Method for waterproofing a layer of paper

Abstract

The invention relates to a method for waterproofing a layer of paper using an aqueous dispersion comprising a combination of particles of a poly(vinyl acetal)-based resin and particles of a resin other than a poly(vinyl acetal)-based resin. It also relates to a bag comprising a paper layer waterproofed by such a method, as well as the use of such a bag for packaging materials in powder form, such as plaster or cement.

Description

Title of the invention: Method for sealing a layer of paper

[0001] The invention relates to a method for sealing a layer of paper, using an aqueous dispersion comprising a combination of particles of a poly(vinyl acetal)-based resin and particles of a resin other than a poly(vinyl acetal)-based resin. It also relates to a bag comprising a layer of paper sealed by means of such a method, as well as the use of such a bag for packaging materials in powder form, such as plaster- or cement-based powders.

[0002] Paper bags require protection against the penetration of atmospheric water vapor through the bag layers. Since paper is a porous and hydrophilic substrate, it is difficult to avoid the adsorption and transport of water or water vapor to prevent the loss of quality of the packaged materials. In particular, gypsum and cement-based powders can harden in the presence of water and thus lose their properties. Polyethylene plastic films are usually inserted between two paper plies as a permeability barrier. However, due to their hybrid composition, these packaging bags are not recyclable. One strategy is to replace the current bags with a recyclable two-ply bag without a plastic-based sheet, in which one of the plies would have a coating based on a very thin polymer, to ensure good water barrier properties.

[0003] However, these coatings are generally based on petroleum resources, which contribute to global warming. From a sustainable perspective, there is growing interest in alternative films and coatings with similar sealing properties.

[0004] In response to the demand for more environmentally friendly packaging materials, bio-based polymers have been used to replace petroleum-based coatings. Some of them show an ability to biodegrade or be compostable. Most of the bio-based polymers found in the literature consist of semi-crystalline polymers such as polyesters that are coated onto paper by extrusion or other high-temperature processes. The use of aqueous polyester dispersions allows for application at room temperature using conventional wet coating processes. Although this method allows for easier and more environmentally friendly application, it is necessary, after application, to heat the coating to a temperature above its melting point to obtain homogeneous films. Habi Actually, the temperature of these bio-based polyesters is high and the process is not suitable for the paper application process, due to the degradation of the paper or the limitation of the oven temperature.

[0005] In this context, the inventors have developed an aqueous dispersion overcoming the limitations mentioned above. Specifically, they have demonstrated that an aqueous dispersion combining resin particles based on poly(vinyl acetal), in particular poly(vinyl butyral), with other polymer resin particles, could be deposited on a paper substrate at room temperature, and form a coating offering very good barrier properties to liquid or vapor water. This invention is particularly relevant for paper bags used for packaging construction materials, such as cement or plaster-based powders, or any other moisture-sensitive product. Paper bags comprising such a coating also have the advantage of having a lower carbon footprint than the sealed paper bags currently used.

[0006] Thus, the present invention relates to a method for sealing (in particular for waterproofing) a layer of paper, comprising applying an aqueous dispersion to a surface of said layer of paper to form a coating, and drying said coating,

[0007] wherein said aqueous dispersion comprises water, particles of a poly(vinyl acetal)-based resin, particles of a resin other than a poly(vinyl acetal)-based resin, one or more plasticizers, and one or more emulsifiers, wherein the total content of resin particles is 5 to 50% by weight.

[0008] In some embodiments, the poly(vinyl acetal)-based resin is poly(vinyl butyral)-based.

[0009] In some embodiments, the poly(vinyl butyral)-based resin comprises residual alcohol and acetate functionalities.

[0010] In certain embodiments, the poly(vinyl butyral)-based resin is obtained from the recycling of laminated glazing.

[0011] In certain embodiments, said aqueous dispersion further comprises one or more crosslinking agents, preferably chosen from silanes, preferably from alkoxysilanes optionally comprising a functional group reactive with respect to carboxylic acid, carboxylate or hydroxy functions, such as an epoxy or amine group.

[0012] In certain embodiments, the weight ratio between the content of crosslinking agent and the content of poly(vinyl acetal)-based resin is between 0.001 and 0.10, in particular between 0.002 and 0.06.

[0013] In certain embodiments, the aqueous dispersion further comprises one or more fillers, the filler content preferably being between 5 and 80% by weight. weight, relative to the total weight of the dispersion.

[0014] In some embodiments, the fillers are selected from calcium carbonate, calcium stearate, clays, talc, dolomite, mica, silica sands, ground basalt, barium sulfate, kaolin, and mixtures of two or more of these compounds.

[0015] In some embodiments, said resin other than a poly(vinyl acetal) based resin is selected from styrene-acrylic resins, styrene-butadiene resins, acrylic resins, alkyd resins, and a mixture of two or more of these resins, preferably from alkyd resins.

[0016] In certain embodiments, the aqueous dispersion further comprises from 0.1 to 20% by weight of pigments, relative to the total weight of the dispersion.

[0017] In certain embodiments, the total weight content of emulsifier(s) is between 0.1% and 10%.

[0018] In certain embodiments, the weight ratio between the plasticizer content and the poly(vinyl acetal)-based resin content is between 0.1 and 1, in particular between 0.2 and 0.5.

[0019] In certain embodiments, the amount of water is between 10 and 70% by weight, in particular between 20 and 60% by weight, relative to the total weight of the dispersion.

[0020] In certain embodiments, the weight ratio between the content of particles of resin other than a poly(vinyl acetal)-based resin and the content of particles of a poly(vinyl acetal)-based resin is between 0.05 and 20, preferably between 0.1 and 10, or even between 1.0 and 5.0.

[0021] Another object of the present invention is a bag comprising a paper layer, and a sealing layer at least partially covering the paper layer, the sealing layer being formed from the aqueous dispersion described in the present application, more particularly formed by the method as defined in the present application.

[0022] Another object of the present invention is a use of a bag as defined in the present application, for the packaging of plaster or cement-based powders. DETAILED DESCRIPTION

[0023] The aqueous dispersion used in the process of the invention comprises water, particles of a poly(vinyl acetal)-based resin, particles of a resin other than a poly(vinyl acetal)-based resin, one or more plasticizers, and one or more emulsifiers, wherein the total content of resin particles is 5 to 50% by weight (for example, 7 to 48%, especially 10 to 45%, or even 15 to 40%, or even 20 to 35%).

[0024] Preferably, the poly(vinyl acetal) based resin is poly(vinyl butyral) based, also called PVB.

[0025] Preferably, the resin based on poly(vinyl acetal), in particular based on poly(vinyl butyral), is made of poly(vinyl acetal), in particular poly(vinyl butyral).

[0026] Preferably, the resin based on poly(vinyl acetal), in particular poly(vinyl butyral), comprises residual alcohol and acetate functions. These residual functions come from the resin manufacturing process, which is generally carried out by hydrolysis of poly(vinyl acetate) into poly(vinyl alcohol) and then acetalization of the latter. The presence of alcohol functions makes it possible in particular to crosslink the resin and improve its properties.

[0027] The poly(vinyl butyral)-based resin is advantageously obtained from the recycling of laminated glazing. These use PVB as a lamination interlayer between two sheets of glass. The use of recycled materials helps reduce the carbon footprint of the sealing system.

[0028] The resin content by weight corresponds to the mass percentage of resin in dry extract in the aqueous dispersion, therefore to the weight of resin relative to the total weight of aqueous dispersion. Generally speaking, unless otherwise stated, the contents of the different constituents of the dispersion are given by weight, relative to the total weight of aqueous dispersion.

[0029] The aqueous dispersion further comprises particles of a resin other than a poly(vinyl acetal)-based resin. This resin may in particular be chosen from styrene-acrylic resins (in particular carboxylated styrene-acrylic resins), styrene-butadiene resins, acrylic resins, alkyd resins, and a mixture of two or more of these resins.

[0030] In a preferred embodiment, the particles of a resin other than a poly(vinyl acetal) based resin are particles of styrene-acrylic resin (in particular carboxylated styrene-acrylics) or alkyd resin, better still alkyd resin.

[0031] The weight ratio between the content of particles of resin(s) other than poly(vinyl acetal) and the content of particles of resin based on poly(vinyl acetal) is preferably between 0.05 and 20, for example between 0.1 and 10, in particular between 1.0 and 5.0.

[0032] The poly(vinyl acetal) resin particles and the particles of the other resin preferably have a volume size distribution such that the d50 is between 50 and 300 nm, in particular between 100 and 250 nm. The particle size distribution is in particular determined by dynamic light diffraction.

[0033] In a particular embodiment, said resin other than a poly(vinyl acetal)-based resin is bio-sourced. The bio-sourced nature of the latter contributes to the reduction of the carbon footprint of dispersion.

[0034] When the resin other than the poly(vinyl acetal) based resin is an alkyd resin, it is advantageous to use a drier, in particular organic salts of cobalt, lead or zinc oxides (preferably, an organic salt of cobalt).

[0035] The plasticizer(s) is (are) advantageously chosen from polyethylene glycol esters, adipates, sebacates, phthalates, benzoate esters and mixtures of two or more of these compounds. Examples include tri(ethylene glycol) di(2-ethylhexanoate), tri(ethylene glycol) di(2-ethylbutyrate), tri(ethylene glycol) di(n-heptanoate), tetra(ethylene glycol) di(n-heptanoate), bis(2-butoxyethyl) adipate, dibutyl sebacate, dibutyl phthalate or dioctyl phthalate.

[0036] Preferably, the weight ratio between the plasticizer content and the poly(vinyl acetal)-based resin content is between 0.1 and 1, in particular between 0.2 and 0.5.

[0037] The emulsifier(s) is (are) advantageously chosen from ionic emulsifiers (cationic or anionic) and non-ionic emulsifiers. The anionic emulsifiers are in particular carboxylates or sulfonates. The carboxylates are for example salts of saturated or unsaturated fatty acids such as stearates, oleates and laurates, rosin salts such as for example potassium oleate. The sulfonates are for example alkyl sulfonates, aryl sulfonates, alkyl aryl sulfonates or sulfonated esters such as for example sodium dodecyl sulfate. The non-ionic emulsifiers are in particular polyoxyethylene alkylphenyl ethers.

[0038] The emulsifier(s) is(are) preferably anionic. Such emulsifiers in fact make it possible to obtain a lower water uptake than non-ionic emulsifiers.

[0039] Preferably, the total weight content of emulsifier(s) is between 0.1% and 10%, for example between 2% and 6% or between 4 and 8%.

[0040] The glass transition temperature of the aqueous dispersion is preferably between 5 and 40°C, in particular between 10 and 30°C, in order to maintain good flexibility after drying. The glass transition temperature is in particular measured by differential scanning calorimetry. The glass transition temperature can be modified in particular by varying the quantity of plasticizer.

[0041] The minimum film formation temperature (generally referred to by its acronym "MFFT") of the dispersion is preferably less than 30°C, in particular less than 20°C, or even less than 10°C and even less than 0°C, in order to allow film formation and coalescence of the coating at room temperature in different climatic conditions. The minimum film formation temperature can be modified in particular by varying the amount of plasticizer, or even by adding coalescing agents.

[0042] In some embodiments, the aqueous dispersion further comprises one or more fillers.

[0043] The fillers are advantageously of mineral nature. The fillers are preferably chosen from calcium carbonate, calcium stearate, clays, talc, dolomite, mica, silica sands, ground basalt, barium sulfate, kaolin and mixtures of two or more of these compounds. The fillers preferably have a particle size ranging from 0.5 to 500 μm, in particular from 1 to 200 μm, measured by laser granulometry.

[0044] The total weight content of fillers is preferably between 5 and 80%, for example between 6 and 70%, in particular between 7 and 65%, or even between 8 and 50%, or even between 9 and 40% or between 10 and 25%, or even between 11 and 24%.

[0045] In some embodiments, the aqueous dispersion further comprises one or more pigments.

[0046] The pigments are preferably chosen from inorganic pigments (for example titanium dioxide or iron oxide), organic pigments (for example carbon black), and mixtures of two or more of these compounds.

[0047] The total weight content of pigments is preferably between 0.1 and 20%, in particular between 2 and 10%.

[0048] Preferably, the aqueous dispersion further comprises one or more crosslinking agents.

[0049] The crosslinking agent(s) is (are) advantageously chosen from water-soluble organometallic compounds, water-insoluble metal oxide or hydroxide particles, organic compounds reactive with hydroxyl groups, and silane compounds.

[0050] Organic compounds reactive with hydroxyl groups are in particular polyfunctional molecules reactive with hydroxyl groups, such as for example poly(carboxylic acids), polyisocyanates or even polyaldehydes. Mention may in particular be made of glutaraldehyde or citric acid.

[0051] The water-soluble organometallic compounds are preferably complexes of zirconium, titanium, zinc or boron. Examples of water-soluble organometallic compounds are ammonium bis(carbonato-) dihydroxy-zirconate, ammonium bis(lactato-) dihydroxytitanate, titanium lactate or titanium triethanolaminate.

[0052] The water-insoluble metal oxide or hydroxide particles are in particular oxides or hydroxides of zinc, zirconium or aluminum (preferably zinc oxides). These particles may have a size of between 0.5 and 100 pm, in particular between 1 and 50 pm. The particle size is typically de- finished by laser granulometry.

[0053] In a particular embodiment, the crosslinking agent is a silane, preferably an alkoxy silane. Better still, the crosslinking agent is an alkoxysilane comprising a functional group reactive with respect to carboxylic acid, carboxylate or hydroxy functions, such as an epoxy group (i.e. an epoxy-alkoxysilane) or amine group (i.e. amino-alkoxysilane). The total weight content of crosslinking agent(s) is preferably between 0.01% and 10%, for example between 0.5% and 2%.

[0054] The crosslinking agent allows several resin particles to be crosslinked. This results in slightly faster drying and, above all, lower water absorption, particularly after immersion in water.

[0055] The amount of water in the composition is preferably between 10 and 70% by weight, in particular between 20 and 60% by weight, for example between 30 and 50% relative to the total weight of the composition.

[0056] In a particular embodiment, the composition further comprises one or more additives, notably chosen from:

[0057] - defoaming agents (for example of the silicone, fluoro-silicone, oil type mineral, acrylic, vinyl polymers),

[0058] - coalescing agents (for example of the glycol type such as propylene glycol or diethylene glycol, glycol ethers such as dipropylene glycol n-butyl ether or propylene glycol methyl ether acetate, alcohol esters such as 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, pyrrolidone such as N-methyl-2-pyrrolidone or N-butyl-2-pyrrolidone),

[0059] - rheological agents (for example of the polyurethane / polyurea, polyacrylic type, polyamide, based on castor oil, or based on clays or cellulose ether),

[0060] - dispersing agents (for example of the silicone, polyacrylate, polyether type),

[0061] - bactericidal or algicidal agents (in particular of the isothiazolinone type such as example benzisothiazolinone or methylisothiazolinone, or halogenated type).

[0062] The total weight content of these additives is preferably between 0.1 and 10%, preferably between 0.2 and 5% relative to the total weight of the aqueous dispersion.

[0063] Dispersing agents are useful to help disperse fillers and pigments. As previously indicated, coalescing agents allow the minimum film formation temperature to be adjusted if necessary.

[0064] The aqueous dispersion used in the process of the invention is normally a single-component composition, that is to say it does not require the addition of another composition before or after application.

[0065] In the method according to the invention, the aqueous dispersion described above is applied to a surface of a layer of paper to form a coating, and then said coating is dried. The dry coating forms a sealing coating (or "sealing layer"), in particular waterproofing.

[0066] The term "paper" encompasses any type of paper such as white paper, coated paper, frictioned paper, textured paper, kraft paper (for example, white, unbleached or brown kraft paper), preferably kraft paper. The grammage of a layer of paper may be between 40 and 300 g / m2, for example between 50 and 150 g / m2.

[0067] The application of the aqueous dispersion can be done by roller, brush, spraying, flexographic printing, or by coating using a size press. It can be done in several layers (in particular two layers).

[0068] The coating is then dried. Drying can normally be done in the air, naturally, at room temperature, therefore without heating or blowing, in a period typically ranging from a few minutes to a few hours. “Room temperature” here refers to a temperature typically between 5°C and 40°C, more particularly between 15°C and 35°C.

[0069] Alternatively, the coating may be heated, for example to a temperature of between 50°C and 130°C, in particular between 70°C and 120°C, typically for a period of a few seconds (for example between 1 and 20 seconds) to a few minutes (for example between 1 and 10 minutes). It is particularly advantageous to heat to such temperatures when the dispersion comprises a crosslinking agent.

[0070] The final (dry) coating may result from the application of several successive layers (in particular two layers). When it results from the application of several successive layers, each layer may independently be dried at room temperature or at a temperature between 50°C and 130°C (in particular between 70°C and 120°C). The dry weight of the coating is preferably between 2 and 40 g / m2, in particular between 5 and 25 g / m2.

[0071] The invention also relates to a bag comprising a paper layer (or equivalently "ply") and a sealing layer (in particular, water-proofing) at least partially covering the paper layer, said sealing layer being formed from the aqueous dispersion described above, more particularly by the method as defined above. In particular, said paper layer may comprise two faces, and the sealing layer covers one or two faces of said paper layer, preferably only one face of said paper layer (typically covers at least 90% or even at least 95%, or even at least 99% of said face).

[0072] In a preferred embodiment, the bag comprises an inner paper layer and an outer paper layer, each having an inner face and an outer face, and a sealing layer covering the inner face of the inner paper layer. or the inner face of the outer paper layer (typically covers at least 90% or even at least 95%, or even at least 99% of said face).

[0073] The “inner paper layer” corresponds to the paper layer on the inner side of the bag. The “outer paper layer” corresponds to the paper layer on the outer side of the bag. The “outer face” of the inner paper layer is the face intended to be in contact with the material packaged in the bag. The “inner face” of the inner paper layer is the face facing the inner face of the outer paper layer. The “inner face” of the outer paper layer is the face facing the inner face of the inner paper layer. The “outer face” of the outer paper layer is the face intended to be in contact with the external environment.

[0074] The presence of the sealing layer on an inner face of one of the two layers of paper has the particular advantage of protecting the sealing layer from the external environment or from the material to be packaged, the pH of which can affect the performance of the sealing layer.

[0075] More preferably, said sealing layer covers the inner face of the outer paper layer (typically covers at least 90% or even at least 95%, or even at least 99% of said face).

[0076] The two layers of paper may or may not be made of the same type of paper.

[0077] The outer paper layer may be printed on at least a portion of its face. externally, for example with ink, especially water-based ink.

[0078] In a particular embodiment, the bag comprises three layers of paper, each having two faces, and said sealing layer covers at least one of the faces of at least one of the layers of paper, preferably only one face of a single layer of paper (typically covers at least 90% or even at least 95%, or even at least 99% of said face). In such an embodiment, it is preferred that the face(s) covered by said sealing layer is not the face intended to be in contact with the packaged material nor the face intended to be in contact with the external environment. The three layers of paper may be made of the same type of paper or not.

[0079] The bag according to the invention may also comprise a valve, and possibly a bottom strip on the bottom side of the bag and / or a bottom strip on the valve side of the bag. The bag may also comprise a carrying handle and / or easy opening means (for example a weakening line) and / or easy closing and opening means (for example a zipper, associated male-female reclosable strips or self-adhesive or self-gripping strips). The valve may comprise, or even consist of, a layer of paper and / or one or more heat-sealable complexes. The valve allows on the one hand the filling of the bag during the packaging of the product, and on the other hand the closing of the bag at the end of filling, by folding, flap or welding depending on the type of bagging machine. When present, the bottom strips may include or be made of at least one layer of paper, and provide a printing support and increase the bag's hold on the bottom side and / or valve side.

[0080] Another object of the present invention is the use of a bag as defined in the present application for the packaging of plaster or cement-based powders.

[0081] The following examples illustrate the invention in a non-limiting manner. EXAMPLES

[0082] Abbreviations

[0083] MVTR: Moisture Vapor Transmission Rate

[0084] HR: Relative Humidity

[0085] The PVB-based aqueous dispersion comprises PVB, triethylene glycol-bis(2-ethylhexanoate) as a plasticizer and potassium oleate as an emulsifier. The solids content is 50%.

[0086] The emulsion of the carboxylated styrene-acrylic polymer is bio-sourced and has a solids content of 50%.

[0087] The alkyd resin emulsion has a solids content of 50%.

[0088] The dispersions of PVB and the second resin (i.e. acrylic or alkyd styrene) are mixed at room temperature, optionally in the presence of 2% by weight of an epoxysilane. When the second resin is an alkyd resin, a cobalt-based drier was used. A film a few tens of micrometers thick (the thickness of the film before drying is adjusted to obtain the desired dry coating weight) is then deposited on a sheet of white kraft paper and dried at room temperature or for 2 min at 80°C. The coatings can also be applied in two stages, a first application with drying at room temperature or 80°C, followed by a second application with drying at room temperature or 80°C. The water vapor permeability properties under temperate (23°C, 50% RH) and tropical (38°C, 90% RH) conditions are then measured.

[0089] In comparison, the mixture of PVB and styrene-acrylic dispersions in functionalization by a silane compound is deposited with a thickness similar to the previous embodiment and dried at room temperature.

[0090] [Tables 1] MVTR (g / m2 / day) White paper Weight (g / m2) 50% RH - 23°C 90% RH - 38°C No deposit / «400 «2000 PVB resin 6 218 1519 PVB'bc resin 6 224 1527 Styrene-acrylic resin / PVB 90 / 10 11 46 63 Styrene-acrylic resin / PVB 85 / 15 17.5 46 95 Styrene-acrylic resin / PVB 70 / 30 11 84 120 Styrene-acrylic resin / PVB 70 / 30 20 (2 * 10) 43 107 Alkyd resin / PVB 70 / 30(a) 10 73 742 Alkyd resin / PVB 70 / 30(ac) 12 60 604 Styrene-acrylic resin / PVB 90 / 10 (b,c) 16.5 (2x8.5) 7 25 Styrene-acrylic resin / PVB 85 / 15 (b,c) 21 (2*10.5) 13 44

[0091] a) siccative used: cobalt salt; b) presence of a silane-type crosslinking agent; c) heating for 2 min at 80°C

[0092] Table 1 above shows that the combination of PVB particles with particles of a second resin, such as a styrene-acrylic or alkyd resin, makes it possible to obtain better sealing of the paper layer, whether in temperate or tropical conditions, in particular compared to PVB particles alone. A step of brief heating of the sealing layer in the presence of a crosslinking agent, in particular of the silane type, makes it possible to further improve its performance.

Claims

Claims

1. A method for sealing a paper layer, comprising applying an aqueous dispersion to a surface of said paper layer to form a coating, and drying said coating, wherein said aqueous dispersion comprises water, particles of a poly(vinyl acetal)-based resin, particles of a resin other than a poly(vinyl acetal)-based resin, one or more plasticizers, and one or more emulsifiers, wherein the total content of resin particles is 5 to 50% by weight.

2. The method of claim 1, wherein the poly(vinyl acetal) based resin is poly(vinyl butyral) based.

3. The method of claim 2, wherein the poly(vinyl butyral) based resin comprises residual alcohol and acetate functions.

4. Method according to claim 2 or 3, in which the poly(vinyl butyral)-based resin comes from the recycling of laminated glazing.

5. Method according to any one of claims 1 to 4, in which said aqueous dispersion further comprises one or more crosslinking agents, preferably chosen from silanes, preferably from alkoxysilanes optionally comprising a functional group reactive with respect to carboxylic acid, carboxylate or hydroxy functions, such as an epoxy or amine group.

6. Method according to claim 5, in which the weight ratio between the content of crosslinking agent and the content of poly(vinyl acetal)-based resin is between 0.001 and 0.10, in particular between 0.002 and 0.

06.

7. A method according to any one of claims 1 to 6, wherein the aqueous dispersion further comprises one or more fillers, the filler content preferably being between 5 and 80% by weight, relative to the total weight of the dispersion.

8. A method according to claim 7, wherein the fillers are selected from calcium carbonate, calcium stearate, clays, talc, dolomite, mica, silica sands, ground basalt, barium sulfate, kaolin, and mixtures of two or more of these compounds.

9. A method according to any one of claims 1 to 8, wherein said resin other than a poly(vinyl acetal) based resin is selected from styrene-acrylic resins, styrene- butadiene, acrylic resins, alkyd resins, and a mixture of two or more of these resins, preferably among alkyd resins.

10. A method according to any one of claims 1 to 9, wherein the aqueous dispersion further comprises from 0.1 to 20% by weight of pigments, relative to the total weight of the dispersion.

11. A method according to any one of claims 1 to 10, wherein the total weight content of emulsifier(s) is between 0.1% and 10%.

12. / 0. Method according to any one of claims 1 to 11, in which the weight ratio between the plasticizer content and the poly(vinyl acetal) resin content is between 0.1 and 1, in particular between 0.2 and 0.

5.

13. Process according to any one of claims 1 to 12, in which the quantity of water is between 10 and 70% by weight, in particular between 20 and 60% by weight, relative to the total weight of the dispersion.

14. A method according to any one of claims 1 to 13, wherein the weight ratio of the content of particles of resin other than a poly(vinyl acetal)-based resin to the content of particles of a poly(vinyl acetal)-based resin is between 0.05 and 20, preferably between 0.1 and 10, or even between 1.0 and 5.

0.

15. A bag comprising a paper layer, and a sealing layer at least partially covering the paper layer, the sealing layer being formed by the method as defined in any one of claims 1 to 14.

16. Use of a bag as defined in claim 15, for packaging plaster or cement-based powders.