple times, e.g., one to ten times, preferably one, two or three times. Preferably, the fiber-based product is a cellulose-based product, and the liquid water-based coating composition is based on silica and / or waxes and / or polymers, e.g. styrenic or acrylic polymers, in form of emulsions or dispersions. In particular, the compositions MF of Papkot™, Sunstar from Sun Chemical,RHOBARR™ from Dow Chemical, Topscreen™ from Solenis and Aquaseal™ and Nowax™ from Paramelt are suitable to be used as liquid water-based coating composition.

[0017] In a second aspect, the present invention provides a coated, molded fiber-based product that is obtainable by the method of the present invention. The coated product prepared by the method of the present invention is characterized by its closed layer structure having no or almost no pinholes.

[0018] In a third aspect, the present invention relates to the use of the coated, molded fiberbased product of the present invention as a foodstuff container, in particular as trays, plates, bowls, mugs, cups, casseroles, clamshells, lids etc.

[0019] Preferred embodiments are set forth in the appended dependent claims and in the following detailed description taken in connection with the examples provided therein and the accompanying figures.BRIEF DESCRIPTION OF FIGURES

[0020] FIG. 1 schematically shows the process steps (i) to (iv) of the method of producing a coated, molded fiber-based product according to the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention is based on the finding that coating of a molded fiber-based product with a liquid water-based coating composition according to the present invention leads to advantageous properties of the surface of the coated, molded fiber-based products. For example, a smoother surface characterized by a more uniform distribution of the coating over the surface and the presence of less pinholes is obtained, which further increases the resistance of the obtained products against water and / or grease. Further, the coated, molded fiber-based products are considered highly sustainable due to the use of a fiber-based molded product and a coating prepared from a liquid water-based coating composition.

[0022] As used herein, the term “foodstuff container” is synonymous with “food container” and refers to all kinds of containers, receptacles, articles or products suitable for enclosing, storing, packaging, receiving or supporting foodstuff or foodstuff product, including, but notlimited to trays, plates, bowls, mugs, cups, casseroles, clamshells, and lids. In the context of the present invention, a foodstuff container is preferably used for enclosing or supporting foodstuff, in particular used to fill it with foodstuff such as solid food or beverages.

[0023] The term “foodstuff” or “foodstuff product”, as used herein, is synonymous with “food” or “food product” and refers to any liquid or solid consumable product, i.e. , any edible material that may be ingested by a human for purposes including, but not limited to nourishment, sustenance, non-nutritive appetite satiation and combinations thereof.

[0024] The term “molded fiber-based product”, as used herein, refers to any product substantially made from any fiber and optionally additives and has been already shaped into its final three-dimensional form. In particular, within the present invention, the molded product is preferably a product prepared by a molded pulp process based on wet pulp.

[0025] The pulp used for preparing the molded fiber-based product may be pulp stemming from one or more sources as for example but not limiting to bagasse pulp and wood pulp.

[0026] In one embodiment the pulp mixture consists of 70 wt.-% bagasse pulp and 30 wt.-% wood pulp (adding up to 100 wt.-% pulp combined with the additives based on polyluminium chloride (PAC) and rosins to generate water resistance of the final (molded) product. In a first aspect, the present invention provides a method of producing a coated, molded fiber-based product, the method comprising the steps of:(i) providing a molded fiber-based product having one or more surfaces each defining a depression for contacting foodstuffs,(ii) filling the depression of the molded fiber-based product with a liquid water-based coating composition to allow the coating composition to contact the surface defining the depression at least partially,(iii) removing excess liquid water-based coating composition to obtain a wet coating film on the at least partially contacted surface,(iv) curing the wet coating film to produce the coated, molded fiber-based product.

[0027] Satisfactorily coated, molded fiber-based products may be achieved after conducting the method according to the invention once.

[0028] Further, the method according to the invention may be conducted multiple times to ensure obtaining a surface having no or nearly no pinholes left. A low number of pinholes is desirable since this ensures a fully closed coating the surface underlying the depression of the molded fiber-based product. Therefore, the method according to the invention may be repeated one to ten times, or one to five times, or one to three times, in particularly once, twice or three times.

[0029] A fully closed coating layer is advantageous in order to improve grease and / or water resistance and thus, the durability during food and / or beverage contact of the coated, molded fiber-based products obtained from the method according to the invention.

[0030] Further, it is advantageous that the present method according to the invention can be integrated in any known of molded fiber-based product manufacturing process. For example, the method according to the invention may be used in-line or off-line together with known processes of the prior art.

[0031] In step (i) of the method according to the invention, there is provided a molded fiberbased product having one or more surfaces each defining a depression for contacting foodstuffs. The term “depression for contacting foodstuff”, as used herein, refers to any depression which may, e.g., receive, store, enclose or support foodstuff or which comes into contact with foodstuff in any other way, intentionally or unintentionally. Within the context of the present invention, the depressions are generally coated to provide a barrier against the foodstuff, in particular against water and / or grease (fat) contained in the foodstuff.

[0032] The term "one or more surfaces each defining a depression" refers to one or more surfaces of one single molded fiber-based product that are molded inwardly to create an inner volume. Within the present invention, this term is to be understood as meaning that the molded fiber-based product may have one or more than one, i.e. multiple, depressions. The term "inner volume" refers to the volume that would be enclosed by one depression of the molded fiberbased product if an imaginary top surface were placed on top of said depression. The inner volume is generally intended to receive foodstuff. It should be understood that the molded fiber-based product can have, and generally has, other surfaces beside the one or more surface each defining depressions.

[0033] The fibers of the molded fiber-based product provided in step (i) according to the invention may comprise or consist of natural fibers, preferably cellulose fibers, pulped fibers as e.g. softwood fibers or sugarcane bagasse fibers, wastestream fibers as e.g. rice husk and rice straw, sugar beets, flax fibers, palm or mixtures thereof. In general, the fibers used for the molded fiber-based product can be pulped or not be pulped, i.e. other natural occurring substances as for example lignin, hemicellulose, other types of polysaccharides and the like can still be present or already be removed. Particularly preferred, the molded fiber-based product comprises cellulose fibers or is based on cellulose fibers, still more preferably comprises only cellulose fibers and no other fibers. Said cellulose fibers can be regenerated cellulose fibers and / or micro- or nanofibrillated cellulose. Furthermore, the fibers may be pretreated according to methods known in the art.

[0034] In addition, all fibers used for the molded fiber-based product may be biodegradable and / or compostable.

[0035] The term “biodegradable”, as used herein, refers to materials which can be biodegraded to at least at least 70% by weight, in particular 90% by weight, 95% by weight, 99% by weight or can be degraded completely, i.e. to 100% by weight. The biodegradeability is measured in accordance with ISO 14855-1 :2012 orASTM 5338:2015.

[0036] Further, the molded fiber-based product may or may not comprise any further additives known in the art in its fiber-matrix to alter the properties of the molded fiber-based product. Non-limiting examples for additives are carbonate salts; polysaccharides; minerals, such as silica; paraffins, such as microcrystalline waxes, low-molecular polyolefins and polyterpenes; polyvinyl alcohol; silicone oils resins or elastomers; chromium chloride complexes; aluminium, calcium, sodium, potassium, and ammonium salts; casein; mannogalactanes; sodium salt of carboxymethyl cellulose; methyl cellulose; hydroxyethyl cellulose; alginates; xanthane; copolymer structures; basic potassium zirconium carbonate; imidazolium compounds; phosphoric acid ester of ethoxylated perfluoropolyetherdiol; modified polyethylene terephthalates, perfluoropolyether dicarbonic acid; polyhexafluoropropylene oxide, alkyl ketene dimers (AKD), rosins, paraffinic waxes, synthetic and natural waxes (e.g., synthetic and natural paraffinic waxes).

[0037] The molded fiber-based product provided in step (i) of the method according to the invention may be pretreated according to any pretreatment method known in the prior art.

[0038] In step (ii) of the method according to the invention, the depression of the molded fiber-based product provided in step (i) of the method according to the invention is filled with a liquid water-based coating composition.

[0039] The term “liquid water-based coating composition” refers to any form of aqueous systems solutions, emulsions, suspensions, dispersions, slurries or sol-gels. The term “aqueous” thereby refers to systems containing water as liquid phase, either as a single solvent or as part of a solvent mixture, wherein the water is selected from the group consisting of tap water, deionized water demineralized water and distilled water. The solvent mixture may further comprise short chained alcohols and / or ketones. The term “sol-gel” refers to a colloidal suspension of inorganic substances such as but not limited to siliciumoxide, or siliciumoxide containing salts or compounds as for example sodium silicate, hydrolyzed sodium silicate, or mixtures thereof.

[0040] The particle size in the liquid water-based coating composition can vary in the range of from <1 nm to 750 pm, or of from 10 nm to 750 pm, or from 25 nm to 750 pm.

[0041] It should be understood that step (ii) of the method according to the present invention may involve filing more than one depression because the molded fiber-based product may have more than one, i.e. multiple, surfaces each defining a depression for contacting foodstuffs as stipulated in step (i) of the method of the present invention.

[0042] The liquid water-based coating composition used in the method according to the present invention is preferably based on one or more compounds selected from polysaccharides, proteins, cellulose, acrylic polymers, sytrenic polymers, waxes, polyhydroxyalkanoate (PHA), polylactates (PLA), polybutylene succinate (PBS), polybutylene adipate-terephthalate (PBAT), polycaprolactone (PCL), alginate, starch, natural waxes, shellac, chitin and cellulose or mixtures thereof.

[0043] Suitable non-limiting examples of compositions that may be used in the method of the present invention are based silica and / or waxes and / or polymers. In particular, MF of Papkot™, Sunstar from Sun Chemical, RHOBARR™ from Dow Chemical, Topscreen™ vonSolenis and Aquaseal™ and Nowax™ from Paramelt may be used in the method of the present invention.

[0044] In addition, the liquid water-based coating composition may contain one or more additives selected from surfactants, organic additives such as glycerol, natural waxes, natural polymers, polysaccharides, proteins, pH buffers or mixtures thereof.

[0045] Preferably, the liquid water-based coating composition contains a surfactant to control the surface tension of the liquid water-based coating composition. Suitable non-limiting examples for surfactants that may be added to the liquid water-based coating composition include for example Tergitol™ TMN-6 and EcoSurf™ from Dow.

[0046] Preferably, the surface tensions of the liquid water-based coating composition and the molded fiber-based product results in similar ranges. This is advantageous for a more uniform wetting of the surface of the molded fiber-based product and reduces the number of repetitions of the method according to the invention required to obtain a fully closed coating layer. Preferably, the contact angle between the liquid water-based coating composition and the molded fiber-based product is below 100°, more preferably below 90°.

[0047] Although molded fiber-based products having advantageous surface properties with the method according to the invention may be obtained when using non-biodegradable liquid water-based coating compositions, the application of biodegradable water-based compositions is preferred.

[0048] Suitable biodegradable liquid water-based coating compositions for use herein are based on one or more compounds selected from polysaccharides, proteins, cellulose, polyhydroxyalkanoate (PHA), polylactates (PLA), polybutylene succinate (PBS), polybutylene adipate-terephthalate (PBAT), polycaprolactone (PCL), alginate, starch, natural waxes, shellac, chitin or mixtures thereof.

[0049] Some coating properties resulting from the method according to the invention can be controlled by adjusting the viscosity of the applied liquid water-based composition in suitable ranges. In case the viscosity of the said liquid water-based composition is too low more pinholes are the result and thus, the surface will not be completely closed, which may require a higher number of repetitions to be closed to avoid reduced resistance to water and grease. Incase the viscosity of said water-based composition is too high, the liquid water-based coating composition flows down the walls after removing excess liquid water-based coating composition and gathers on the bottom of the fiber-based products resulting in non-uniform distribution of the wet coating film. In both cases, a surface appearing not smooth to customers results.

[0050] For optimal results, the viscosity of the liquid water-based compositions is preferably in the range of from 25 cP to 750 cP, preferably from 40 cP to 500 cP, still more preferably from 50 cP to 400 cP, yet more preferably from 60 cP to 300 cP, and most preferably from 70 cP to 200 cP. The viscosity is determined with a Brookfield viscometer with a spindle L2 at 100rpm at 25 °C.

[0051] Further, the liquid water-based coating compositions preferably has solids contents of at least 5% by weight, at least 10% by weight, or at least 15% by weight, up to 50% by weight, 40% by weight or 30% by weight. A preferred range is 5% to 50% by weight or more preferably 10% to 40% by weight, particularly preferred 15% to 30% by weight, based on the total weight of the liquid water-based coating composition.

[0052] The liquid water-based composition is preferably biodegradable and / or compostable.

[0053] Further, either the molded fiber-based product, or the liquid water-based coating composition, or both may be heated prior to and / or during step (ii) to a temperature in the range of from 20°C to 95°C, or from 40°C to 80°C, or from 50°C to 70°C.

[0054] The term “filling” refers to topping up the inner volume of a depression of the molded, fiber-based product with the liquid water-based coating composition by e.g. pouring excessive amounts of the liquid water-based coating into the depression. The surface being inundated by the liquid water-based coating determines the area of the surface of the depression of the molded, fiber-based product that is coated. Preferably, “filling” does not include the application of the liquid water-based coating by one of spraying, roll-to-roll, brushing and curtain coat processes. More preferably, “filling” does not include the application of the liquid water-based coating by two, three or all of spraying, roll-to-roll, brushing and curtain coat processes.

[0055] The depression of the molded fiber-based product can be filled essentially entirely with the liquid water-based coating composition. The term “essentially entirely” refers to thefilling degree to which the depression to which is filled, i.e. up to 100% of the inner volume of the depression is filled with the liquid water-based coating composition, in order to ensure entirely contacting the underlying surface of the depression in step c), i.e the bottom and side walls are completely wetted by the liquid water-based coating composition. The depression may be filled to at most 80% of the inner volume of the depression, or to at most 60% of the inner volume of depression, or to at most 40% of the inner volume of the depression, or to at most 20% of the inner volume of the depression; or to at least 10% of the inner volume of the depression; or to at least 20% of the inner volume of the depression; or to at least 30% of the inner volume of the depression; or to at least 40% of the inner volume of the depression; or to at least 50% of the inner volume of the depression. The filling degree in step b) of the method according to the invention mostly depends on the desired coated area of the coated, molded fiber-based product.

[0056] In step (iii) of the method according to the invention, the excess liquid water-based coating composition is removed from the depression of the molded fiber-based product to obtain a wet coating film on the at least partially contacted surface of said depression.

[0057] The excess liquid water-based coating composition may be removed from the depression of the molded fiber-based product by any technical means known in the prior art. For example, the excess liquid water-based coating composition may be poured out, ejected by rotation (e.g. centrifuged) or sucked out, e.g. by a suction tool or a sponge having the positive shape matching the respective depression or a tube at the lowest part of the depression.

[0058] The excess liquid water-based coating composition removed from the depression of the molded fiber-based product in step (iii) of the method according to the invention may be recovered.

[0059] The excess liquid water-based coating composition removed from the depression of the fiber-based product in step (iii) of the method according to the invention may further be recycled and / or re-used.

[0060] In step (iv) of the method according to the invention, the wet coating film remaining on the surface of the inner volume of the depression of the molded fiber-based coating composition is cured. The term “cure” refers within this application to a heat treatment of thewet coated molded fiber-based product during which excess solvent is removed and a closed film, the final coating, is formed from the deposited micro- and nanoparticles, for example but not limited to crosslinking of reactive groups of active components or fusing particles comprised in the wet coating film together.

[0061] The curing of the wet coating film in step (iv) of the method according to the invention may be conducted at temperatures of at least 20°C, or at least 25°C, or at least 30°C, or at most 250°C, or at most 200°C, or at most 170°C, or in the range of from 20°C to 250°C, or from 25 °C to 200 °C, or from 30 °C to 170 °C.

[0062] The curing of the wet coating film in step (iv) of the method according to the invention may be further conducted for a time of at least 10 s, or at least 15 s, or at least 20 s, or at least 30 s, or at most 15 min, or at most 10 min, or at most 5 min, or at most 3 min, or in the range of from 10 s to 15 min, or from 15 s to 10 min, or from 20 s to 5 min, or from 30 s to 3 min.

[0063] The temperature for curing the wet coating film in step (iv) of the method according to the invention may be reached by any technical means known in the prior art as for example by, but not limited to, IR irradiation, microwave irradiation, convectional hot air or combinations thereof. Preferably, convectional hot air is used to reach the temperature for curing the wet coating film in step (iv) of the method according to the invention.

[0064] The coating weight of the coating resulting from curing the wet coating film in step (iv) of the method according to the invention is at least 1 g / m2, or at least 3 g / m2, or at least 5 g / m2, and is up to 100 g / m2, or 75 g / m2, or 50 g / m2, or 20 g / m2. Preferably, the coating weight is in the range of from 1 g / m2to 100 g / m2, more preferably from 3 g / m2to 50 g / m2, and most preferably from 5 g / m2to 20 g / m2.

[0065] In a second aspect, the present invention provides a coated, molded fiber-based product obtainable by the method according to the present invention.

[0066] The coating of the coated, molded fiber-based product according to the invention can be characterized by the absence of any pinholes, i.e. by a completely closed surface.

[0067] In a third aspect, the present invention relates to the use of the coated, molded fiberbased product of the present invention as a can be used directly for customers in food-contactapplications or can be further treated according to any treatment methods known in the prior art.

[0068] The present invention further relates to the use of the coated, molded fiber-based product according to the invention as a foodstuff container, in particular as trays, plates, bowls, mugs, cups, casseroles, clamshells, or lids.ExamplesExample 1 : Preparation of coated, molded fiber-based productA liquid water-based coating solution is prepared comprising 50 w% Dow Rhobarr 135 and 50 w% water and stirred to a homogenous mixture.Next, a molded fiber-based product comprising bagasse / softwood fibers is provided. In this case, a casserole for oven use is prepared, which can withstand foodstuff under oven conditions such as 220 °C for 30 min.The casserole is filled to 80% of the inner volume with the coating solution. Next, the coating solution is removed by pouring out the solution into a container for recuperation. During pouring, the product is tilted and kept in this position for at least 10 seconds to ensure as much as possible of the excess coating is removed from the surface. This will decrease the total coating weight on the product, decrease the drying time and decrease the effect of excess coating flowing down the product wall to the product bottom when the product returns to its horizontal position. Afterwards, the product is placed in a convectional hot air oven at 95 °C for 90 seconds.This procedure is repeated twice to a total number of repetitions of three.The casserole for oven use obtained can withstand foodstuff under oven conditions such as 220 °C for 30 min.Example 2: Coating propertiesSurface properties of the coating resulting from the method according to the invention (wet coated sample) was tested by the absorbance of vegetable oil. The obtained results were compared to an uncoated molded cellulose fiber sample and a commercially available, spray coated, molded cellulose fiber sample. Therefore, an identical amount of vegetable oil was applied to each sample. The resistance of each treated sample was assessed after 30 min at room temperature (table 1):Table 1 : Results of the coating comparison test with vegetable oilThe above table 1 impressively shows the advantageous coating achieved by the present invention being consistent on all treated areas independent from incline. The method according to the invention therefore provides a closed surface having no / almost no pinholes and thus, being reliable protective to the underlying fiber-matrix.

Claims

CLAIMS1 . A method of producing a coated, molded fiber-based product, the method comprising the steps of:(i) providing a molded fiber-based product having one or more surfaces each defining a depression for contacting foodstuffs,(ii) filling the depression of the molded fiber-based product with a liquid water-based coating composition to allow the coating composition to contact the surface defining the depression at least partially,(iii) removing excess liquid water-based coating composition to obtain a wet coating film on the at least partially contacted surface,(iv) curing the wet coating film to produce the coated, molded fiber-based product.

2. The method of claim 1 , wherein steps (ii) to (iv) are repeated one to ten times.

3. The method of claim 1 or 2, wherein the fiber-based product comprises natural fibers, preferably cellulose fibers.

4. The method of any one of the preceding claims, wherein the liquid water-based coating composition is based on one or more compounds selected from cellulose, polyhydroxyalkanoate (PHA), polylactates (PLA), polybutylene succinate (PBS), polybutylene adipate-terephthalate (PBAT), polycaprolactone (PCL), alginate, starch, natural waxes, shellac, and chitin.

5. The method of any one of the preceding claims, wherein the liquid water-based coating composition has a viscosity in the range of 25 cP to 750 cP.

6. The method of any one of the preceding claims, wherein the solids content of the liquid water-based coating composition is in the range of from 5% to 50% by weight, based on the total weight of the liquid water-based coating composition.

7. The method of any one of the preceding claims, wherein the molded fiber-based product and the liquid water-based coating composition are both biodegradable and / or compostable.

8. The method of any one of the preceding claims, wherein the molded fiber-based product and / or the liquid water-based coating composition are heated prior to and / or during step (ii).

9. The method of any one of the preceding claims, wherein, in step (ii), the depression is essentially entirely filled with the liquid water-based coating composition; and / or wherein the excess liquid water-based coating composition removed in step (iii) is recovered and re-used or recycled; and / or wherein step (iv) is conducted at a temperature in a range of 25°C to 200°C, preferably in a range of 30°C to 170°C.

10. The method of any one of the preceding claims, wherein step (iv) is conducted for a time of from 15 s to 10 min.

11. The method of any one of the preceding claims, wherein the curing of step (iv) is performed by IR irradiation, microwave irradiation, or hot air, preferably hot air.

12. The method of any one of the preceding claims, wherein the curing of step (iv) results in a solid coating film having a coating weight of 1 g / m2to 100 g / m2.

13. A coated, molded fiber-based product obtainable by the method according to any one of claims 1 to 12.

14. The coated, molded fiber-based product of claim 13, wherein the coating is characterized by the absence of any pinholes, i.e. by a completely closed surface.

15. Use of the coated, molded fiber-based product of claims 13 or 14 as a foodstuff container, in particular as trays, plates, bowls, mugs, cups, casseroles, clamshells, or lids.

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