Methods for manufacturing packaging board

The invention is related to methods for manufacturing liquid-tight and gas-tight packaging board and a package, and products provided according to the said methods. According to the invention, a polymerizing reaction mixture is spread on paper or a board base of paperboard or cardboard, the mixture containing at least one silicon compound forming an inorganic, chain or crosslinked polymeric backbone containing alternating silicon and oxygen atoms, and at least one reactive, organic compound forming organic side chains and/or crosslinks in the polymeric backbone. The reaction mixture may form a colloidal solution in which, along with the polymerization, gelling takes place, whereupon the thus created gel is dried, densified and cured to form a liquid-tight and gas-tight layer of coating. In addition to oxygen and silicon, the said chain-like or crosslinked polymeric backbone can contain metal atoms which replace the silicon, and the organic compound can contain, as a reactive group, an epoxy, an amino, a carboxyl, a carbonyl, a vinyl or a methacrylate group. Furthermore, a joint-forming polymeric coating can be spread on the previously obtained, tight glassy layer of coating to close the manufactured package. Products, to which the paper or the board coated according to the invention can be applied, include milk and juice containers or similar packages of liquid foodstuffs, bag-type foodstuff packages, heat-sealed, peelable covers of containers and boxes, and microwave and conventional oven trays.

The object of the invention is a method for manufacturing packaging board,
 in which a board base of paperboard or cardboard is provided with at least
 one silicon-based, liquid-tight and gas-tight layer of coating. Another
 object of the invention is a method based on the coating of paper or a
 board base to manufacture liquid-tight and gas-tight packages, and
 products provided by using the methods, including foodstuff packages and
 trays.
 In order to be useful, for packages of liquid and other wet foodstuffs or
 foodstuffs which spoil easily the board or the paper must be provided with
 a liquid-tight and gas-tight coating. The coating prevents the oxygen in
 the air from penetrating the package and spoiling the product, and it also
 prevents the package from getting wet and the aromas of the product
 escaping from the package. Corresponding gas tightness can be required
 from medicine, cosmetics, and detergent packages.
 An effective way to render liquid packages, such as juice containers,
 liquid-tight and gas-tight is to provide the board of the container with a
 thin aluminium foil. Aluminium as such has also been used for peelable
 covers of yoghurt and curdled milk cups and butter and margarine boxes.
 However, aluminium foil has disadvantages: high manufacturing costs, it is
 not biologically decomposable, there are difficulties in regenerating the
 packaging material, and the package cannot be heated in a microwave oven.
 Another problem with detachable aluminium covers is that they tear and
 burst easily.
 An alternative solution for tightening the board or the paper used for
 packages is to provide it with one or more layers of polymeric coating.
 The number of layers and the material used depend on the requirements set
 by the packaged product. The best coating materials have essentially
 reached a tightness corresponding to that of aluminium foil and, as
 substitutive materials, they have eliminated the above-mentioned
 disadvantages connected with aluminium. However, it has been necessary to
 combine various polymeric materials in these substitutive solutions so
 that they comprise, for example, an oxygen-tight, water vapor-tight, and
 aroma-tight barrier layer, heat-seal layers on both sides of the paper or
 the board, and one or more layers of binding material to bind the polymers
 to the paper or the board and to each other. Therefore, the structure of
 the packaging paper or board becomes complex and the consumption of
 polymeric material is extensive.
 Examples of packages tightened according to the above description include
 containers which are intended to be used as packages of milk, cream, sour
 milk, juice or other similar, liquid foodstuffs and which are entirely
 made of board provided with layers of polymeric coating. In these
 containers, the board is typically provided with four or even five layers
 of polymeric coating so that, for example, the board comprises an
 oxygen-tight and aroma-tight barrier of, e.g., polyamide, a layer of
 binding material on top of that, and, at the very top, a heat-seal layer
 of polyethylene, for example, and another heat-seal layer of polyethylene
 is provided on the opposite side of the board. Another typical package
 application is a portion package of, for example, milk, curdled milk,
 yoghurt, water, juice, desserts or ice-cream, in which the package is in
 the form of a small cup or a container, is typically made of plastic or
 coated paperboard, and is provided with a heat-sealed, peelable cover. The
 cover material is paper which is coated with an oxygen-tight and
 aroma-tight barrier consisting of, for example, polyamide, ethylene vinyl
 alcohol copolymer (EVOH) or polyethylene terephtalate (PET), with a layer
 of binding material, and with a heat-seal layer which is against the mouth
 of the container or the cup and which consists of, for example,
 styrene-modified copolymer of ethylene and methacrylic acid, making the
 product both heat-sealable and easy to be peeled off. Cosmetic products
 and pharmaceutical pills have been packed in a similar manner, using
 plastic or glass containers provided with a peelable paper cover which is
 sealed with a polymeric coating.
 Patent publication U.S. Pat. No. 5,340,620 describes paperboard provided
 with a silicon-based polymeric coating, in which the polymer serves as an
 oxygen-tight barrier. According to the publication, the coating is
 provided by polymerizing organosilane by using UV irradiation, whereby, in
 addition to an inorganic polymeric backbone, organic bonds are formed in
 the coating when the organic groups of the silane react with each other.
 However, the portion of the inorganic polymeric backbone is prevalent in
 the coating, which is why it can be too fragile to withstand, for example,
 the creasing which is part of the manufacture of paperboard or cardboard
 containers; furthermore, there is no mention of the water vapor-tightness
 of the coating. It is obvious that the coating material of the embodiment
 of the publication cannot provide paperboard or cardboard suitable for
 liquid packages. Moreover, organosilanes are an expensive raw material for
 the coating.
 Silicon-based coatings have also been described e.g. in the published
 applications DE 4 020 316 and 4 025 215, which cite paper as one possible
 substrate of the coating but which describe in detail the coating of
 plastic or metal only, and according to the publications, the purpose of
 the coating is to provide resistance to wear so that the film-like
 substrate still maintains its flexibility. Therefore, the publications are
 not concerned with packaging technology which is the object of the present
 invention.
 Another use of tightened packaging board are foodstuff underlayers, such as
 ovenable microwave or conventional oven trays which can be part of
 consumer packages of foodstuffs, such as casserole foods intended to be
 heated, or which can be sold as separate products. Such underlayers must
 be impermeable to water and grease; and in addition to this, sufficient
 heat-resistance is required from ovenable trays. Polyester coated
 paperboard has been used in oven trays; however, its disadvantages include
 the thickness of the required polymeric layer and the fact that it is very
 difficult for the polymeric coating to withstand typical oven temperatures
 of more than 200.degree. C. Polypropylene has been used as the polymer
 coating in microwave ovenable trays.
 The purpose of the invention is to present a new solution to provide a
 board base of paperboard or cardboard intended to be used as packaging
 material with a polymeric layer of coating which renders the package
 liquid-tight and gas-tight. The purpose is particularly to provide a
 simple structure of coated board and savings in coating material, while at
 the same time making the coating tough enough to withstand the creasing
 required of paperboard or cardboard containers without breaking. The
 invention is characterized by the steps of providing a polymerizing
 reaction mixture containing at least one silicon compound to form an
 inorganic, chain-like or crosslinked polymeric backbone containing
 alternating silicon and oxygen atoms, and at least one reactive organic
 compound to form organic side chains and/or crosslinks to the polymeric
 backbone, spreading said mixture on the board base, and curing said
 mixture to form a layer of coating.
 The process according to the invention can be implemented, starting from a
 silicon compound, such as silane, an organic compound reacting with it,
 water, and a possible catalyst, whereby the hydrolyzed groups of the
 silicon compound are first partly condensed, forming colloidal particles
 in the solution. With the sol ageing and/or with a catalyst being added,
 the reaction continues with the particles growing and being combined,
 resulting in a chained or crosslinked gel covering the surface of the
 board, the gel being finally dried and cured by heating or irradiating it
 using UV, IR, laser or microwave radiation to form a thin, tight coating
 on the board. Depending on the circumstances, the curing time may vary
 from fractions of a second to several hours. The coating thus obtained
 simultaneously features typical characteristics of both an inorganic and
 an organic substance, and the properties of the coating can be adjusted
 particularly by properly selecting the organic component which forms
 crosslinks or side chains.
 The organic compound as used is a purely organic carbon based compound. The
 organic compound is capable of forming organic, carbon based side chains
 or crosslinks through the reactive sites of the polymeric backbone formed
 by the silicon compound. Said organic compounds are thus distinct from the
 silico-organic compounds such as organosilanes which polymerize by
 hydrolysis and condesation of the alkoxy groups into an essentially
 inorganic chain or network structure.
 In the invention, a considerable portion of the polymeric layer can be
 formed of suitable reactive organic compounds which are essentially
 cheaper than organosilanes. Furthermore, an organic compound, which
 preferably is added to the reaction mixture at a relatively late stage,
 advances the completion of the polymerization. The polymeric backbone
 which is created when only organosilane is used can constitute a steric
 hindrance to the mutual reactions of the reactive substituents of silane,
 while a free organic compound which is present is presumably able to
 continue the reaction even after this, forming more side chains and/or
 crosslinks between the inorganic silicon-oxygen chains. By adjusting the
 amount of the organic compound used, the degree of organicity of the
 coating thus created and the properties depending on it can also be
 adjusted at the stage of polymerization.
 According to the invention, an oxygen-tight and water vapor-tight and tough
 layer of coating is provided which does not break when bent, withstands
 creasing, and can be made very thin without creating small visually
 unperceivable pin holes in the coating, during the forming stage or later
 when heated or jointed, which constitute a problem in present coating
 materials and because of which the layers of coating have had to be made
 relatively thick. On the basis of preliminary tests, a tight layer of
 coating can be provided on a smooth board base by as low an amount of
 coating as 1 g/m.sup.2, and in practice, a preferred amount of coating is
 in the range of about 2 to 6 g/m.sup.2. A further advantage is that a
 polymeric sealing layer can be spread directly on top of the silicon-based
 layer of coating without needing a binding agent between these layers. In
 known organic coating combinations, simply the weight of a gas-tight
 barrier which can be made of polyamide, PET or EVOH is typically at least
 about 20 g/m.sup.2, and these materials require a separate layer of
 binding material between the barrier and the heat-seal layer. Therefore,
 the invention can be used to accomplish essential savings in material and
 a decrease in the weight of the board as compared with the said, known
 technology. Another advantage of the invention is that the spreading of
 the coating mixture is easy to accomplish using the methods commonly used
 in paper and paperboard or cardboard industry, such as rod or blade
 coating techniques or spraying. The spreading of the coating may thus be
 effected in the board machine by using the "on-line" principle as part of
 the manufacturing process of the board, by using the same application
 technology that is used in spreading normal coatings. The coating can also
 be effected on premoulded tray blanks or in connection with the moulding.
 When needed, the mixture can be extended with filling material, the most
 preferable materials including scale- or slate-like filling materials,
 such as talc, mica or glass flakes. When the coating is formed, these
 substances settle in the direction of the surface and contribute to its
 properties of impermeability. The adhesion of the coating to the filling
 agents is excellent. It is also possible to dye the coating by adding
 pigments or organic colouring agents to the mixture, or by mixing organic
 and/or inorganic fibres or particles into the coating formulation, the
 fastening of which to the coating can be improved by coupling agents.
 Furthermore, it is possible to include, in the formulation, an organic,
 polymerizing agent which forms a separate polymeric structure with respect
 to the inorganic chain or crosslinked structure, according to the
 invention, and which intermeshes with it. In addition to the board
 machine, the spreading of the coating can be carried out, in connection
 with a printing process, for example, on a finished board which does not
 necessarily have to be dried first. In this case, the board can be
 precoated with any kind of coating commonly used in paper and board
 industry.
 The chain or crosslinked backbone of the polymeric coating provided
 according to the invention can consist of silicon and metal atoms and
 oxygen atoms which alternate with them. Preferably, the structure mainly
 consists of silicon and oxygen, and fairly small amounts of metal atoms
 can be combined with the same structure as substituents for the silicon.
 The metals can preferably include, for example, Ti, Zr, and Al. The
 organic groups that are combined with the polymeric structure can mainly
 include substituted or unsubstituted alkyl and aryl groups.
 The polymerizing reaction that creates the inorganic polymeric backbone of
 the coating, according to the invention, can be described by way of an
 example by the following formula:
 ##STR1##
 in which:
 Me refers to a tetravalent metal atom,
 R refers to an alkyl group or hydrogen,
 X refers to, for example, an alkyl or aryl body or chain,
 Y refers to a reactive substituent which can be, for example, an amino, a
 hydroxyl, a carbonyl, a carboxyl, a vinyl, an epoxy or a methacrylate
 group,
 u, v, and w are integer numbers, and
 n and m are integers from 1 to 3.
 In the organic polymerization of the coating composition which is
 preferably carried out at the drying and setting stage of the coating, an
 organic compound can combine with the reactive substituent Y of
 organosilane to form an organic side chain, by using an addition reaction.
 The reaction can also be a condensation depending on the reacting groups.
 The reactive group at the end of the chain can further react with
 substituent Y of organosilane in the polymerization, whereby an organic
 crosslink is created between the silicon chains. It is also possible that
 substituents Y of organosilane react directly with each other to form a
 crosslink between the silicon chains. The number and the length of the
 crosslinks, i.e., the degree of organicity of the coating, can be adjusted
 with the aid of the quality and the fraction of the organic compound
 included in the reaction mixture. Particularly suitable crosslinking
 organic compounds include epoxides which contain two epoxy groups in an
 alkyl or aryl body or chain, and diols.
 The liquid medium needed in the process according to the invention may
 contain, for example, water, alcohol, and/or liquid silane. The
 hydrolyzation carried out in the above reaction example binds water,
 providing that water is present, while at the same time alcohol is
 released in the reaction, converting into a liquid phase.
 Organosilanes which comprise hydrolyzing and condensing groups, or their
 hydrolyzates are suitable for starting materials of the process according
 to the invention.
 Correspondingly, compounds containing metal centre atoms, such as Zr, Ti,
 Al, B, etc., or compounds of these metals and silicon, or mixtures of the
 compounds can be used. E.g. silanes of the following type can be used:
EQU (YX).sub.a (HX.sup.1).sub.b Si(OR).sub.4-a-b (1)
 in which
 Y=a reactive organic group which is an epoxy group, a vinyl group or other
 polymerizing, organic group,
 X and X.sup.1 =a hydrocarbon group containing 1 to 10 carbon atoms,
 R=a hydrocarbon group containing 1 to 7 carbon atoms, an alkoxyalkyl group
 or an acyl group containing 1 to 6 carbon atoms,
 a=number 1 to 3,
 b=number 0 to 2, providing that a+b.ltoreq.3.
 Organic polymerization can be described by way of an example in the
 following way:
 a) the reactive groups of the organosilane of the coating composition (Y in
 the above reaction equation) crosslink the coating when they are
 polymerized.
 A polyethylene oxide crosslink formed by epoxy silane is presented as an
 example:
 ##STR2##
 b) the added, organic, reactive prepolymer reacts with the reactive group
 of the organosilane
 ##STR3##
 c) the added, organic, polymerizing substance reacts when the molecules of
 the substance in question polymerize with each other
 ##STR4##
 d) all alternatives a, b, and c can have an effect together.
 The number and the length of the crosslinks, i.e., the degree of organicity
 of the coating can also be adjusted by the quality and the fraction of the
 organic compound included in the reaction mixture. The organic compound
 can be a monomer which can be prepolymerized to a varying degree and/or
 combined with the silane at the time of spreading the mixture. The organic
 compound can also be in the form of a prepolymer when added to the
 reaction mixture. The amount of the organic compound can be, calculated as
 a monomer, 5 to 80, preferably 10 to 70, and most preferably 10 to 50
 molar percent of the total amount of the polymerizing starting materials
 of the reaction mixture.
 The epoxysilanes according to formula (1), containing one glycidoxy group
 can include, for example: glycidoxymethyltrimethoxysilane,
 glycidoxymethyltriethoxysilane, .beta.-glycidoxyethyltriethoxysilane,
 .beta.-glycidoxyethyltrimethoxysilane,
 .gamma.-glycidoxypropyltrimethoxysilane,
 .gamma.-glycidoxypropyltriethoxysilane,
 .gamma.-glycidoxypropyltri(methoxyethoxy)silane,
 .gamma.-glycidoxypropyltriacetoxysilane,
 .delta.-glycidoxybutyltrimethoxysilane,
 .delta.-glycidoxybutyltriethoxysilane, glycidoxymethyldimethoxysilane,
 glycidoxymethyl(methyl)dimethoxysilane,
 glycidoxymethyl(ethyl)dimethoxysilane,
 glycidoxymethyl(phenyl)dimethoxysilane,
 glycidoxymethyl(vinyl)dimethoxysilane,
 .beta.-glycidoxyethyl(methyl)dimethoxysilane,
 .beta.-glycidoxyethyl(ethyl)dimethoxysilane,
 .gamma.-glycidoxypropyl(methyl)dimethoxysilane,
 .gamma.-glycidoxypropyl(ethyl)dimethoxysilane,
 .delta.-glycidoxybutyl(methyl)dimethoxysilane and
 .delta.-glycidoxybutyl(ethyl)dimethoxysilane.
 Silanes containing two glycidoxy groups can include, for example:
 bis-(glycidoxymethyl)dimethoxysilane, bis-(glycidoxymethyl)diethoxysilane,
 bis-(glycidoxyethyl)dimethoxysilane, bis-(glycidoxyethyl)diethoxysilane,
 bis-(glycidoxypropyl)dimethoxysilane, and
 bis-(glycidoxypropyl)diethoxysilane.
 Examples of compounds according to formula (1), containing other reactive
 groups include: vinyltriethoxysilane,
 vinyl-tris(.beta.-methoxyethoxy)silane, vinyltriacetoxysilane,
 .gamma.-methacryloxypropyltrimethoxysilane,
 .gamma.-aminopropyltriethoxysilane,
 N-.beta.-(aminoethyl)-.gamma.-aminopropyltrimethoxysilane,
 N-bis(.beta.-hydroxyethyl)-.gamma.-aminopropyltriethoxysilane,
 N-(.beta.-aminoethyl)-.gamma.-aminopropyl(methyl)dimethoxysilane,
 .gamma.-chloropropyltrimethoxysilane,
 .gamma.-mercaptopropyltrimethoxysilane and
 3.3.3-trifluoropropyltrimethoxysilane.
 Examples of silicon compounds which are described by general formula (2)
EQU (HX).sub.n Si(OR).sub.4-n (2)
 include dimethyldimethoxysilane, methyltrimethoxysilane, tetraethoxysilane,
 phenyltrimethoxysilane and phenylmethyldimethoxysilane.
 These compounds can be used as separate components or as mixtures of two or
 more compounds.
 Other possible compounds include, for example, colloidal silica, i.e., a
 colloidal solution which contains a certain fraction of very fine-grained
 silica-anhydride powder and which is dispersed in water or alcohol, for
 example, and in which the particle diameter is preferably 1 to 100 nm.
 Prepolymers can be used as crosslinking organic compounds and the reactive
 groups of organosilanes preferably react with the prepolymers so that
 similar reactive groups react mutually to form crosslinks which combine
 inorganic oxygen silicon chains. For example, epoxide resin or aromatic
 diols can be used to react with silanes containing epoxy groups.
 Aromatic alcohols, such as Bisphenol A, Bisphenol S, and 1.5-dihydroxy
 naphtalene can be used as diols. Acrylates can be used to react with
 silanes containing acrylic groups or acryloxy groups. Prepolymers which
 have reactive double bonds are used with vinyl silanes or other silanes
 containing polymerizable double bonds, as well as with silanes containing
 mercapto groups. Polyols are used with silanes containing isocyanate
 groups. Isocyanates are used with silanes containing hydroxy groups and
 epoxide resin is used with aminosilanes.
 Mineral fillers, such as talc and mica can be used as filling material.
 Furthermore, coupling agents, tensides, and other additives which are used
 to prepare composites and coatings can be added to the mixture.
 The hydrolyzates of the silicon compounds according to formulas (1) and (2)
 can be manufactured by hydrolyzing corresponding compounds in a solvent
 mixture, such as a mixture of water and alcohol in the presence of acid,
 of which the method is commonly known. When the silicon compounds
 according to general formula (1) and (2) are used in the form of
 hydrolyzates, a better result is generally obtained by mixing and
 hydrolyzing the silanes together.
 A curing catalyst makes the coating cure at a relatively low temperature
 and has an advantageous effect on the properties of the coating.
 The following substances, for example, can be used as the curing catalysts
 of silanes containing epoxy groups: Broensted acids, such as hydrochloric
 acid, nitric acid, phosphoric acid, sulphuric acid, sulphonic acid, etc.;
 Lewis acids, such as ZnCl.sub.3, FeCl.sub.3, AlCl.sub.3, TiCl.sub.3, and
 the metal salts of these organo complex acids, such as sodium acetate, and
 zinc oxylate; organic esters of boric acid, such as methyl borate and
 ethyl borate; alkalis, such as sodium hydroxide and caustic potash;
 titanates, such as tetrabutoxy titanate and tetraisopropoxy titanate;
 metal acetyl acetonates, such as titanyl acetyl acetonate; and amines,
 such as n-butyl amine, di-n-butyl amine, guanidine, and imidazole.
 Latent catalyzers can possibly also be used, such as salts of inorganic
 acids and carboxylic acids, such as ammonium perchlorate, ammonium
 chloride, and ammonium sulphate, ammonium nitrate, sodium acetate, and
 aliphatic fluorosulphonates.
 The selection of the most suitable curing catalyst depends on the desired
 properties and the use of the coating composition.
 Furthermore, the coating can contain solvents, such as alcohols, ketones,
 esters, ethers, cellosolves, carboxylates or their mixtures. Lower
 alcohols from methanol to butanol are particularly recommended. Methyl
 cellosolve, ethyl cellosolve, and butyl cellosolve, lower carboxylic acids
 and aromates, such as toluene and xylene, and esters, such as ethyl
 acetate and butyl acetate, are also commonly used. However, the use of
 solvents is preferably minimized, for example, by using silanes as
 solvents because the evaporation of solvent vapors in connection with the
 coating of the paperboard causes extra arrangements.
 To obtain a smooth coating, a small amount of a flow regulating agent (such
 as block-copolymer of alkylenedioxide and dimethylsiloxane) can be added
 if needed.
 Antioxidants and substances which protect against UV-light can also be
 added to the coating.
 Non-ionic tenside can be added to the coating solute to adjust its wetting
 properties and hydrophilic properties.
 The silicon-based coating layer provided according to the above description
 has a glassy outward appearance and it is also tight and flexible, does
 not crack or form holes, is heat-resistant and chemically resistant. The
 coating is oxygen-tight, grease-tight, aroma-tight, and water vapor-tight,
 and it is not sensitive to moisture. In the recycling of material carried
 out by pulping, the minor amounts of coating material present do not harm
 the recycled pulp thus obtained.
 The curing of the coating layer and removing the remaining liquid phase are
 preferably carried out by heating the coating to a temperature range of
 about 100 to 200.degree. C. Heating removes the porosity from the coating,
 giving it the required liquid-tightness and gas-tightness.
 As mentioned earlier, a joint-forming polymeric coating can be spread on
 top of the layer of coating provided according to the invention without a
 laminating adhesive between the layers. For example, when container-type
 packages are manufactured from paperboard or cardboard, the heat-sealing
 polymer serves as an adhesive that seals the joints of the container. To
 ensure the tightness of the joints, both sides of the board are preferably
 coated with heat-sealing polymer.
 As the thin, glassy coating layer provided according to the invention is
 transparent, the pictures and the text that have been printed on the board
 before the coating process will be visible. This is an advantage in food
 trays in which the glassy coating constitutes the outer surface of the
 product.
 The coated packaging board manufactured according to the invention can be
 used as the oxygen-tight and aroma-tight material of containers or small
 cups intended for packages of liquid foodstuffs. The layer of coating
 withstands, without breaking, the creasing of the coated paperboard to
 provide the edges of containers which have the shape of a rectangular
 prism or a tetrahedron.
 Another special application of the packaging board coated according to the
 invention is grease-tight, heat-resistant material of foodstuff bases,
 such as microwave or conventional oven trays. In this case, too, the
 paperboard is subjected to creasing and folding and the coating must
 withstand the treatment without breaking. Furthermore, one special
 advantage of the coating of ovenable trays, provided according to the
 invention, is the good heat-resistance of the coating. The paperboard can
 be shaped into a tray by pressing at a high temperature and the trays
 easily withstand the normal temperatures of kitchen stoves and microwave
 ovens, and even temperatures exceeding 300 .degree. C. at which the
 paperboard will begin to char. At the same time, the layers of coating
 protect the paperboard from the softening effect of steam coming from the
 food when heated so that the tray maintains its form. When baked, the food
 does not stick to the coating according to the invention. The tray
 provided in accordance with the invention can be part of the consumer
 packaging of prepared food, for example, whereby the food is intended to
 be heated in the tray after opening the package, or the trays can be sold
 to consumers as such.
 Furthermore, the invention comprises a method for manufacturing a
 liquid-tight and gas-tight package, which is characterized in that a
 polymerizing reaction mixture is spread on paper or a board base of
 paperboard or cardboard, said mixture comprising at least one silicon
 compound forming an inorganic, chain or crosslinked polymeric backbone
 which contains alternating silicon and oxygen atoms, and at least one
 reactive, organic compound forming organic side chains and/or crosslinks
 to the polymeric backbone, that the reaction mixture is cured to form a
 layer of coating, and that the package is partly of fully formed of the
 thus obtained polymer coated paper or board.
 It should be mentioned in this context that the board base in the present
 invention refers to a fairly stiff fibre-based packaging material which is
 sufficiently self-supporting to be suitable for container-like packages or
 foodstuff bases, for example, which are manufactured entirely of the said
 material. The weight of such a board is at least about 170 g/m.sup.2, and
 generally in the order of 225 g/m.sup.2 or higher. A board in the weight
 range of 170-250 g/m.sup.2 is conventionally referred to as paperboard and
 a board having a weight of 250 g/m.sup.2 or more is referred to as
 cardboard. The paper in the invention refers to a thinner and lighter
 fibre-based material which is suitable packaging material, for example,
 for heat-sealed, peelable covers of portion packages or boxes.
 What is presented above in connection with the manufacturing method of
 packaging board according to the invention, is mainly applicable as such
 to the manufacturing method of the package according to the invention.
 This is related, for example, to the forming of the silicon-based layer of
 coating, its chemical structure and composition, and to a possible
 spreading of a jointing polymeric coating on top of the glassy silica
 coating.
 Products according to the invention, manufactured according to the methods
 described above, include particularly sealed paperboard and cardboard
 packages of liquid foodstuffs, such as milk, cream, sour milk or juice
 containers and small cups, sealed paper foodstuff packages, such as soup
 mix powder pouches, coffee, and spice packages, paperboard microwave or
 conventional oven food trays which can be part of prepared food packages,
 paperboard or cardboard detergent packages, and the heat-sealed paper
 covers of glass, plastic or paperboard foodstuff, medicine, and cosmetics
 packages, particularly the covers of yoghurt, milk, juice, water,
 ice-cream or dessert cups, and those of curdled milk containers or butter,
 margarine or prepared foodstuff boxes.

The consumer package of yoghurt according to the invention presented in
 FIGS. 1 and 2 preferably consists of small plastic cup 1 with an
 oxygen-tight and aroma-tight cover paper 3 heat-sealed on its mouth 2.
 Cover paper 3 comprises paper layer 4, silicon-based, oxygen-tight and
 aroma-tight polymeric layer 5 made by using a sol-gel process, and, for
 example, heat-seal layer 6 of styrene-modified copolymer of ethylene and
 methacrylic acid. Heat-seal layer 6 secures cover paper 3 tightly to
 flange 2 encircling the mouth of the pot. At the same time, heat-seal
 layer 6 allows cover paper 3 to be peeled when the cup is opened. The
 weight of paper layer 4 of the cover paper can be, for example, 40 to 80
 g/m.sup.2, the weight of the oxygen-tight and aroma-tight layer of coating
 5 is preferably about 2 to 5 g/m.sup.2, and the weight of heat-seal layer
 6 can be, for example, about 20 g/m.sup.2.
 Ovenable tray 7 according to FIGS. 3 and 4 which can be applied to a
 package of prepared food, for example, comprises paperboard layer 8 and
 glassy, silicon-based polymeric layers 9 formed by a sol-gel process on
 the inner and outer surfaces of the tray. The weight of the paperboard
 layer is at least about 225 g/m.sup.2, and the weight of both glassy
 polymeric layers 9 is preferably about 2 to 5 g/m.sup.2. Polymeric layers
 9 render the tray water-tight and grease-tight and they withstand the
 conventional kitchen stove operating temperatures of 200 to 250.degree. C.
 without being damaged. The polymeric layer of the inner surface of the
 tray specifically prevents the food from sticking and the polymeric layer
 of the outer surface of the tray mainly protects the tray against the
 grease on the bake sheet and against the splatters coming from the food
 when heated. In some instances, the polymeric layer of the tray outer
 surface can be omitted. The illustrated tray 7 as such can also be used in
 microwave ovens.
 Milk container 10 which is illustrated in FIGS. 5 and 6 and which is mainly
 shaped as a rectangular prism is made entirely of coated, liquid-tight and
 gas-tight packaging board. The packaging board comprises a polymeric
 heat-sealing layer 11 on the outer surface of container 10, paperboard
 layer 12, a silicon-based, oxygen-tight and aroma-tight polymeric layer 13
 made by a sol-gel process and placed inside of the paperboard layer, and a
 heat-sealing layer 14 which constitutes the inner surface of the
 container. Heat-sealing layers 11, 14 of e.g. polyethylene at the joints
 of container 10 secure the overlapping paperboard layers tightly to each
 other. The weight of paperboard 12 of the container is at least about 225
 g/m.sup.2, the weight of the oxygen-tight and aroma-tight polymeric layer
 13 is preferably about 2 to 5 g/m.sup.2, and the weight of both
 heat-sealing layers 11, 14 is, for example, about 20 g/m.sup.2.
 The packaging board according to FIG. 6 which constitutes the wall of the
 container can be provided with an extra polymeric layer (not shown)
 between paperboard layer 12 and sol-gel layer 13 which possibly also
 contains pigments and fillers. Examples of preferred polymers include
 polyolefins and styrene acrylates. The said polymeric layer can be used to
 decrease the material thickness of sol-gel layer 13 because the polymeric
 surface is smoother and tighter than the paperboard layer.
 The invention and the polymeric coating materials it employs are described
 by the following application examples.
 EXAMPLE 1
 Barrier Coating
 182 g of 2.2-bis(4-hydroxyphenyl) propane is dissolved by mixing in 473 g
 of gamma-glycidyloxypropyltrimethoxysilane at room temperature. 24 g of
 0.1N hydrochloric acid is gradually added to this mixture, agitating it at
 the same time. Agitation is continued for about two hours, during which
 time about 20 g of colloidal silica is added. When needed, 1 g of a flow
 regulating agent is added. The solution thus prepared is usable for at
 least one month. 16 g of methyl imidazole (a Lewis acid) is added by
 mixing for about one hour before the solution is used. This solution is
 usable for about 24 hours.
 The coating was effected by using the rod coating method on the following
 paperboard grades:
 1. Pigment Coated SBS Paperboard
 Basis weight 235 g/m.sup.2
 Thickness 314 .mu.m
 2. Styrene Butadiene Dispersion Coated Paperboard
 3. Cup Board with Smooth Surface
 Basis weight 230 g/m.sup.2
 Thickness.about.300 .mu.m
 The coating was heat-cured in a furnace at 160.degree. C. for 2 minutes.
 Test Results
 The coating solution according to Example 1 was used in the tests conducted
 on paperboard grades 1, 2, and 3. The results indicate that the coating
 solution with this viscosity suited smooth and less porous paperboard
 grades the best (samples 1 and 2).
 When assessed visually, the coating is clear, transparent, and it has a
 good film forming ability. On the basis of an electron microscope study,
 the coating in samples 1 and 2 is whole and continuous. The coating in
 sample 3 is partly absorbed by the pores, causing holes.
 The physical properties of the coating are shown in Table 1.
 TABLE 1
 The test results of Example 1
 Penetration
 of water Pene- Resistance Resistance
 vapor tration to oil to temper-
 Paper- Thickness g/m.sup.2 /24 h, of oxygen and ature,
 board of coating 23.degree. C., 50% cm.sup.3 /m.sup.2 /24 grease, DSC
 25-
 grade .mu.m RH h, 23.degree. C. KIT-TEST 300.degree. C.
 1. Pigment 5 9 23 12 No
 SBS changes
 2. 4 3 30 12 No
 Dispersion changes
 coated
 3. Smooth 6 25 420 8 No
 cup board changes
 EXAMPLE 2
 The solution is prehydrolyzed as in Example 1.
 Instead of colloidal silica, small amounts of fine-grained talc, totalling
 180 g, are added by agitating continuously, 98% of the grain size of the
 talc being less than 10 .mu.m (Finntalc C10).
 After methyl imidazole had been added to the mixture, its viscosity was
 adjusted to suit the rod coating by adding about 7 g of colloidal silica
 to it.
 The coating solution was used to coat paperboard grades 1 and 3 according
 to Example 1. The coating was hardened and dried in the same conditions as
 in Example 1.
 Test Results
 When assessed visually, the coating is slightly matte and it has a good
 film forming ability.
 The physical properties of the coating are presented in Table 2.
 TABLE 2
 The test results of Example 2
 Pene- Resistance Resistance
 Penetration tration to oil to temper-
 Paper- Thickness of water of oxygen and ature
 board of coating vapor cm.sup.3 / grease DSC 25-
 grade .mu.m g/m.sup.2 /24 h m.sup.2 /24 h KIT-TEST 300.degree. C.
 1. Pigment 10 11 33 12 No
 SBS changes
 3. Smooth 12 9.8 29 12 No
 cup board changes
 EXAMPLE 3
 35.6 g of phenyltrimethoxy silane, 276.6 g of
 glycidyloxypropyltrimethoxysilane and 19.8 g of aminopropyltriethoxysilane
 were mixed in a vessel in an ice bath. 6 g of water was gradually added to
 this mixture by dropping and agitation in the ice bath was continued for
 15 minutes, whereupon 12 g of water was added in small amounts and the
 mixture was further agitated in the ice bath for 15 minutes. Then 97.2 g
 of water was added by dropping it faster and agitation was continued for
 two hours at room temperature. Then 43.6 g of epoxy resin (Dow Corning
 D.E.R. 330) was added to this hydrolyzate. Coating was carried out on
 paperboards 1 to 3 according to Example 1 by using the rod coating method.
 The coating was cured in a furnace at 160 .degree. C. for three minutes.
 TABLE 3
 The test results of Example 3
 Penetration
 of water Pene- Resistance Resistance
 vapor tration to oil to temper-
 Paper- Thickness g/m.sup.2 /24 h of oxygen and ature
 board of coating 23.degree. C., 50% cm.sup.3 /m.sup.2 /24 grease DSC
 25-
 grade .mu.m RH h 23.degree. C. KIT-TEST 300.degree. C.
 1. Pigment 4 10 25 12 No
 SBS changes
 2. Dis- 4 4 32 12 No
 persion- changes
 coated
 3. Smooth 6 12 35 12 No
 cup board changes
 EXAMPLE 4
 The solution was prehydrolyzed as in Example 3. 147 g of mica (Kemira Mica
 40) was added to the hydrolyzate. The coating solution was used to coat
 the paperboard grades 1, 2, and 3 according to Example 1. The coating was
 cured and dried as in Example 3.
 Test Results
 When examined visually, the coating is slightly matte and it has a good
 film forming ability. The physical properties of the coating are presented
 in Table 4.
 TABLE 4
 The test results of Example 4
 Penetration
 of water Pene- Resistance Resistance
 vapor tration to oil to temper-
 Paper- Thickness g/m.sup.2 /24 h, of oxygen and ature
 board of coating 23.degree. C., 50% cm.sup.3 /m.sup.2 /24 grease DSC
 25-
 grade .mu.m RH h 23.degree. C. KIT-TEST 300.degree. C.
 1. Pigment 5 8 20 12 No
 SBS changes
 2. 6 4 25 12 No
 Dispersion changes
 coated
 3. Smooth 6 10 30 12 No
 cup board changes
 It is clear to those skilled in the art that the different embodiments of
 the invention are not limited to the examples described above but can vary
 within the appended claims.