Method for the production of a paper article made impermeable with a silica-based coating and article thus obtained

Abstract

A method is described for producing paper products made impermeable to liquids and fats by means of a surface coating of silica-based material, wherein adhesion between different parts of the same sheet or of multiple sheets of paper is obtained without the use of polymeric adhesives.

Description

[0001] “METHOD FOR THE PRODUCTION OF A PAPER ARTICLE MADE IMPERMEABLE WITH A SILICA-BASED COATING AND ARTICLE THUS OBTAINED”

[0002] FIELD OF THE INVENTION

[0003] The present invention refers to the sector of paper products, often of the disposable type, which for their production require the bonding of two separate paper parts or of two sections or edges of the same part on itself.

[0004] STATE OF THE ART

[0005] Numerous products and artifacts are formed by joining together, through adhesive bonding, different paper parts that may be initially separate parts or sections of a single part, such as a single sheet. For example, bonding separate parts is used to produce cardboards or sheets of paper of relatively high thickness and weight by bonding together sheets of a thickness lower than desired; bonding edges of the same sheet is used, instead, to create objects of a complex shape, such as cups, capsules, containers, bags, and the like.

[0006] The use of paper products has also accelerated in recent years as a result of the push to replace plastic with alternative products that are not based on plastic polymers, or have a reduced plastic content, and ensuring good recyclability, biodegradability, and compostability. For example, the Single Use Plastic (SUP) Directive, which bans the use of certain single-use plastic products for which commercially available alternatives exist, came into force in the European Union in 2021.

[0007] A direction taken by research with the aim of replacing plastic products is that of paper coated with silica or modified silica, using the sol-gel technique. In this technique, a solution or suspension (sol) of silica precursors is prepared in an aqueous or hydroalcoholic solvent; the precursors react in solution to form a gel, i.e. a viscous liquid phase that is spread over the paper and adhering to it because of its viscosity; the gel-coated paper is then subjected to a heat treatment at temperatures, typically in the range of 120-200 °C, causing the evaporation of the volatile components of the gel and the final solidification of the inorganic portion, thus forming a vitreous layer on the paper that is impermeable to liquids, and resistant even to the temperatures of hot drinks. Paper and paper products made waterproof in this way are described, for example, in the patent applications WO 2020 / 261198 Al and WO 2022 / 171893 Al, both in the name of the Applicant.

[0008] Currently, also the manufacturing of products with paper made impermeable by a silica coating requires the use of adhesives to obtain complex shapes.

[0009] The adhesives currently used for bonding paper parts, whether or not coated with silica, include cold adhesives (of the vinyl, acrylic, polyurethane type, etc.), hot melt adhesives, thermolacquers (of the vinyl, acrylic type), and UV adhesives.

[0010] Patent application EP 4043362 Al describes the production of silica-coated paper cups, wherein two edges of a sheet are closed to form a truncated cone shape and the bottom is attached to the smaller base of the truncated cone using polymeric adhesives.

[0011] Patent application WO 2023 / 156368 Al describes disposable paper cups made impermeable with a silica coating obtained by a sol-gel process, wherein the edges of a sheet and the bottom are bonded together using a “primer”, which is said to be an adhesive or a varnish, preferably heat-sealable, of the types available on the market, for example hot-melt adhesives which may be based on polyvinyl acetate resins, ethylene vinyl acetate resins, or mixtures thereof with polyethylenes, polyterpenes or starches.

[0012] The use of adhesives requires the use of dedicated equipment in paper product manufacturing plants, such as devices (rollers, dispensers, etc.) that allow the adhesive to be applied to selected areas of the paper sheet, which will then be used in the bonding operation; this also requires the manufacturing process to include a dedicated step for spreading the adhesive onto a selected surface of the sheet (register printing). Similarly, at least for some types of adhesives, an additional dedicated device and an additional process step are required for the adhesive consolidation, such as a UV lamp and an irradiation step for adhesives that cure in this way. Furthermore, the use of adhesives does not allow to completely eliminate the presence of plastic materials in disposable paper products.

[0013] The object of the present invention is to provide a simplified process for the production of paper products made impermeable by coating with siliceous material, which requires at least one bonding step between two paper parts or two edges of the same paper part, but in which polymeric adhesives are not used.

[0014] SUMMARY OF THE INVENTION

[0015] This and other objects are achieved with the present invention, which in a first aspect thereof relates to a process for the production of a paper product covered on at least one surface with siliceous material, said product comprising at least one area of adhesion of two areas of two sheets or of the same sheet of paper, said process comprising the following steps: a) preparing one or more sheets of paper having at least one coated side or of calendered or supercalendered paper, wherein said one or more sheets are optionally shaped at any time before step d); b) covering at least one side of each of said one or more sheets of coated, calendered or supercalendered paper with a solution of one or more silica or modified silica precursors, said solution comprising a tetraalkoxysilane, an alkyl-trialkoxysilane, nanometric silica, or a mixture thereof, in a solvent consisting of water or a hydroalcoholic mixture, provided that if said sheet or sheets of paper are coated on one side only, the side covered with said solution is the coated one; c) causing the drying of the sheet or sheets of paper covered with the solution of step b), thus obtaining the formation of a layer of silica or modified silica on said at least one side of said one or more sheets of paper; d) placing two edges of a single sheet or of two sheets, obtained in step c), next to each other in such a way that the layer of silica or modified silica present on a first edge of the single sheet comes into direct contact with the layer of silica or modified silica present on a second edge of said single sheet, or in such a way that the layer of silica or modified silica present on a edge of a first sheet comes into direct contact with the layer of silica or modified silica present on a edge of a second sheet; e) subjecting an area of said adjacent edges to a heat-compression treatment at a temperature of at least 100 °C while exerting a pressure of at least 300 N, or to an ultrasonic treatment applying a pressure of at least 150 kPa, thus causing the interposed siliceous material to adhere.

[0016] In the second aspect thereof, the invention relates to the products obtained with the method described above.

[0017] BRIEF DESCRIPTION OF THE FIGURES

[0018] The invention will be described in detail below with reference to the figures, where:

[0019] - Fig. 1 depicts a schematic cross-section of a sheet of paper which is coated on one side only;

[0020] - Fig. 2 depicts a schematic cross-section of a sheet of paper which is coated, calendered, or supercalendered on both sides;

[0021] - Fig. 3 depicts a schematic cross-section of a sheet of coated paper and with a siliceous material coating on one side only;

[0022] - Fig. 4 depicts a schematic cross-section of a sheet of coated, calendered, or supercalendered paper with a siliceous material coating on both sides;

[0023] - Fig. 5 depicts a schematic cross-section of the adhesion area of two sheets of paper having only one of the two sides coated with siliceous material;

[0024] - Fig. 6 depicts a schematic cross-section of the adhesion area of two sheets of paper which have a siliceous material coating on both sides;

[0025] DETAILED DESCRIPTION OF THE INVENTION

[0026] In the present description, the following terms have the specified meanings:

[0027] - “sheet of paper”: is intended to mean a paper part having a thickness much smaller than its lateral dimensions; this portion may be flat (thus forming a proper sheet, as commonly called), or it may have been previously deformed to obtain a three-dimensional part; for example, it may have been rolled, folded, etc.

[0028] - “edge”: is intended to mean a peripheral area of a sheet of paper on one or more sides thereof;

[0029] - “area”: is intended to mean the zone of the edges where the adhesion of the invention is actually achieved; the area may coincide with the surface of the edges or be a part of them; - “finishing”: is intended to mean the modification of the characteristics of a raw paper surface obtained following coating, calendering or supercalendering of said surface

[0030] In the figures cited in the following description, an equal number corresponds to an equal element. Furthermore, the figures are not to scale and, in particular, the thicknesses are greatly increased to highlight the layers that make up the coated paper sheets of the invention.

[0031] The invention is based on the inventor’s observation that, surprisingly, if a silica material produced by the sol-gel process is deposited on a coated, calendered or supercalendered paper surface, the deposit exhibits adhesive properties when subjected to a heat-compression or ultrasonic treatment.

[0032] This phenomenon was completely unexpected; in the field, it was believed that to achieve adhesion between two paper parts coated with siliceous material (or between two areas of the same paper part), the use of a traditional (polymeric) adhesive was necessary; in fact, the inventor is not aware of any documents describing the adhesive properties of the siliceous material layer alone. Furthermore, the inventor has observed that the siliceous material layer exhibits these adhesive characteristics only when produced on coated, calendered or supercalendered paper; a similar siliceous deposition produced on paper not having these characteristics, such as uncoated paper, kraft paper, and the like, exhibits no adhesiveness.

[0033] This observation by the inventor allows the step of dispensing adhesives on selected areas to be eliminated from the paper product manufacturing processes, as well as the elimination of associated dispensing devices from the machines. The invention also allows the elimination of traditional polymeric adhesive, thus helping to reduce the amount of plastic-like material in the final product.

[0034] The first step of the invention, a), consists of preparing one or more sheets of coated, calendered or supercalendered paper. In the case of coated paper, this can be coated on one side only or on both sides; the coating consists of covering the surface of the sheet with a mixture of materials, including kaolinite, calcium carbonate, bentonite and talc, optionally with the addition of a binder or a polymer. In the case of calendered or supercalendered paper, obviously, both sides of the sheet have the same characteristics because the calendering process symmetrically applies a compressive force on both sides.

[0035] Fig. 1 shows a schematic cross-section of a sheet 10 of one-side coated paper; the paper support, 11, is covered on one side with a layer of coating, 12, while the opposite side has no coating or finishing. Fig. 2 shows, in a view similar to that of Fig. 1, a sheet 20 of paper with finishing on both sides; in this case the two finishing layers, 21 and 21’, can be coating layers or layers of more compressed cellulose fibers obtained in calendering or supercalendering processes.

[0036] The sheet or sheets, 10 or 20, may or not be shaped, depending on the final product desired. If this is simply a cardboard obtained by joining two or more sheets across their entire surface, the shaping step is not required. In the more commercially important case of producing an article with a particular shape, such as a cup, the sheet of paper has to be shaped; for example, in the case of a cup, the shape is a circular crown section, so that by overlapping the two straight sides, a truncated cone shape is obtained, to which the bottom of the cup is then added at the smaller base. The shaping can occur at any time before the edges are joined to be adhered to each other, therefore at any time before step d) of the process.

[0037] In step b) of the process of the invention, the paper from step a) is covered on at least one side with a solution of one or more silica or modified silica precursors, according to methods known in the sol-gel technique. In the case of one-side coated paper, the coating with said solution is performed on the coated side.

[0038] In this technique, a solution of at least one precursor of the final silica layer is prepared in a solvent selected between water or a hydroalcoholic mixture; this precursor is typically a tetraalkoxysilane, an alkyl-trialkoxysilane, nanometric silica, or mixtures thereof. When the sheet has to be subsequently folded, rolled, or otherwise deformed, the precursor should contain at least one alkyl-trialkoxysilane.

[0039] Tetraalkoxysilanes are compounds of general formula Si(OR)4, wherein R is an alkyl radical. For the purposes of the present invention, R is a C1-C4 alkyl radical, preferably methyl and even more preferably ethyl; the tetraalkoxysilanes corresponding to these alkyl radicals are tetramethoxysilane, also known by the abbreviation TMOS, and tetraethoxysilane, also known by the abbreviation TEOS, respectively.

[0040] Alkyl-trialkoxysilanes are compounds of general formula R’-Si(OR”)3, wherein R’ and R”, equal or different from each other, are C1-C4 alkyl radicals; preferably R’ is a C1-C3 radical, and even more preferably it is methyl (Cl). A preferred compound for the purposes of the present invention is the alkyl-trialkoxy silane in which R’ = methyl and R” = ethyl, i.e., the compound methyltri ethoxy silane, known in the art by the abbreviation MTES.

[0041] Nanometric silica is a form of amorphous silica consisting of nanometric silica particles (i.e., smaller than 1 micrometer, pm, typically between about 5 and 100 nm), generally aggregated to form micrometric size secondary particles i.e., larger than one micrometer, typically between 1 and 100 pm).

[0042] This material is also known in the art as “colloidal silica” or “fumed silica.” For food contact applications (e.g., in the case of cups), nanometric silica should have a purity of at least 99.5%. Nanometric silica is widely available commercially and is sold, for example, by Evonik Resource Efficiency GmbH, Essen, Germany, under the name AEROSIL® (e.g., the product AEROSIL® OX 50), or by Cabot Corporation, Boston, Massachusetts, USA, under the name Cab-O-Sil®.

[0043] The solution used to cover the paper is generally prepared in an acidic environment, with a pH of about 2-3, typically achieved by adding mineral acids such as HC1 or HNO3. The solution may also contain, as optional components, one or more C1-C6 alcohols, a base (selected, e.g., between NaOH or KOH) in such an amount to adjust the pH in the range of between 2.3 and 4.5, and glycerin pigmented with a food-safe colorant.

[0044] A preferred solution for the purposes of the present invention contains, by weight, between 35% and 45%, preferably 40% MTES, between 15% and 25%, preferably 20% TEOS, and between 35% and 45%, preferably 40% water, in addition to a small amount of an acid solution (e.g., a 1 N HC1 solution) sufficient to adjust the pH within the range of 2-3.

[0045] Covering with the solution the at least one side of the sheet, whether already shaped or not, can be done using any known method, such as dipping, spraying, brushing, and the like. Alternatively, for flat sheets, it is possible to use methods typical of flexographic printing, wherein the solution described above is collected from a tank and, thanks to a roller system, is transported, dosed via an anilox roller (for the purposes of the invention, a roller with a low number of surface cells per centimetre, i.e. about 80-250, is sufficient) and deposited on the substrate to be coated. Finally, another method for transferring the solution to the paper surface is described in patent application WO 2020 / 261198 Al, wherein the sheet of paper is passed between two rollers, at least one of which is hollow and filled with the solution, and its cylindrical surface is perforated and covered with felt, so that the solution soaks the felt, which distributes it across the sheet of paper as it rolls.

[0046] Once the solution has been distributed on at least one surface of the paper sheet, in step c), the resulting deposit is dried. In theory, this could be achieved by simply leaving the sheets flat or arranged in a suitable conditioned chamber, even at room temperature; to achieve rapid drying, compatible with industrial process timelines, this operation is performed by treating the sheet at a temperature of between 100 and 250 °C, typically between 120 and 200 °C. Although the temperature of 250 °C is higher than the spontaneous combustion temperature of paper, a treatment at this temperature for very short periods is possible because the paper is protected from combustion, as long as it is wetted by the solution.

[0047] In this step, the liquid components of the solution (water, alcohols whether voluntarily added or resulting from the hydrolysis of tetraalkoxysilanes or alkyl -trialkoxy silanes) are eliminated, and a layer of vitreous silica is formed; in the preferred case of using a portion of the alkyl-trialkoxysilane, the vitreous layer is not made of pure silica but of a silica chemically modified by the presence of alkyl groups bonded to some of the silicon atoms; this condition favours the vitreous layer flexibility. For the sake of simplicity, herein below, the term “silica” will be used to indicate both pure silica (i.e., with the exact composition SiCh) and modified silica. The paper obtained at this point of the process is impermeable to water, oils, and fats, even at the temperatures of hot foods, and partially to gases. Figures 3 and 4 schematically represent, respectively, a sheet 30 (obtained by treating a sheet of the type 10) coated with a silica layer, 31, on one side only, and a sheet 40 (obtained by treating a sheet of the type 20) coated with silica layers, 31 and 31’, on both sides. In step d) of the process of the invention, two edges of a single sheet, or edges of two sheets obtained in step c), are brought in contact.

[0048] In the simplest case of forming sheets thicker than the initial sheets, two or more sheets are simply overlapped so as to coincide (z.e., such that no part of one sheet protrudes beyond the adjacent sheet); in this specific case, the “edges” of the sheets coincide with the surface thereof.

[0049] In the most commercially interesting case, in which a product of more complex shape is to be created, two edges of a single sheet or two sheets are placed next to each other.

[0050] The two edges should be placed next to each other in such a way that each of them faces the other with a surface covered with a silica layer obtained in the previous step. According to the invention, the silica layer present on a first edge is placed in direct contact with the silica layer present on a second edge, without interposing any material between these two silica layers. In the case when only one surface of the sheet is covered with silica, the sheet should be folded in such a way that two edges of the same surface face each other; this situation is schematically represented in Fig. 5, which shows a cross-section of the overlapping and adhesion area of the sheet folded on itself. More commonly, both surfaces of the sheet are covered with silica, and in this case the two edges can simply be overlapped, obtaining a flat adhesion area; this situation is schematically represented in Fig. 6; in this figure, for clarity of representation, only the numerical references of the two silica layers, 31 and 31’, which determine the adhesion of the two sheets of paper, are shown.

[0051] Finally, in the last step of the process of the invention, e), a treatment is applied to an area of the adjacent edges that causes the facing silica layers to adhere. The adhesion area may coincide with the edges or be a portion of them.

[0052] The treatment may involve heat-sealing, in which the area requiring adhesion is compressed with suitable heated tools, such as a hot-plate welder, pliers, jaws, and the like. The silica should be heated to a temperature of at least 100 °C, preferably between 120 and 200 °C, and the pressure exerted by the tool should be of at least 300 N. Under these conditions, the time required to achieve adhesion of the two surfaces is less than 1 s, and for the highest temperatures and pressures, even less than 0.5 s.

[0053] Alternatively, the adhesion of an area of the two silica-coated surfaces can be achieved through ultrasonic treatment. In this case, a sonotrode with a surface intended to come into contact with the paper, and whose shape corresponds to the adhesion area, is required. The sonotrode is the part of an ultrasonic welding apparatus that comes into direct contact with the part to be welded and transfers the ultrasonic energy to that part. Using the ultrasonic welder does not require the application of temperature, and the part to be welded is possibly heated by mechanical agitation due to vibration; the pressure to be applied with the ultrasound should be of at least 150 kPa.

[0054] The reasons why these treatments unexpectedly result in the adhesion of the two silica layers have not been clarified; it can be speculated that the two layers contain free -OH groups on their surfaces, which during these treatments react with each other, forming Si-O-Si bridges between the two different silica layers, giving rise to adhesion.

[0055] Step e) can be repeated multiple times to obtain objects with complex shapes. For example, in the case of a cup, a first welding is performed to secure the two straight edges of a sheet shaped as a section of a circular crown, thus obtaining a truncated cone with two free bases; in a second step, a bottom, also made of silica-coated paper on at least one surface, is attached to the smaller diameter base using the same method.

[0056] In a second aspect thereof, the invention relates to the products that can be manufactured with the process described above; these can be bags, envelopes, tea bags, trays, cutlery, capsules, or other thermoformed products with complex geometries involving overlaps. For example, in the case of a bag, this can be produced starting from a sheet of paper shown in Fig. 3, folded over itself so that the two halves mutually face each other with the silica-coated surfaces, and welding the two sides perpendicular to the fold line. Other products, such as capsules and trays, can be obtained by welding together the silica-coated surfaces of previously shaped paper sheets.

[0057] The invention will be further described in the following experimental section.

[0058] EXAMPLE 1 This example refers to the making of a welding through heat-compression of coated paper covered with modified silica.

[0059] A solution was prepared comprising, by weight, 40% MTES, 20% TEOS, and 40% water, to which 0.38% by weight of a 1 N HC1 solution was added.

[0060] The solution was manually applied with a felt roller to both surfaces of a 30 x 30 cm sheet of commercial coated paper (Burgo 270); the amount of solution applied, measured by weight difference before and after coating, was of about 10 g / m2per side of the sheet.

[0061] The sheet thus covered was dried in oven for 2 minutes at 150 °C.

[0062] The dry sheet was rolled up, overlapping the two short sides by 0.5 cm, and subjected to heat-sealing at 120 °C, with a force of 500 N for 0.3 s, using a Twin 406 hot-plate welder manufactured by Raima S.r.l. of Cernusco sul Naviglio (MI).

[0063] The welding efficacy was verified with a tensile test carried out according to DIN EN ISO 1924-3 standard using an Instron® 8801 instrument; during the test, the two bonded paper edges were pulled and their resistance to being pulled apart was assessed. The test resulted in the paper being delaminated when the applied tensile force reached 5.8 N, but the two sealed edges were not pulled apart; this means that the welding withstands tensile forces greater than the last value recorded in the test.

[0064] EXAMPLE 2

[0065] Example 1 was repeated, with the only difference that welding was performed with a PSX 20 ultrasonic welder from Sonomax, Fenegro (CO), applying a force of 300 kPa for 0.3 s.

[0066] During the tensile test, the welding did not open, while the paper delaminated when the tensile force reached 4.1 N.

Claims

CLAIMS1. Process for the production of a paper product covered on at least one surface with siliceous material, said product comprising at least one area of adhesion of two areas of two sheets or of the same sheet of paper, said process comprising the following steps: a) preparing one or more sheets (10; 20) of paper having at least one coated side or of calendered or supercalendered paper, wherein said one or more sheets are optionally shaped at any time before step d); b) covering at least one side of each of said one or more sheets of coated, calendered or supercalendered paper with a solution of one or more silica or modified silica precursors, said solution comprising a tetraalkoxysilane, an alkyl-trialkoxysilane, nanometric silica, or a mixture thereof, in a solvent consisting of water or a hydroalcoholic mixture, provided that if said sheet or sheets of paper are coated on one side only, the side covered with said solution is the coated one; c) causing the drying of the sheet or sheets of paper covered with the solution of step b), thus obtaining the formation of a layer (31, 31’) of silica or modified silica on said at least one side of said one or more sheets of paper; d) placing next to each other two edges of a single sheet or of two sheets obtained in step c), in such a way that the layer (31) of silica or modified silica present on a first edge of the single sheet comes into direct contact with the layer (31 ’) of silica or modified silica present on a second edge of said single sheet, or in such a way that the layer (31) of silica or modified silica present on a edge of a first sheet comes into direct contact with the layer (31’) of silica or modified silica present on a edge of a second sheet; e) subjecting an area of said adjacent edges to a heat-compression treatment at a temperature of at least 100 °C while exerting a pressure of at least 300 N, or to an ultrasonic treatment applying a pressure of at least 150 kPa, thus causing the interposed siliceous material to adhere.

2. Process according to claim 1 , wherein said tetraalkoxysilane has general formula Si(OR)4,wherein R is a C1-C4 alkyl radical, and said alkyl-trialkoxysilane has general formula R’-Si(OR”)3, wherein R’ and R”, equal to or different from each other, are C1-C4 alkyl radicals.

3. Process according to any one of the preceding claims, wherein when said one or more sheets are to be folded, rolled or deformed after step b), said solution of one or more silica or modified silica precursors comprises at least one alkyl-trialkoxysilane.

4. Process according to any one of the preceding claims, wherein said silica or modified silica precursor solution comprises, by weight, between 35% and 45% of CH3- Si(OCH2CH3)3, between 15% and 25% of Si(OCH2CH3)4, between 35% and 45% of water, and an acid in such an amount as to bring the pH of the solution in the range of 2- 3.

5. Process according to any one of the preceding claims, wherein step b) is carried out by immersion, spraying, brushing, flexographic printing or by passing said one or more sheets of paper between two rollers, at least one of which is hollow, filled with the solution, and has a perforated cylindrical surface covered with felt to allow the passage of the solution from inside the cylinder towards said one or more sheets of paper.

6. Process according to any one of the preceding claims, wherein step c) is carried out by treating said one or more sheets at a temperature comprised between 100 and 250 °C.

7. Process according to any one of the preceding claims, wherein said sheet of coated, calendered or supercalendered paper is shaped in the form of a circular crown section, such that the superposition of the two rectilinear sides gives rise to a three-dimensional truncated cone shape.

8. Paper product produced according to the method of any one of the preceding claims, selected from a bag, an envelope, a tea bag, a tray, a cutlery or a capsule.

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