Paper-based composite as food packaging material

Abstract

Composite as food packaging material, comprising a substrate consisting of a paper carrier and a primer based on an aqueous polymer dispersion, and a barrier layer comprising a metal oxide layer and a hybrid polymer layer of an organically modified silicic acid (hetero)polycondensate.

Description

Field of the invention

[0001] The present invention relates to a paper-based composite as a food packaging material with a barrier function and to the production and use of the composite. State of the art

[0002] There is a great demand for flexible packaging materials that are cost-effective and require little material to produce. For sustainability reasons, substrates based on fossil-based polymers are increasingly being replaced with paper. Of particular importance in this regard is the use of paper materials from the waste paper cycle.

[0003] The barrier performance of polymeric substrate films can be increased by applying metal oxides, for example, using physical vapor deposition (PVD). PVD coating is also widely used on paper substrates, but is primarily used for decorative purposes. One reason for this is that such coatings can only increase barrier performance as long as the coatings are virtually defect-free. However, since paper has a very rough surface, inorganic coatings always contain some defects that allow small amounts of gas to pass through. The roughness of the paper can be reduced by precoating.

[0004] A hybrid polymer layer made of inorganic-organic polymers based on silanes can serve as a smooth planarization layer. The low viscosity of the coatings and the drying effects create very smooth surfaces that are ideal for the subsequently applied inorganic coatings.

[0005] Another critical factor is the hygroexpansion of the paper. PVD coating takes place under dry conditions, where the paper shrinks. After the coating process, the paper absorbs moisture and expands, which can cause stress in the inorganic layer and, consequently, defects such as cracks. Furthermore, the metal oxide layer must be very thin to prevent the formation of cracks and flaking.

[0006] Of utmost importance for maintaining the barrier performance of the coated paper is its flexibility under mechanical stress during the production process, transport or use. Problems to be solved by the invention

[0007] It is the object of the present invention to provide a paper packaging which has and maintains a very high barrier performance even under mechanical influence.

[0008] Furthermore, it is the object of the present invention to provide a coating for paper substrates which is both environmentally friendly and flexible and which, together with the known barrier layers, enables a higher barrier performance. Summary of the invention

[0009] The object was achieved by providing the composite according to the invention, which has a primer based on an aqueous polymer dispersion directly on the paper substrate.

[0010] The present invention is defined in the appended claims. Advantages of the invention

[0011] The use of a primer based on an aqueous polymer dispersion improves the barrier performance of known barrier layers on paper substrates.

[0012] Primer or sealing varnishes based on an aqueous polymer dispersion known in the state of the art can be used as pre-coat varnishes.

[0013] The composite according to the invention is flexible, so that the barrier performance is largely maintained under mechanical stress.

[0014] By using an aqueous polymer dispersion, its properties can be adjusted. For example, the viscosity can be adjusted by varying the water content of the aqueous polymer dispersion. Aqueous polymer dispersions do not require reactive diluents, which adversely affect the mechanical properties of polymers. Eliminating reactive diluents makes the aqueous polymer dispersion more environmentally friendly and results in a less sticky primer.

[0015] Due to the excellent barrier performance of these combined layers, the substrate itself does not require any barrier performance, so there are no particular restrictions regarding the composition and properties of the substrate. Therefore, both original paper substrates and recycled materials or other fiber-based materials (fiber encapsulants) can be used as substrates.

[0016] Due to these properties, the composite according to the invention is excellently suited as a food packaging material. Embodiments of the invention

[0017] The composite according to the invention comprises a paper substrate, a primer based on an aqueous polymer dispersion applied thereto, and a barrier layer on the primer. The barrier layer is a metal oxide layer and / or a hybrid polymer layer.

[0018] The composite according to the invention may hereinafter be referred to simply as "the composite." The primer based on an aqueous polymer dispersion may be referred to simply as "the primer." The barrier layer consisting of the metal oxide layer and the hybrid polymer layer may be referred to simply as "the barrier layer."

[0019] The composite is a sheet with a preferred thickness of 1 mm or less, more preferably less than 300 µm. The composite is preferably flexible enough to be folded and bent without cracking, making it suitable for packing and wrapping objects, particularly foodstuffs. Preferably, the composite can be folded to form a folded edge, with no cracks occurring in the paper substrate, the precoat, or the barrier layer.

[0020] In the composite, not only the paper substrate and the primer, but preferably all components, i.e. also the one or more layers of the barrier layer, are arranged directly on top of one another, ie without an intermediate layer or adhesive or the like.

[0021] The layer sequence can depend on the drying process of one layer on top of the underlying layer. The increased temperature during drying can lead to expansion, shrinkage, or viscosity changes of the layer. By adjusting the drying process of a layer, the defect density can be reduced.

[0022] The primer used can also be used as a sealing layer in the resulting packaging and as a finishing layer to protect the underlying metal oxide layer.

[0023] The composite according to the invention not only provides a barrier against water vapor and oxygen, but also against other migratable molecules, such as aromas from the packaged food or undesirable external odors. Furthermore, a certain barrier exists against other substances, such as impurities contained in printed packaging materials or recycled materials.

[0024] The metal oxide layer and the hybrid polymer layer can independently cover the underlying layers completely or partially. There are no particular restrictions on the respective thicknesses of the layers. They can vary and be flexibly adapted depending on the application method of the hybrid polymer layer, the nature of the substrate surface, and the application of the coating.

[0025] Due to the very brittle metal oxide layer and the stiff hybrid polymer layer, the barrier performance in conventional products is lost upon folding. Surprisingly, this disadvantage is overcome in the composite according to the invention, especially when it contains a combination consisting of a precoat, a metal oxide layer, and a hybrid polymer layer in that order. This combination results in good adhesion between the precoat and the metal oxide layer, and good adhesion between the metal oxide layer and the inorganic groups of the hybrid polymer. One possible explanation for the resistance to folding stresses could be the laminate theory. The laminate theory explains the forces acting on a laminate when it is bent. Tensile stress acts on the outer layer, and compressive stress on the inner layer. The fragile layer is the brittle metal oxide layer.It should be located in the center, i.e., in the neutral plane, where the strains and stresses occurring during bending have the least influence. Both the substrate thickness and the layer thickness play an important role here, as these factors determine the position of the brittle metal oxide. Furthermore, the precoat and the hybrid polymer layer act as buffer layers, improving the fracture properties of the brittle film. A buffer layer with greater stiffness than the substrate, such as a hybrid polymer layer, can improve crack resistance. The more flexible precoat and the stiffer hybrid polymer layer act as optimal buffer layers. Changing the order of the layers is also possible and can have certain advantages.When the metal oxide layer is applied directly onto the primer and the hybrid polymer layer as a topcoat, defects (e.g., pinholes) in the metal oxide layer can be closed by the hybrid polymer due to the low viscosity and covalent bonding of hybrid polymer layers to metal oxide layers. Hybrid polymer layers are chemically inert and also highly abrasion-resistant, and can protect the brittle and sensitive metal oxide structure.

[0026] When the metal oxide is applied to the hybrid polymer layer, the very flat surface of the hybrid polymer layer can reduce the number of defects in the metal oxide.

[0027] The paper substrate is coated with a primer on one of its surfaces. The other surface can be free of any coating or can have a coating, e.g., also with a primer or a planarization layer or a barrier layer. If a recycled material is used as a paper substrate, it must be sealed on the inside, towards the food, to prevent undesirable substances from migrating into the food. On the outside, the base substrate can be coated with a planarization layer to smooth its surface so that it can be printed, for example. A planarization layer can also be used if a barrier layer, e.g., made of a metal oxide layer and / or a hybrid polymer layer, is provided.

[0028] The composite according to the invention is suitable for use as a food packaging material. However, it is not limited to this. With the properties demonstrated herein, a composite according to the invention is suitable for numerous applications requiring flexible, paper-based packaging with high barrier performance. Examples of such applications include cosmetic items such as individually packaged care products or disinfectant wipes, as well as cleaning agents such as detergents in tablet or powder form. Furthermore, the composite is suitable for contact with food, for example, for powdered nutritional supplements, instant coffee, portioned ketchup, or tea bags. The composite according to the invention can therefore also be referred to as follows: "Composite as packaging material, in particular for food, cosmetics, or cleaning agents."

[0029] In this specification, layers, compounds, components, and the like are generally described in the singular, e.g., "a" component or "contains a" component. It is emphasized that, unless otherwise stated, these formulations are in no way intended to exclude the presence of more than one of the specified layers, components, compounds, or the like.

[0030] Percentages (%) herein refer to percentages by mass unless otherwise stated.

[0031] Unless otherwise stated, the standards and norms referred to herein refer to the latest version available at the time of filing this application. Paper substrate

[0032] The composite according to the invention is based on a paper substrate. The paper substrate is suitable as a base material for the food packaging material with a barrier function.

[0033] Paper is a cellulose-based sheet material. In the present invention, the term "paper" or "paper substrate" encompasses all cellulose-based sheet materials, including thicker papers such as cardboard or paperboard. The paper can be a thin sheet material with a basis weight of less than 150 g / m². 2 , but also a cardboard in the range 150 g / m 2 up to 600 g / m 2 Cardboard can be even heavier than cardboard. A paper, e.g., calendered paper, precoated paper, release paper, or similar, with a basis weight of 20-120 g / m² is preferred. 2 , preferably 35-90 g / m 2 and more preferably 50-70 g / m 2 .

[0034] The thickness of the paper substrate is preferably more than 10 µm, for example, 10 µm to 10 mm. More preferably, the paper substrate is a thin sheet with a thickness of 10 µm to 1 mm, even more preferably 10 µm to 200 µm.

[0035] The paper substrate is preferably flexible. The surface of the paper substrate coated with the primer is preferably smooth. This surface preferably has a mean roughness Ra of less than 2 µm or from 0.1 to 10 µm. Furthermore, it preferably has a roughness Rq of 1 to 10 µm and Rz of 5 to 20 µm. The mean roughness of the paper substrate coated with the primer can be determined, for example, using scanning electron microscopy (SEM), laser scanning microscopy (LSM), or atomic force microscopy (AFM).

[0036] A smooth surface can be achieved through mechanical treatment, such as calendering, and / or coating. Examples of smooth paper include supercalendered kraft (SCK) paper such as glassine, clay-coated kraft (CCK), PE-coated kraft (PCK), machine-glazed kraft (MG), fiber-molded paper, release liner, or similar.

[0037] The paper substrate can be, for example, original paper or a recyclate, i.e., waste paper. Original paper contains no or only minimal amounts of impurities. A paper-based recyclate may contain unwanted short-chain sugar chains, paint, or printing ink as impurities that are unnecessary for the intended function of the composite or may even be detrimental. An example of impurities is a content of at least 5% of low-molecular-weight substances with a molecular weight of less than 1000 Da. Primer

[0038] The paper substrate is provided on at least one of its surfaces with a primer based on an aqueous polymer dispersion. Herein, an aqueous polymer dispersion is a composition containing water as the main phase and a polymer as the secondary phase, and includes dispersions of a liquid in another immiscible liquid, as well as colloidal solutions of polymer particles in water. The aqueous polymer dispersion is referred to as a primer coating and is preferably highly flexible. A primer coating is, for example, an aqueous polymer dispersion of a high-molecular-weight propylene copolymer, a modified polyvinyl alcohol, or a modified vinyl acetate copolymer. An aqueous polymer dispersion of an ethylene-acrylic acid (EAA) copolymer is preferred. Sealing layers for packaging systems can generally be used as primers.

[0039] The primer is applied directly to the paper substrate so that the primer and the paper substrate lie directly on top of each other in the composite.

[0040] The primer reduces surface roughness, acts as a water vapor barrier, increases surface energy, and seals the surface, preventing later applied layers from penetrating the paper structure. The primer creates stronger adhesion between the paper substrate and the metal oxide, thus increasing the metal oxide's resistance during bending or folding, thus reducing cracking. Particularly strong adhesion is achieved when the primer contains acrylic acid, which leads to good adhesion to the hydroxyl groups of the paper and the hybrid polymer. The metal oxide SiO xCan be covalently bonded to polymers via CO-Si and / or C-Si bonds. The alkyl content influences the properties of the precoat. A higher alkyl content leads to better adhesion between the substrate and the polymer resin, higher overall stiffness, a lower melting temperature, and greater flexibility, which is important for sealability.

[0041] The primer can smooth the rough surface of the paper substrate. It should therefore preferably be at least thick enough to cover all surface roughness of the paper substrate. The result is preferably a surface consisting entirely of primer material.

[0042] The primer can be applied by reverse gravure printing, blade coating, curtain coating, or slot die coating. Additionally, the paper substrate can be corona-treated. The primer is preferably applied in a thickness of 1-30 g / m². 2 or 3-10 g / m 2 , preferably 4-8 g / m 2, more preferably 5-7 g / m 2 applied and is dried at 80-110° C and cured if necessary. The average thickness of the primer is preferably 1 µm to 20 µm, more preferably 2 µm to 10 µm. The thickness of the layer is determined by scanning electron microscopy.

[0043] During the composite production process, the primer is applied to the paper substrate. The water-based primer partially penetrates the paper substrate, creating an intimate bond between the two components. After the primer dries, this results in strong adhesion between the paper substrate and the primer.

[0044] By using a precoat, even a very thin metal oxide layer with a thickness of, for example, 20 to 60 nm can achieve high barrier performance. The precoat makes the barrier layer structure independent of the nature of the paper-fiber substrate to be coated. This also allows recycled materials to be used as paper substrates because potential migration of monomers or residual substances (MOSH / MOAH) from the paper can be prevented, and the coating creates defined surfaces.

[0045] The aqueous polymer dispersion contains a polymer. Herein, the term "polymer" includes both a fully polymerized polymer and a prepolymer that is extended to the polymer after application to the paper substrate. The prepolymer can be an oligomer or a polymer with a weight-average molecular weight (Mw) of at least 2000 g / mol or even more than 5000 g / mol. A lower Mw results in a lower viscosity.

[0046] The primer is produced using an aqueous dispersion of a polymer as the starting material. The dispersibility of the polymer in water requires hydrophilic components, e.g., acidic or ionic residues in the polymer molecule. Hydrophilic components would interfere with non-aqueous, e.g., purely organic compositions, and are therefore not present in polymers of such compositions. Therefore, the aqueous polymer dispersion used according to the invention differs from conventional non-aqueous primer coatings. Consequently, the primers obtained from it also differ.

[0047] The aqueous polymer dispersion preferably contains 10 to 90% water and 90 to 10% polymer, more preferably 40 to 80% water and 20 to 60% polymer. Preferably, the total water content and polymer content constitute 80 to 100% of the aqueous polymer dispersion.

[0048] The dispersion is aqueous, i.e., it is water-based. It is preferably solvent-free, i.e., free from solvents other than water. In some embodiments, a small amount of solvent, e.g., 0.01% to 10% or 0.1% to 5%, may be present in the aqueous polymer dispersion. After application to the paper substrate, the aqueous polymer dispersion is dried. The water in the aqueous polymer dispersion can be removed by heating and / or air convection. Herein, the term "drying" encompasses a process for partially or completely removing water. A dried dispersion may contain small amounts of water or no water, e.g., 0 to 5% or 0.1 to 5%, preferably 0 to 2% water.

[0049] The dispersion may contain other components such as a reactive diluent, colorant or photoinitiator.

[0050] The photoinitiator can be selected from the group consisting of thioxanthones, ketosulfones, (alkyl)benzoylphenylphosphine oxides, 1-hydroxyalkylphenyl ketones, or 2,2-dimethoxy-1,2-diphenylethan-1-one. The photoinitiator can be present in an amount of 0.1% to 10%, particularly 0.5% to 5% on a dry matter basis. Herein, a percentage on a "dry matter basis" means the percentage based on the solids content, i.e., excluding water and any other solvents present.

[0051] The viscosity of the aqueous polymer dispersion can be adjusted by varying the water and / or reactive diluent content. The dispersion preferably has a viscosity of 0.01 to 10 Pa s, particularly preferably 10 to 3000 mPa s. A viscosity within this range results in good flowability of the aqueous polymer dispersion and a uniform coating of the substrate surface. Since the viscosity of the aqueous polymer dispersion depends on its water content, the viscosity increases upon drying. In the present invention, the viscosity is measured at 23°C according to DIN EN ISO 2555 (Brookfield method).

[0052] A reactive diluent is reactive because it is polymerizable and becomes part of the molecule of the cured polymer structure, and it is a diluent because it reduces the viscosity of the dispersion. The reactive diluent preferably has a weight-average molecular weight Mw of less than 500 g / mol, thus differing from the prepolymer with a weight-average molecular weight Mw of more than 2000 g / mol. The reactive diluent can be selected from aliphatic (meth)acrylates or polyether (meth)acrylates. Since the viscosity can be adjusted by varying the water content, a reactive diluent is not required. Therefore, the aqueous polymer dispersion preferably does not contain a reactive diluent. However, a reactive diluent may be present. The dispersion may contain 0 to less than 2%, preferably less than 0.1%, more preferably 0% of a reactive diluent on a dry mass basis.

[0053] In one embodiment, the aqueous polymer dispersion contains a water-dispersible prepolymer having, for example, at least one polymerizable carbon-carbon double bond. It can be cured, i.e., polymerized, resulting in a cured polymer. The aqueous polymer dispersion can contain the prepolymer as the only polymerizable component. Alternatively, the aqueous polymer dispersion can contain the prepolymer and other copolymerizable components, e.g., a chain extender, which can be monomeric. In this case, the finished polymer is formed, for example, by chain extension and by curing the polymerizable CC double bonds. In any case, preferably, all components necessary for preparing the polymer are present in the aqueous polymer dispersion, and no component needs to be added to carry out the polymerization.In other words, the precoat is preferably obtainable by a process consisting of reducing the water content of the aqueous polymer dispersion, e.g., by heating, and curing the dried dispersion. The prepolymer is curable by heating or irradiation, i.e., treatment with UV light or electron beams. Preferably, the prepolymer is UV-curable. The radical polymerization can be initiated by the photoinitiator.

[0054] The prepolymer with a polymerizable CC double bond can be selected from the group consisting of acrylates, methacrylates, vinyl ethers, allyl ethers, propenyl ethers, alkenes, dienes, unsaturated esters, allyl triazines, allyl isocyanates, and N-vinylamides. Herein, the term "acrylate" or "acrylic" is understood to include "(meth)acrylate" or "(meth)acrylic," respectively.

[0055] The polymer contained in the aqueous polymer dispersion is dispersible in an aqueous medium, preferably water. For this purpose, it exhibits a certain degree of hydrophilicity. It preferably contains hydrophilic radicals derived from corresponding hydrophilic compounds that are capable of rendering the polymer dispersible in an aqueous medium, either directly or after reaction with a neutralizing agent to form a salt. The hydrophilic compounds are generally selected from polyols. They may contain an ionic or nonionic hydrophilic group. A polyol may be preferred that contains one or more anionic salt groups, such as carboxylate and sulfonate salt groups, or acid groups that can be converted into an anionic salt group, such as carboxylic acid or sulfonic acid groups. Examples are hydroxycarboxylic acids of the formula (HO) x R(COOH) y, where R represents a straight-chain or branched hydrocarbon radical having 1 to 12 carbon atoms, and x and y are independently integers from 1 to 3. Typically, the aqueous polymer dispersion requires the neutralization of the hydrophilic radicals into salts. This is usually achieved by adding an organic or inorganic neutralizing agent, or mixtures thereof, to the polymer or water.

[0056] The mechanical properties of the resulting primer are influenced not only by the chemical composition of the polymer or curable prepolymer, but also by the respective crosslink density. A higher crosslink density typically results in a harder and more brittle material, while a lower crosslink density results in a softer and more conformable material. Hybrid polymer layer

[0057] The composite according to the invention can contain a "hybrid polymer layer composed of an organically modified silicic acid (hetero)polycondensate." This term means that the hybrid polymer layer consists of the organically modified silicic acid (hetero)polycondensate or consists of at least 50% of it. The inorganic silicic acid (hetero)polycondensate is modified with organic groups, which are preferably organically polymerized, and is sometimes referred to herein simply as a "hybrid polymer." Preferably, the hybrid polymer is both inorganically and organically cross-linked. Unless otherwise stated, the term "hybrid polymer" also includes polymers with bio-based and / or biodegradable components. An example of a hybrid polymer is ORMOCER® or bioORMOCER® (tradename of the Fraunhofer Society for the Promotion of Applied Research eV, Munich).

[0058] The inorganic component of the hybrid polymer consists of silicon cations, optionally in combination with other cations such as aluminum, zirconium, titanium, or boron, as well as combinations thereof, which are linked together via oxygen bridges to form a network. This constitutes an organically modified silica ion or, in the presence of additional metal ions, an organically modified silica heteropolycondensate. An example of such a heteropolycondensate is a condensate containing silicon and aluminum. The term "silica (hetero)polycondensate" is used as a collective term for pure silica polycondensates and polycondensates containing heteroatoms.

[0059] The hybrid polymer is typically produced by hydrolytic condensation of silanes, possibly in combination with co-condensable compounds of other metal ions. These silanes can carry hydrolyzable groups such as alkoxides or hydroxide groups. However, they can also carry organic groups directly bonded to silicon via carbon. These groups remain bonded to the respective silicon atoms during hydrolytic condensation, so that the polycondensate is modified with the organic groups.

[0060] The organic groups can be derivatives of one or more types of natural or synthetic organic polymers. The organic groups can be at least partially bound into the condensate via Si-C bonds. The organic groups bound into the condensate via Si-C bonds can be thermally or photochemically organically polymerizable groups, particularly epoxy groups. However, the organic groups can also be bonded to silicon via oxygen. They can be biodegradable.

[0061] The hybrid polymer layer is preferably biodegradable. Hybrid polymers with good biodegradability are formed by replacing non-biodegradable organic components with biodegradable components. The Si-O-Si bonds are generally acid-stable but base-labile, so that the barrier layer can be degraded. This base-lability can also be exploited in the recycling of a composite according to the invention by removing the hybrid polymer from the substrate through alkaline washing. Biodegradability can also be adjusted by reducing the degree of crosslinking, especially the degree of inorganic crosslinking (i.e., the Si-O-Si or Si-O-metal bonds). For the biodegradability of the coating, it is necessary that as large a proportion as possible, preferably at least 10%, of the organic groups incorporated into the inorganic network of the hybrid material be biodegradable.Therefore, preferably at least some of the organic groups comprise a hydrocarbon chain with 2 to 8, preferably 2 to 6, carbon atoms located between two groups selected from ether, ester, amide, and urethane groups. In particular, at least some of the organic groups may comprise at least two hydrocarbon chains with 2 to 8 carbon atoms that are branched to one another. Particularly preferably, at least some of the inorganic, biodegradable groups comprise the component [O-(CH2). m-C(O)-O]n, where m is an integer between 2 and 8, n is 0, 1, 2 or 3 or greater than 3 and, if the said component is present multiple times in the organic, biodegradable group, can assume different values, with the proviso that n is 1 or greater than 1 in the only or in at least one of the said components. The organic groups can also be selected from the group consisting of polycaprolactone triol (of fossil origin but biodegradable), chitosan, cellulose, cellulose derivatives, hemicelluloses and cellulose building blocks and other bioorganic resources. In order to enable the integration of the degradable components used into the hybrid polymer network, they are partially chemically modified. By varying the biopolymer content, the degradation rates can also be adjusted.This and the choice of the biopolymer itself open up the possibility of controlling the rate of degradation.

[0062] By using the material class based on hybrid polymers and bio-based and biodegradable hybrid polymers, excellent barrier improvements can be achieved while maintaining compostability.

[0063] The hybrid polymer layer is created by applying a suitable coating varnish to the desired substrate. If it contains solvent, this can be removed if necessary. Alternatively or cumulatively, thermal post-treatment or irradiation with light / UV is possible. Specifically, the hybrid polymer layer can be produced using a process that comprises applying a composition, optionally present in a diluent and / or solvent and containing hybrid polymer, to a metal oxide layer, for example by spraying, slot die, gravure roller, and drying and / or curing the composition. The coating varnish is preferably applied in a thickness of 1-30 g / m 2 or 2-9 g / m 2 , preferably 3-8 g / m 2 , more preferably 4-5 g / m 2applied and thermally cured at 80-120° C. The average thickness of the hybrid polymer layer is preferably 500 nm to 30 µm, more preferably 1.0 to 10 µm, and even more preferably 1.0 to 5.0 µm. The thickness of the layer is determined, for example, by scanning electron microscopy.

[0064] In addition to the hybrid polymers, the hybrid polymer layer can contain other polymers and / or fillers. The fillers are preferably inert and chemically non-reactive. However, they can also perform an active function (e.g., antimicrobial function, active barrier function). They should be soluble or dispersible in the hybrid sol, non-hygroscopic, and have a particle size that does not affect the optical appearance of the coating. Typical fillers are starch, chemically modified starch, dextrin, microcrystalline cellulose, insoluble cellulose derivatives, and inorganic compounds (e.g., talc, TiO2, SiO2, silicates, clay materials, insoluble carbonates, and phosphates). The filler content (preferably in an amount of 1–25% of the coating) depends on the material. Starch or dextrin improve the mechanical properties and facilitate processability.Inorganic fillers such as silicates, aerosils and dust particles improve the moisture barrier and can be incorporated in quantities of up to 50%.

[0065] Due to reactive functional groups, the hybrid polymer adheres very well to adjacent interfaces. The organic component preferably predominates in the coatings, thus increasing flexibility even in the cured coating. Furthermore, the use of longer spacers (three or more chain atoms) between the alkoxysilane group and the organic functionality further maximizes flexibility while maintaining barrier performance. Furthermore, flexibility can be increased by conducting the synthesis at pH values ​​below 7, which preferably allows for the creation of a more linear network. Metal oxide layer

[0066] The term "metal oxide layer" means that the metal oxide layer consists of the deposited metal oxide or consists of at least 50% of it. Metal oxides can be SiO2, SiO, MgO, CaO, TiO2, ZnO, AlOx, or MnO, or mixtures thereof. Silicon oxide SiO is preferred. x , which contains silicon monoxide (SiO) and silicon dioxide (SiO2), so that "x" is 1.0 to 2.0. Values ​​above 2.0 are also possible. The higher the density of the hydroxy functionalities of the deposited SiO measured by XPS, the higher the x particles, the larger "x." In one embodiment, "x" is greater than 2.0, preferably greater than 2.2, even more preferably greater than 2.5.

[0067] The metal oxide layer provides a permeation barrier against both water vapor and oxygen. The average thickness of the metal oxide layer is preferably 5 nm to 200 nm, more preferably 10 nm to 100 nm or 20 nm to 100 nm. The layer must have a certain minimum thickness for stability reasons. However, layers that are too thick are more brittle.

[0068] For high-performance barrier structures, a combination of a vacuum-deposited barrier layer with a hybrid polymer layer is possible. The inorganic or metallic layer is applied in a vacuum by physical vapor deposition (PVD) or by vacuum chemical vapor deposition (VCVD). In addition to the vacuum process for applying inorganic or metallic barrier layers, it is also possible to use metal oxides such as SiO xThe process of flame pyrolysis or CCVD (combustion chemical vapor deposition) belongs to the group of chemical vapor deposition or CVD (chemical vapor deposition) and enables the deposition of functional thin layers at atmospheric pressure, for example when using SiO x as metal oxide, the content of silanol groups is higher at the interface, the internal interfaces and surfaces and in the bulk of the metal oxide layer.

[0069] The metal oxide layer can completely or partially cover the underlying layer. There are no specific restrictions on the thickness of the layer. It can vary depending on the application method, the nature of the substrate surface, and the application of the coating.

[0070] The metal oxide is modified, for example, by CCVD, which creates, among other things, hydroxyl groups, which are located on the surface of the deposited metal oxide particles. These can form covalent bonds with the hybrid polymers of the adjacent layers, so that, for example, in the case of the use of SiO xthan the metal oxide, additional -Si-O-Si bonds are formed. The resulting layer of modified metal oxide is referred to in the present invention simply as a metal oxide layer. The metal oxide layer produced by CCVD is very rich in surface OH groups, which serve as chemical anchors for adhesion groups of other substances. This enables a simple, large-area, and continuous coating of the precoat on the paper substrate. The flame pyrolytic process enables the combination of two or three chemically related layers, namely a metal oxide layer (especially when using silicon oxide), a hybrid polymer layer, and a precoat based on hydrophilic polymers. The covalent bonds at the interfaces of the layers result in synergy effects for maximum barrier performance. Synergistic effects at the interface of the two layers result in the formation of covalent bonds (e.g.Si-O-Si), which compensate for defects and / or porosities in the inorganic layers and lead to stable interlocking of the layers. Excellent wetting properties and covalent bonds also allow the microscopic holes (pinholes, cracks) in the metal oxide layers that arise during CCVD application to be sealed. Furthermore, using adapted process technology, the metal oxide coating of highly temperature-sensitive substrates can be realized without any loss of properties. This allows substrates for packaging purposes to be produced easily and cost-effectively without vacuum technology and avoiding batch processes. Examples

[0071] The specified water vapor permeability (WVTR) is determined according to DIN 53 122-1 (23°C, 85% rh, relative humidity).

[0072] The quantitative characterization of the paper coatings was performed as follows: Barrier damage caused by packaging machine stresses, folding, abrasion of the product packaging, and puncture by sharp packaging edges was simulated in a FlexCrack resistance test. For this purpose, the coated substrate is folded inward 180° in both the machine direction (MD) and the cross direction (CD). The fold is created by a 2 kg metal cylinder. Samples treated in this way are referred to as "folded" after folding.

[0073] The following varnishes were used: Primer (A): An aqueous polymer dispersion of an EAA copolymer; solids content: 20%; pH: 8 to 9. Primer (B): An aqueous polymer dispersion of a high molecular weight propylene copolymer; solids content: 55%; pH: 10. Barrier coating (C): A silane-based hybrid polymer with thermally curable organic groups; solids content: between 30 and 45%; pH: between 3 and 5; viscosity: 10 to 20 mPas.

[0074] Smooth paper was used as the substrate. Example 1 (Reference example): The varnish (A) was applied to a release liner paper using reverse gravure printing and cured with a hot air dryer at 100° C. The primer was applied by PVD with a SiO x -layer coated. Example 2 (Reference Example): Instead of varnish (A), varnish (B) was used and diluted with water to a solids content of 25%. The coating was applied as in Example 1. Example 3: Example 1 was repeated, but the SiO x -layer was applied between varnish (A) and varnish (C). Example 4: The coating was carried out as in Example 3, but the lacquer (A) was applied in two layers with a total thickness as in Example 1. Example 5: The coating was carried out as in Example 3, but the lacquer (C) was placed between the SiO x -layer and varnish (A) was applied.

[0075] Table 1 shows the results of the examples. Table 1 Layer structure WVTR [g / m 2 d]at 23 °C / 50 %Unfolded WVTR [g / m 2 d]at 23 °C / 50 %Folded Comparison example Paint A 13 Example 1 (Reference example) Lack A / SiO x 1,2 4,5 Example 2 (Reference example) Lack B / SiO x 4,5 6,6 Example 3 Lack A / SiO x / LackC 0,2 0,6 Example 4 2x Lack A / SiO x / Lack C 0,8 0,9 Example 5 Lack A / Lack C / SiO x 0,4 0,8

[0076] In further examples it was shown that composites of (i) paper substrate / primer / SiO x -layer / hybrid polymer layer a very low WVTR below 3 g / m 2 d and that the WVTR was increased by up to a factor of 2 by folding. Composites of (ii) paper substrate / precoat / hybrid polymer layer / SiO x -layer showed a similar WVTR to composites (i), with the WVTR being increased more by folding than in composites (i).

[0077] The double application of the primer significantly increased the resistance of the water vapor barrier effect to wrinkles in all composites.

[0078] All composites showed excellent barrier properties against water vapor in both directions, i.e. from the paper substrate side to the barrier layer side and from the barrier layer side to the paper substrate side.

Claims

[1] Composite as food packaging material, comprising a substrate consisting of a paper carrier and a primer based on an aqueous polymer dispersion, and a barrier layer comprising a metal oxide layer and a hybrid polymer layer of an organically modified silicic acid (hetero)polycondensate. [2] The composite of claim 1, wherein the surface of the paper substrate coated with the primer has a mean roughness of less than 2 µm. [3] Composite according to claim 1 or 2, wherein the precoat has a thickness of 1 µm to 10 µm. [4] A composite according to any one of the preceding claims, wherein the aqueous polymer dispersion is an aqueous polymer dispersion of an ethylene-acrylic acid copolymer, propylene copolymer or polyvinyl alcohol. [5] A composite according to any one of the preceding claims, which comprises or consists of the paper substrate, the precoat, the metal oxide layer and the hybrid polymer layer in this order; or which comprises or consists of the paper substrate, the precoat, the hybrid polymer layer and the metal oxide layer in this order. [6] Composite according to one of the preceding claims, wherein the metal oxide layer comprises a SiO x -layer. [7] Composite according to one of the preceding claims, wherein the organically modified silicic acid (hetero)polycondensate in the hybrid polymer layer is bio-based and / or biodegradable. [8] A method for producing a composite according to any one of the preceding claims, comprising the following steps: (a) providing a paper substrate; (b) applying a primer based on an aqueous polymer dispersion to a surface of the paper substrate; (c) drying the paper substrate coated with the primer; and (d) applying a barrier layer comprising a metal oxide layer and a hybrid polymer layer comprising an organically modified silicic acid (hetero)polycondensate in any order to the dried primer. [9] The method of claim 8, wherein the method comprises, between steps (a) and (b), the step (a') of smoothing a surface of the paper substrate, and in step (b) the precoat is applied to the surface of the paper substrate smoothed in step (a'). [10] A method according to claim 8 or 9, wherein the smoothing of a surface in step (a') comprises mechanical smoothing and / or coating. [11] A method according to any one of claims 8 to 10, wherein the surface of the paper substrate coated with the precoat in step (b) has a mean roughness of 0.05 to 2 µm. [12] Composite obtainable by the process according to any one of claims 8 to 11. [13] Use of a composite according to any one of claims 1 to 7 and 12 for packaging a food product. [14] Use according to claim 13, wherein the barrier layer of the composite contacts the food. [15] Use according to claim 13 or 14, wherein packaging a food product comprises folding the composite.