MXPA99007122A - Biaxially oriented polypropylene film that has more than one layer, its use and procedure for its production - Google Patents

Abstract

The present invention relates to a biaxially oriented polypropylene film having very good optical properties and good processing performance and which, after it has been metallized or has been coated with oxidic materials, has a high oxygen barrier value, and whose The structure has at least one base layer B and at least one layer A applied to this base layer, wherein this layer A has a specified number of specified height elevations and specified diameter, the invention further relates to the use of the film and a procedure for its production

Description

B1AX1ALLALLY ORIENTED POLYPROPYLENE FILM THAT HAS MORE THAN ONE LAYER, ITS USE AND PROCEDURE FOR ITS PRODUCTION DESCRIPTIVE MEMORY This invention relates to a biaxially oriented polypropylene film having good optical properties and which, after it has been metallized or has been coated with oxidic materials, has a high oxygen barrier value, and whose structure has at least one base layer B and at least one layer A applied to this base layer, wherein this layer A has a specified number of specified height elevations and specified diameter. The invention also relates to the use of the film and a process for its production. Biaxially oriented polypropylene films are used in the packaging sectors and in the industrial sectors, especially where there is a need for their advantageous properties, i.e., good optical properties, high mechanical strengths, good barrier action, in particular with respect to gases, good dimensional stability when heated, and excellent flat character. For most applications it is furthermore convenient, for example, for reasons of promotional effectiveness, to improve the optical properties of the polypropylene film, in particular its brightness and turbidity, while retaining its good processability. In the same way, it is convenient to improve the barrier properties of polypropylene films, for example, to achieve new applications. The prior art describes how the optical properties, in particular the brightness and turbidity, of the biaxially oriented polypropylene film can be improved. DE-A 43 06 155 discloses a transparent, non-sealable, oriented polyolefin film having more than one layer, with a base layer and with at least one non-sealable outer layer made of polypropylene, wherein the outer layer it comprises particles of silicon that have had a subsequent organic treatment and have an average particle size of 2 to 6 μm. The film's turbidity and processing properties are improved, but the text does not teach any improvement in the barrier properties of the film. Nor does it give any indication of how the topography of a film of this type should be adjusted for the simultaneous improvement of the brightness and the oxygen barrier. I films with oxidic materials (for example, SiOx or AlxOy) or with water glass. Essentially, the coatings used are transparent. The effectiveness of the barrier against the substances mentioned above depends essentially on the type of the polymers in the film and the quality of the barrier layers applied. Thus, a very high barrier efficiency against gases, such as oxygen and flavors, is achieved in biaxially oriented metallized polyester films. An effective barrier against water vapor is best achieved in biaxially oriented metallized polypropylene films. There are other applications in which a very effective water vapor barrier and an acceptable oxygen barrier are convenient. In the prior art, neither is there sufficient knowledge of the detailed bases for the barrier effect in metallized or oxidically coated polypropylene films, nor how this can be decisively improved. The variables that are clearly important are the surface of the substrate, the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties. The thesis of H. Utz (Technische Universität München, 1995: "Barriereeigenschaften aluminiumbedampfter Kunststoffolien" [Barrier properties of aluminum-metallized plastic films]), gives detailed results of investigations on the influence of the surface of the substrate on the barrier properties in several plastic films.

An object of the present invention was to provide a film of : biaxially oriented co-extruded polypropylene having very good optical properties, i.e. reduced haze and in particular high gloss, at least on one surface of the film (hereinafter referred to as the film's A surface). An A surface of this type would, for example, be particularly advantageous for printing or metallization. The high gloss of the film is transferred to the print or to the metal layer, and thus gives the packaging a particularly glossy appearance. After metallization or after coating with oxidic materials on the surface A, the film should also have an oxygen barrier value better than in the prior art, and should be easy to produce and process. In summary, the objective was to provide a film with the following combination of characteristics: high gloss, in particular on the surface A of the film - low turbidity, and low oxygen permeation of the film after metallization or after coating with oxidic materials on the surface A of the film. The brightness of the surface A of the film must be greater than 120, and the turbidity of the film must be less than 2.0. A low oxygen permeation means in the present case that less than 30 cm3 of oxygen per square meter per day must be diffused through the metallized film when it is exposed to air at a pressure of 1 bar. The other properties of the film should be at least equivalent to those of known packaging films of this type. They should be, for example, simple and effective in costs to produce, and be easy to process in conventional machinery (ie, without blocking, for example). The objective is achieved by a biaxially oriented coextruded polypropylene film having a base layer B, of which at least 80% by weight is formed of thermoplastic polypropylene, and having one or more other layers, wherein less one layer A facing outwards has a number of elevations / projections N per mm2 of surface area of the film, which is related to their respective heights h and diameters d by the following equations: log N / mm2 < Ah - Bh * log h / μrp, 0.05 μm < h < 1.0 μm (1) Ah = 1.4; Bh = 2.5 log N / mm2 < Ad-Bd * log h / μm, 0.2 μm < d < 10.0 μm (2) For the purposes of the present invention, the elevations / projections are conical elevations / protrusions projecting outwardly from the flat surface of the film. To achieve the high oxygen barrier values in metallized or oxidically coated films, as required by the objective, the number of elevations / protrusions N per mm 2 of surface area A of the film must be less than a particular numerical value, as the one given by equations (1) and (2). This numerical value is given only by the right sides of equations (1) and (2) as a function of the height h and the diameter d of the elevations / protrusions. The biaxially oriented polypropylene films described by the above equation have a relatively small number of elevations / protrusions in the A layer that will be metallized or oxidically coated. On the scale of h < 0.5 μm, particularly on the h < 0.4 μm, and very particularly on the scale of h < 0.3 μm, the number of elevations / protrusions is markedly less than that known from the prior art. If the number N of elevations per unit area in layer A of the film to be metallized or oxidically coated in greater than the right sides of equations (1) or (2), this implies greater oxygen permeation, which is inconvenient with respect to the objective of the present invention. In this case, moreover, the brightness of the surface of the film (in the non-metallized or uncoated state) is no longer high, and this is too inconvenient with respect to the object of the present invention. In a preferred embodiment of the novel movie, the constant Ah of the equation (1) mentioned above has the value of 1.18, and in a particularly preferred embodiment, it has the value of 1.0. In an equally preferred embodiment of the novel film, the constant Bh in equation (1) mentioned above has the value of 2.2, and in a particularly preferred embodiment has the value of 2.1. In a preferred embodiment of the novel film, the constant A in equation (2) mentioned above has the value of 3.0, and in a particularly preferred embodiment, has the value of 2.6. In an equally preferred embodiment, the constant Bd of equation (2) mentioned above has the value of 2.3, and in a particularly preferred embodiment has the value of 2.2. In the preferred and particularly preferred embodiments, the layer A according to the invention has extremely few N elevations / protrusions per unit area area. In this case, the metallized or oxidically coated film has particularly good oxygen barrier properties, and the values for brightness on this side are extremely high. The subclaims give preferred embodiments of the invention. These are also described below. According to the invention, the film has at least two layers. Their layers are then a layer B and layer A. In a preferred embodiment of the invention, the film has three layers and has layer A on one side of layer B (= base layer) and has, on the other side of layer B, another layer C made of polypropylene and comprising the pigments required to produce and process the film. In this case, the two layers A and C form the outer layers A and C.

In principle, several raw materials can be used as materials for the different layers. However, for the production of the individual layers, it is preferred that they be made from polypropylene raw materials. The base layer of the novel film having more than one layer comprises polyolefins, preferably propylene polymers, and other additives if desired, in effective amounts in each case. The base layer generally comprises at least 50% by weight of the propylene polymers, preferably from 75 to 100% by weight, in particular from 90 to 100% by weight, based in each case on the base layer. The propylene polymer generally contains from 90 to 100% by weight of propylene units, preferably from 95 to 100% by weight, in particular from 98 to 100% by weight, and generally has a melting point of 120 ° C or more. , preferably from 150 to 170 ° C, and generally has a melt flow rate of 0.5 to 8 g / 10 min, preferably 2 to 5 g / 10 min, at 230 ° C and with a force of 21.6N ( DIN 53 735). Preferred propylene polymers for the base layer are isotactic propylene homopolymers with an atactic ratio of 15% by weight or less, ethylene-propylene copolymers with an ethylene content of 10% by weight or less, propylene copolymers with a -C4-C8 olefins with an a-olefin content of 10% by weight or less, and terpolymers of propylene, ethylene and butylene with an ethylene content of 10% by weight or less and with a butylene content of 15% by weight or less, and particular preference is given to the isotactic propylene homopolymer. The percentages by weight given refer to the respective polymer. A mixture of the aforementioned homo- and / or copolymers and / or terpolymers of propylene and other polyolefins, in particular made of monomers having from 2 to 6 carbon atoms, is also suitable if the mixture comprises at least 50% by weight. weight of propylene polymer, in particular at least 75% by weight. Other poiiolefins which are suitable in the polymer mixture are polyethylenes, in particular HDPE, LDPE and LLDPE if the proportion of these polyolefins, based on the polymer mixture, does not exceed 15% by weight in each case. In a preferred embodiment of the novel film, the propylene polymer of the base layer is peroxidically degraded. A measure of the degree of degradation of the polymer is the degradation factor A, which gives the relative change in the molten material flow rate (in accordance with DIN 53 735) of the polypropylene, based on the starting polymer. A = MFI1 / MFI2 MFI1 = melt flow rate of the propylene polymer before adding the organic peroxide. MFI2 = melt flow rate of the peroxidically degraded propylene polymer. The degradation factor A of the propylene polymer used is generally in the range of 3 to 15, preferably from 6 to 10. Particularly preferred organic peroxides are dialkyl peroxides, where it is understood that the alkyl radicals are the usual straight or branched chain saturated lower alkyl radicals having up to 6 carbon atoms.

Particular preference is given to 2,5-dimethyl-2,5-di (tert-butylperoxy) hexane and di-tert-butyl peroxide. The base layer may generally comprise effective amounts in each case of neutralizing and stabilizing agents, and also, if desired, lubricants, antistatics and / or hydrocarbon resins. Preferred resins are in particular hydrocarbon resins. The hydrocarbon resins can be hydrogenated, to some degree or completely. The possible resins are in principle synthetic resins or resins from natural sources. It has proven to be particularly advantageous to use resins with a softening point higher than 80 ° C (measured in accordance with DIN 1995-U4 or ASTM E-28). Resins with a softening point of 100 to 180 ° C, in particular 120 to 160 ° C, are preferred. The resin is preferably incorporated in the film in the form of a masterbatch, which is added to the extruder (eg, single-worm or cascade extruder). Examples of customary master batches are those comprising from 30 to 70% by weight, preferably 50% by weight, of propylene homopolymer, and from 70 to 30% by weight, preferably 50% by weight, of hydrocarbon resin. The percentage weight data are based on the total weight of propylene polymer and hydrocarbon resin.

Among the numerous resins, preference is given to hydrocarbon resins and specifically to petroleum resins, styrene resins, cyclopentadiene resins and terpene resins. These resins are described in Ullmanns Enzy lopádie der techn. Chemie [Ullman's Encyclopedia of Industrial Chemistry], 4th edition, Vol. 12, pp. 525-555. Petroleum resins are the hydrocarbon resins that are prepared by pounding intensely decomposed petroleum materials in the presence of a catalyst. These petroleum materials usually comprise a mixture of resin-forming substances, such as styrene, methylstyrene, vinyltoluene, indene, methylindene, butadiene, isoprene, piperylene and pentylene. Styrene resins are styrene homopolymers or styrene copolymers with other monomers, such as methylstyrene, vinyltoluene and butadiene. The cyclopentadiene resins are cyclopentadiene homopolymers or cyclopentadiene copolymers, obtained from coal-tar distillates and fractionated gaseous petroleum. These resins are prepared by keeping the materials, which comprise cyclopentadiene, at a high temperature for a long period. Depending on the reaction temperature, dimers, trimers or oligomers can be obtained. Terpene resins are terpene polymers, that is, hydrocarbons of the formula C10Hi6 present in almost all essential oils and plant resins that contain oil, or are resins of terpenes modified with phenol. Specific examples of terpenes that may be mentioned are pinene, α-pinene, dipentene, limonene, myrcene, camphene, and the like. The hydrocarbon resins can also be those known as modified hydrocarbon resins. The modification generally occurs by reacting the raw materials prior to polymerization, introducing specific monomers, or by reacting the polymerized product, in particular for hydrogenation or partial hydrogenation reactions. Other hydrocarbon resins used are styrene homopolymers, styrene copolymers, cyclopentadiene homopolymers, cyclopentadiene copolymers and / or terpene polymers with a softening point greater than 100 ° C in each case. In the case of unsaturated polymers, the hydrogenated product is preferred. Particular preference is given to the use of cyclopentadiene polymers with a softening point of 140 ° C and more in the base layer. The novel polyolefin film comprises at least one outer layer A, which has been applied to the base layer B mentioned above. For this outer layer, use may be made in principle of the polymers used for the base layer. Other materials in addition to these may also be present in the outer layer, in which case the outer layer is then preferably composed of a mixture of polymers, a co-polymer or a homopolymer. The outer layer A according to the invention is generally composed of α-olefins having from 2 to 10 carbon atoms.

The outer layer generally comprises a propylene homopolymer or a copolymer of ethylene and propylene, or ethylene and butylene, or propylene and butylene, or ethylene and another α-olefin having from 5 to 10 carbon atoms, or propylene and other a- oiefin having 5 to 10 carbon atoms, or a terpolymer of ethylene and propylene and butylene, or ethylene and propylene and another α-olefin having 5 to 10 carbon atoms, or a mixture made of two or more of the homo-, co- and terpolymers mentioned, or a mixture made of two or more of the mentioned homo-, co- and terpolymers, if desired mixed with one or more of said homo-, co- and terpolymers. In particular, the outer layer preferably comprises essentially: a propylene homopolymer or a copolymer of ethylene and propylene, or ethylene and 1-butylene, or propylene and 1-butylene, or a terpolymer of ethylene and propylene and 1-butylene, or a mixture made of two or more of the aforementioned homo-, co- and terpolymers, or a mixture made of two or more of the mentioned homo-, co- and terpolymers, if desired mixed with one or more of the homo-, co- and terpolymers mentioned, and preference is given in particular to propylene homopolymers or random copolymers of ethylene-propylene with an ethylene content of 1 to 10% by weight, preferably 2 to 8% by weight, or random copolymers of propylene -1-butylene with a butylene content of 4 to 25% by weight, preferably 10 to 20% by weight, based in each case on the total weight of the copolymer, or random terpolymers of ethylene-propylene-1-butylene with an ethylene content of 1 to 10% by weight, preferably from 2 to 6% by weight, and a content of 1-butylene from 3 to 20% by weight, preferably from 8 to 10% by weight, based in each case on the total weight of the terpolymer, or a mixture made of an ethylene-propylene-1-butylene terpolymer and a propylene-1-butylene copolymer with an ethylene content of 0.1 to 7% by weight and a propylene content of 50 to 90% by weight and a content of 1-butylene from 10 to 40% by weight, based in each case on the total weight of the polymer mixture. The propylene homopolymer used in the outer layer comprises predominantly (at least 90%) propylene, and has a melting point of 140 ° C or more, preferably 150 to 170 ° C. Preference is given to isotactic homopolypropylene with a soluble fraction of n-heptane of 6% by weight or less, based on isotactic homopolypropylene. The homopolymer of component I or the homopolymer present therein generally has a melt flow rate of 0.5 to 15 g / 10 min, preferably 2.0 to 10 g / 10 min. The copolymers used in the outer layer and described above generally have a melt flow rate of 2 to 20 g / 10 min, preferably 4 to 15 g / 10 min. The melting point is on the scale of 120 to 140 ° C. The terpolymers used in the outer layer have a molten material flow index in the range of 2 to 20 g / 10 min, preferably 4 to 15 g / 10 min, and a melting point on the scale of 120 to 140 °. C. The mixture made of co- and terpoiomers and described above, has a melt flow rate of 5 to 9 g / 10 min and a melting point of 120 to 150 ° C. All the melt flow rates given above are measured at 230 ° C and with a load of 21.6 N (DIN 53 735). If desired, the polymers of the outer layer could have been peroxidically degraded in the manner described above for the base layer, and in principle the same peroxides are used. The degradation factor for the polymers of the outer layer is generally in the range of 3 to 15, preferably 6 to 10. If desired, hydrocarbon resins can also be added to the outer layers in the manner described above for the layer of base. The outer layers generally comprise from 1 to 40% by weight of resin, in particular from 2 to 30% by weight, preferably from 3 to 20% by weight.

The embodiments with resin-containing outer layers are particularly advantageous with respect to their optical properties, such as gloss and transparency, and with respect to their water vapor and oxygen barrier properties. The outer layers containing resin should generally comprise anti-blocking agents to ensure easy passage through the machinery. The novel film having more than one layer comprises at least the base layer described above, and at least one outer layer. Depending on its desired application, the film may have another outer layer C on the side of the base layer opposite the outer layer A. If desired, one or more intermediate layers may also be applied between the base and outer layers.

The preferred embodiments of the film have three layers. The structure, thickness and composition of a second outer layer can be selected independently of the outer layer already present. The second outer layer may likewise comprise one of the polymers or polymer blends described above, but this does not have to be the same layer as that of the first outer layer. However, the second outer layer may also comprise other polymers that are commonly used for an outer layer. The thickness of the outer layers is greater than 0.1 μm, preferably in the range of 0.2 to 5 μm, in particular 0.4 to 3 μm, and if there are outer layers on both sides, their thickness may be identical or different. The total thickness of the novel polyolefin film having more than one layer can vary within wide limits, and depends on the desired application. It is preferably from 3 to 100 μm, in particular from 8 to 60 μm, the base layer comprising approximately 50 to 96% of the total thickness of the film. The density of the film is generally 0.9 g / cm 2 or more, preferably from 0.9 to 0.97 g / cm 3. To improve the adhesive properties of the outer layers, at least one surface of the film can be corona treated or flame treated. If desired, identical or different treatments of this type can be carried out on both surfaces.

To further improve certain properties of the novel polyolefin film, the base layer or outer layers may comprise an amount effective in each case of other additives, preferably antistatic and / or antiblocking agents and / or lubricants and / or stabilizers and / or neutralizing agents, compatible with the propylene polymers of the base layer and of the outer layers, except the antibioqueo agents, which are generally incompatible. All amounts given in percent by weight (% by weight) in the following description are based on the respective layer or layers to which the additive may have been added. Preferred antistats are alkali metal alkan sulfonates, polydiorganosiloxanes (ie, ethoxylated and / or propoxylated) modified with polyether (polydialkylsiloxanes, polyalkylphenylsiloxanes, and the like), and / or essentially straight-chain aliphatic tertiary amines saturated with an aliphatic radical which has from 10 to 20 carbon atoms with substitution by hydroxyalkylene groups of C1-C4. Particularly suitable are N, N-bis (2-hydroxyethyl) alkylamines having from 10 to 20 carbon atoms, preferably from 12 to 18 carbon atoms, in the alkyl radical. The effective amounts of antistatics are in the range of 0.005 to 0.5% by weight, and a preferred antistatic is 0.005 to 0.5% by weight of glycerol monostearate. To achieve good barrier values and good metal adhesion, it is useful to keep the antistatic ratio low if possible, or even to completely fill with antistatic.

Suitable antiblocking agents are inorganic additives, such as silica, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate, and the like, and / or incompatible organic polymers, such as polyamides, polyesters, polycarbonates, and the like. Preference is given to benzoguanamide-formaldehyde, silica and calcium carbonate polymers. The effective amounts of antiblock agent are in the range of 0.001 to 2% by weight, preferably 0.01 to 0.8% by weight. The average particle size is from 1 to 6 μm, in particular from 2 to 5 μm. The globular particles, such as those described in EP-A-0 236 945 and DE-A-38 01 535, are particularly suitable. Anti-blocking agents are preferably added to the outer layers. To comply with equation (1), little or no inert pigment filler is present in layer A according to the invention. The concentration of inert particles in layer A is from 0 to 0.08% by weight, preferably from 0 to 0.065% by weight, in particular from 0 to 0.05% by weight, and very particularly from 0 to 0.04% by weight, and depends essentially the size of the particles used. The preferred particles are SiO 2 in colloidal and chain form. In principle, there is no restriction on the diameters of the particles used. However, to achieve the objective, it has been shown to be useful to use particles with an average primary particle diameter less than 60 nm, preferably less than 55 nm, and particularly preferably less than 50 nm, and / or particles with a diameter of average primary particle greater than 1 μm, preferably greater than 1.5 μm, and particularly preferably greater than 2 μm. The lubricants are amides of higher aliphatic acids, esters of higher aliphatic acids, waxes and metal soaps, and also polydimethylsiloxanes. The effective amounts of the lubricants are in the range of 0.001 to 3% by weight, preferably 0.002 to 1% by weight. Particularly suitable is the addition of higher aliphatic acid amides in the range from 0.001 to 0.25% by weight in the base layer and / or in the outer layers. A particularly suitable amide of an aliphatic acid is erucamide. To achieve good barrier values and good metal adhesion, it is useful if the lubricants can be substantially supplied. The stabilizers that can be used are the usual stabilizer compounds for polymers of ethylene, polymers of propylene and other polymers of alpha-olefins. The amounts of these added polymers are from 0.05 to 2% by weight. Particularly suitable are phenolic stabilizers, alkali metal / alkaline earth metal stearates and / or alkali metal / alkaline earth metal carbonates. Phenolic stabilizers are preferred in amounts of 0.1 to 0.6% by weight, in particular 0.15 to 0.3% by weight, and with molar masses greater than 500 g / mol. Particularly advantageous are pentaerythritol tetrakis-3- (3,5-di-tert-butyl-4-hydroxy-phenyl) propionate or 1, 3,5-trimethyl-2,4,6-tris (3,5- di-tert-butyl-4-hydroxybenzyl) benzene. The neutralizing agents are preferably calcium stearate and / or calcium carbonate of average particle size not greater than 0.7 μm, absolute particle size less than 10 μm and specific surface area not less than 40 m2 / g. In the particularly preferred embodiment, the novel polypropylene film has a three layer structure, and thus also comprises a layer C. The two layers A and C then form the outer layers A and C. The structure, thickness and composition of the second layer outer layer C can be selected independently of the outer layer A already present. The second outer layer may likewise comprise the polymers or polymer blends mentioned above, but these do not have to be the same as those of the first outer layer. This second outer layer generally comprises more pigments (ie, higher concentrations of pigments) than the first outer layer A according to the invention. The concentration of pigment in this second outer layer is from 0.02 to 1.0%, advantageously from 0.025 to 0.8%, in particular from 0.03 to 0.6%, and very particularly preferably from 0.035 to 0.5% depending, for example, on the desired processing behavior of the film. It is preferred to select the pigment types, the pigment concentrations and the particle concentrations, and also the thickness ratios of the layer, in order to give good optical properties and easy production and processing of the film. It has been shown that it is useful to describe the ease of production and processing of the film using the following parameters for the C side: a) average roughness, Ra, cb) coefficient of static friction / sliding, μc, on this side with respect to itself, and c) number of elevations / protrusions, Nc / mm2. It is advantageous if the film is structured in such a way that this surface C is opposite to the outer layer according to the invention (or on B in the case of a two-layer film). a) the value of Ra is from 20 to 300 nm b) the coefficient of static friction / slippage μc of this layer with respect to itself is less than 0.5, and c) the number of elevations / protrusions Nc / mm2 is expressed by The equations: Ah2-Bh2 * log h / μm <; Nc / mm2 < Ah3-Bh3 * log h / μm (3) 0.05 μm < h < 1.0 μm Ah2 = -1,000 Bh2 = 3.70 Ah3 = 2.477 Bh3 = 2.22 A 2-Bd2 * log d / μm < Nc / mm2 < Ad3-Bd3 * log d / μm (4) 0.2 μm < d < 10 μm Ad2 = -1,700 Bd2 = 3.86 Ad3 = 4,700 Bd3 = 2.70. In a preferred embodiment, Ra is from 30 to 250 nm, in particular from 35 to 200 nm. In a preferred embodiment, the coefficient of static friction / slippage μc of this layer with respect to itself is less than 0.45, and in particular less than 0.40. In a preferred embodiment, the constants Ah2 to Bh3 in equation (3) have the values of Ah2 = -0.523, Bh2 = 3.523, Ah3 = 2.300 and Bh3 = 2.3, and in a particularly preferred embodiment, the values are Ah2 = 0.00, Bh2 = 3.300, Ah3 = 2.000 and Bh3 = 2.400, and very particularly preferably Ah2 = 1.420, Bh2 = 2.500, Ah3 = 2.000 and Bh3 = 3.000. In a preferred embodiment, the constants A 2 to Bd3 in equation (4) have the values of A 2 = 2.00, Bd2 = 3,630, Ad3 = 4.40 and Bd3 = 2. 70, and in a particularly preferred embodiment, the values are Ad2 = 2,400, Bd2 = 3,720, Ad3 = 4,000 and Bd3 = 2,600, and very particularly preferably A 2 = 3,400, Bd2 = 2,400, Ad3 = 4,000 and Bd3 = 3,300. If desired, there may also be an intermediate layer between the base layer and the outer layers. Again, this layer may be composed of the polymers described for the base layers. In a particularly preferred embodiment, it is composed of the polypropylene used for the base layer. It can also comprise the usual additives described. The thickness of the intermediate layer is generally greater than 0.3 μm, preferably from 0.5 to 15 μm, in particular from 1.0 to 10 μm, and very particularly preferably from 1.0 to 5 μm. In the particularly advantageous three-layer mode of the novel film, the thickness of the outer layers A (and C) is generally greater than 0.1 μm, preferably from 0.2 to 3.0 μm, advantageously from 0.2 to 2.5 μm, in particular from 0.3 to 2 μm, and very particularly preferably from 0.3 to 1.5 μm; and the outer layers A and C may have identical or different thicknesses. The invention further relates to a process for producing the novel film having more than one layer by coextrusion processes known per se. For the purposes of this method, the method is to coextrude, through a die for flat film, the melted materials corresponding to the individual layers of the film, to extract the resulting film on one or more rollers to solidify it, and then stretch (orient) the film biaxially, to heat-fix the biaxially stretched film and, if desired, to corona treat the desired surface layer for corona treatment. The biaxial stretching (orientation) is generally carried out in sequence, and preference is given to the biaxial stretching sequence! which begins with longitudinal stretching (in the machine direction), followed by transverse stretching (perpendicular to the machine direction).

The polymer or polymer mixture for the individual layers are generally firstly compressed and plasticized in an extruder as in the co-extrusion process, wherein the polymer or polymer mixture can this time comprise any of the additives added. In particular, the resins are preferably added in the form of a master batch.

The molten materials are then extruded simultaneously through a die for flat film, and the coextruded film is then extracted on one or more extraction rollers, after which it is cooled and solidified. The resulting film is then stretched longitudinally and transversely in the extrusion direction, and this causes the orientation of the molecular chains. The longitudinal stretching is usually carried out with the help of two rollers running at different speeds, corresponding to the desired stretch ratio, and the transverse stretch with the help of an appropriate tension frame. The longitudinal stretching ratios according to the invention are from 4.0 to 9, preferably from 4.5 to 8.0. The cross stretch ratios should then be selected accordingly, preferably giving a scale of 5.0 to 11.0. The biaxial stretching of the film is followed by its heat setting (heat treatment), wherein the film is maintained at a temperature of 100 to 160 ° C for about 0.1 to 10 sec. The film is then rolled up in the usual way using a winding equipment.

It has been found to be particularly useful that the extraction rollers which cool and solidify the extruded film are maintained at a temperature of 10 to 100 ° C, preferably 20 to 70 ° C, by means of a heating and cooling circuit. The temperatures at which the longitudinal and transverse stretching is carried out can vary on a relatively wide scale, and it depends on the composition of the mixture of the base layer and, respectively, the mixture of the outer layer, and the desired properties of the movie. In general, the longitudinal stretch is preferably carried out at 80 to 150 ° C, and the transverse stretch preferably at 120 to 170 ° C. As mentioned above, one or both surfaces of the film can be corona treated or flame treated by one of the known methods after performing the biaxial stretching, if desired. The intensity of the treatment is generally in the range from 37 to 50 mN / m, preferably from 38 to 45 mN / m. A useful procedure for corona treatment is to pass the film between two conductors that function as electrodes, where the applied voltage between the electrodes, mainly an alternating voltage (of approximately 5 to 20 kV and from 5 to 30 kHz), is high enough to allow corona discharge to occur. The corona discharge ionizes the air above the surface of the film, and it reacts with the molecules on the surface of the film in such a way that it produces polar inclusions in the essentially non-polar polymer matrix. For flame treatment with a polarized flame (see for example US-A-4, 622, 237), a direct voltage is applied between a burner (negative pole) and a cooling roller. The magnitude of the applied voltage is from 400 to 3000 V, preferably from 500 to 2000 V. The applied voltage increases the acceleration of the ionized atoms and the kinetic energy with which they impact on the surface of! polymer. It becomes easier to break the chemical bonds within the polymer molecule, and the formation of free radicals proceeds more quickly. In the case of metallization of the film on the layer A according to the invention, the metal layer is preferably composed of aluminum. However, other materials that can be applied as a thin and coherent layer are also suitable. A particular example of a suitable material is silicon, which, unlike aluminum, gives a transparent barrier layer. The oxidic layer is preferably composed of oxides of the elements of the second, third or fourth main groups of the periodic table, in particular magnesium, aluminum or silicon oxides. The metallic or oxidic materials used are generally those that can be applied under reduced pressure or in vacuum. The thickness of the applied layer is generally from 10 to 100 nm. An advantage of the invention is that the production costs of the novel film are comparable to those of the prior art. The other properties of the novel film relevant to processing and use are essentially unchanged or even improved. In addition, the recirculated material can always be reused to produce the film at concentrations of 20 to 50% by weight, based on the total weight of the film, without any significant adverse effect on the physical properties of the film. The film is highly suitable for packing food products and other consumable items that can be damaged by light and / or by air. In summary, the novel film has high gloss, in particular high gloss on the surface A of the film, and low turbidity. The film also has an excellent oxygen barrier value once the surface A of the film has been metallized or has been coated with oxidic materials. In addition, it has good winding and processing performance. The brightness of the surface A of the film is greater than 120. In a preferred embodiment, the brightness of this side is greater than 125, and in a particularly preferred embodiment, it is greater than 130. This surface of the film is therefore particularly suitable for printing or for metallization. The high brightness of the film is transferred to the print or to the applied metal layer, and thus gives the film the desired appearance, effective for promotional purposes. The turbidity of the film is less than 3.0. In a preferred embodiment, the turbidity of the film is less than 2.5, and in a particularly preferred embodiment, it is less than 2.0.

Once it has been metallized on its surface A, the film has an oxygen barrier value less than 30 cm3 m "2 d" 1 barias "1, preferably less than 25 cm3 m" 2 d "1 barias-1, and particularly preferably less than 20 cm3 m "2 d" 1 barias "1. The coefficient of friction on the side opposite to side A is less than 0.5. In a preferred embodiment, this coefficient of friction of the film is less than 0.45, and in a particularly preferred embodiment, it is less than 0.4. Table 1 below shows once again the most important properties of the films according to the invention.

TABLE 1 Measurement in the non-metallized film The following methods were used to determine the parameters for raw materials and films: (1) Optical density The TD-904 Macbeth densitometer from Macbeth (Division of Kollmorgen Instruments Corp.) was used to measure the optical density. The optical density is defined as DO = -Ig l / lo, where I is the intensity of the incident light, i0 is the intensity of the transmitted light, and i / lo is the transmittance. (2) Oxygen barrier The oxygen barrier of the metallized films was measured using an OX-TRAN 2/20 from Mocon Modern Controls (E.U.A.) in accordance with DIN 53 380, Part 3. (3) Coefficient of friction The coefficient of friction was determined in accordance with DIN 53 375, the coefficient of sliding friction being measured 14 days after production. (4) Surface tension The surface tension was determined using the "ink method" (DIN 53 364). (5) Turbidity The turbidity of the film was measured in accordance with ASTM-D 1003-52. Holz turbidity was determined by a method based on ASTM-D 1003-52, but to use the most effective measurement, scale measurements were made on four pieces of film deposited one on top of the other, and a groove diaphragm was used. 1st instead of a hole for a 4th pin. (6) Brightness Brightness was measured in accordance with DIN 67 530. Reflectance was measured as a characteristic optical value for the surface of the film.

Based on standards ASTM-D 523-78 and ISO 2813, the angle of incidence was adjusted to 20 ° or 60 °. A beam of light hits the flat test surface at the established angle of incidence, and is thus reflected and / or scattered.

A proportional electrical variable is shown that represents the light beams that strike the photoelectric detector. The measured value lacks dimensions, and must be indicated along with the angle of incidence. (7) Determination of particle sizes on the surfaces of the film A scanning electron microscope was used (e.g., DSM 982 Gemini, Leo GmbH (Zeiss)) together with an image analysis system to determine the particle size distribution of the antiblocking agent (particle size distribution) on the surfaces of the film. The increase selected in all cases was 1700. For these measurements, film specimens are placed flat on a specimen holder. These are then metallized obliquely at an angle a with a thin metallic layer (for example, silver). Here, a is the angle between the surface of the specimen and the direction of diffusion of the metal vapor. The particles of the antiblocking agent cast a shadow on this oblique metallization. Since the shadows are not electrically conductive at this stage, the specimen can then be metallized with a second metal (eg, gold), the metal vapor impacting here vertically on the surface of the specimen.

Scanning electron microscope (SEM) images are taken of the specimen surfaces prepared in this way. The shadows of the anti-blocking agent particles are visible due to the contrast between the materials. The specimen is oriented in the SEM, so that the shadows run parallel to the lower edge of the image (x direction). SEM images are taken with this instrument, and they are transferred to an image analysis system, which is used to make precise measurements of the lengths of the shadows (in the x direction) and their maximum extension in the y direction (parallel to the vertical edge of the image). The diameter D of the particles of the antiblocking agent at the surface level of the specimen is equal to the maximum extension of the shadows d in the y-direction. The height of the particles of the antibioqueo agent, measured from the surface of the film, can be calculated from the angle of metallization and the length L of the shadows, given the knowledge of the increase V selected for the SEM image: h = (tan (a) * L) / VA In order to achieve a sufficiently high level of statistical reliability, precise measurements are made on the few thousand particles of the antiblocking agent. Using known statistical methods, frequency distributions are then produced for the diameters and heights of the particles. The class range selected for this is 0.2 μm for the diameter D of the particles, and 0.05 μm for the height h of the particles. (8) Roughness The roughness Ra of the film was measured in accordance with DIN 4768 with a cut of 0.25 mm. (9) melt flow rate The melt flow rate measurement was based on DIN 53 735, with a load of 21.6 N at 230 ° C and, respectively, 50 N at 190 ° C. (10) Melting point DSC measurement, melting curve maximum, heating rate of 20 ° C per minute.

EXAMPLE 1 Co-extrusion was used, followed by gradual orientation in longitudinal and transverse directions, to produce a transparent three-layer film with ABC structure and a total thickness of 20 μm. The thickness of each layer is given in table 2. The outer layer A was a mixture made of: 94.0% by weight of isotactic polypropylene with an MFl of 4g / 10 min, and 6.00% by weight of a master batch made of 99.0 % by weight of polypropylene with an MFl of 4 g / 10 min, 0.5% by weight of Sylobloc 44 (Grace colloidal S1O2) and 0.5% by weight of Aerosil TT 600 (SiO2 in chain form, from Degussa). Base layer B: 99.8% by weight of polypropylene with an MFl of 4 g / 10 min, and 0.2% by weight of N, N-bisetoxyalkylamine. The outer layer C was a mixture made of: 84.0% by weight of isotactic polypropylene with an MFl of 4g / 10 min, and 16.00% by weight of a master batch made of 99.0% by weight of polypropylene with an MFl of 4 g / 10 min, 0.5% by weight of Sylobloc 44 (Grace's colloidal SiO2) and 0.5% by weight of Aerosil TT 600 (S¡O2 in chain form, from Degussa). The processing conditions in the individual steps were: Extrusion Temperatures Layer A 270 ° C Layer B 270 ° C Layer C 270 ° C Die space width 1 mm Extraction roller temperature 30 ° C Longitudinal stretching Temperature 80-140 ° C C Longitudinal stretching ratio 5.0 Transverse stretch Temperature 160 ° C Transverse stretch ratio 10.0 Setting Temperature 150 ° C Duration 2 sec The film has very good optical properties and good processing performance (see table 3). After the film was produced (as in this example and in all the following examples), its side A was metallized with vacuum aluminum in an industrial metallizer. The coating speed was 8 m per second, and the optical density was 2.6. The film had the high oxygen barrier value required. The structure of the film and the properties achieved with films produced in this way are given in tables 2 and 3.

EXAMPLE 2 Co-extrusion was used, followed by gradual orientation in longitudinal and transverse directions, to produce a transparent three-layer film with ABC structure and a total thickness of 20 μm, in a manner similar to that of Example 1. Compared to Example 1, the only change was the outer layer A. The outer layer A was a mixture made of: 98.0% by weight of isotactic polypropylene with an MFl of 4g / 10 min, and 2.00% by weight of a master batch made of 99.0% by weight of polypropylene with an MFl of 4 g / 10 min, 0.5% by weight of Sylobloc 44 H (Grace colloidal SIO 2) and 0.5% by weight of Aerosil TT 600 (S1O2 in chain form, from Degussa). The processing conditions selected for all the layers were as in Example 1.

EXAMPLE 3 Co-extrusion was used, followed by gradual orientation in longitudinal and transverse directions, to produce a transparent three-layer film with ABC structure and a total thickness of 20 μm, in a similar fashion to that of Example 2. Compared to Example 2, the only change was the outer layer A. The outer layer A was a mixture made of: 88.0% by weight of isotactic polypropylene with an MFl of 4g / 10 min, and 10.0% by weight of hydrocarbon resin (ORegalrez 1 139 of Hercules Inc.) with a softening point of 140 ° C and a molecular weight of 2500, and 2. 00% by weight of a masterbatch made of 99.0% by weight of polypropylene with an MFl of 4 g / 10 min, 0.5% by weight of Sylobloc 44 H (SiO2 colloidal of Grace) and 0.5% by weight of Aerosii TT 600 (Si02 in the form of a chain, Degussa). The processing conditions selected for all the layers were as in Example 1.

EXAMPLE 4 Co-extrusion was used, followed by gradual orientation in longitudinal and transverse directions, to produce a transparent three-layer film with ABC structure and a total thickness of 20 μm, in a manner similar to that of example 2. Compared to example 2, the uraco change was the base layer B. The base layer B was a mixture made of: 89.8% by weight of polypropylene with an MFl of 4 g / 10 min, 0.2% by weight of N, N-bisetoxyalkylamine and 10.0% by weight of hydrocarbon resin (®Regalrez 1139 from Hercules Inc.) with a softening point of 140 ° C and a molecular weight of 2500.

TABLE 2 ? ? TABLE 3 ? l} Measured (a) in the non-metallized film Side A: metallized outer layer C side: non-metallic.

Claims (14)

NOVELTY OF THE INVENTION CLAIMS

1. - A biaxially oriented co-extruded polypropylene film having a base layer B, of which at least 80% by weight * is formed of thermoplastic polypropylene, and having one or more other layers, wherein at least one layer The outside facing has a number of elevations / protrusions N per mm2 of surface area of the film, which is related to their respective heights and diameters d by the following equations: log N / mm2 < Ah - Bh * log h / μm, 0.05 μm < h < 1.0 μm (1) Ah = 1.4; Bn = 2.5 log N / mm2 < Ad-Bd * log h / μm, 0.2 μm < d < 10.0 μm (2) Ad = 3.4; Bd = 2.4.

2. The polypropylene film according to claim 1, further characterized in that layer A comprises less than 0.1% of an inert filler.

3. The polypropylene film according to one or more of claims 1 and 2, further characterized in that the thickness of the layer A facing outwards is 0.1 to 3.0 μm.

4. The polypropylene film according to one or more of claims 1 to 3, further characterized in that the film has a two-layer structure and is composed of the base layer B and the outer layer A.

5.- The polypropylene film according to one or more of claims 1 to 4, further characterized in that the film has a 3-layer structure and is composed of an outer layer A facing outwards, the base layer B and a second outer layer C, which has been applied to the base layer B on its side facing from the outer layer A.

6. The polypropylene film according to claim 5, further characterized in that the outer layer C has been pigmented.

7. The polypropylene film according to one or more of claims 1 to 6, further characterized in that the outer layer A has been pigmented.

8. The polypropylene film according to one or more of claims 1 to 7, further characterized in that at least one outward facing outer layer has been corona treated.

9. The polypropylene film according to any of claims 1 to 8, further characterized in that at least one surface has been metallized.

10. The polypropylene film according to claim 9, further characterized in that the oxygen barrier value of the metallized film is < 30 cm3 / m2 barias d.

11. - The polypropylene film according to claim 9, further characterized in that the oxygen barrier value of the metallized film is < 25 cm3 / m2 barias d.

12. The process for producing a biaxially oriented polypropylene film having more than one layer according to claim 1, characterized in that the polypropylene melts corresponding to the compositions of the outer and base layers are fed to a die. of coextrusion and co-extruded therefrom on a cooling roll, and the resulting pre-film is then biaxially oriented and thermofixed, wherein at least one outward facing outer layer has a number of elevations / protrusions N per mm 2 of area of surface of the film, which is related to their respective heights and diameters d by the following equations: log N / mm2 < Ah - Bh * log h / μm, 0.05 μm < h < 1.0 μm (1) Ah = 1.4; Bh = 2.5 log N / mm2 < Ad-Bd * log h / μm, 0.2 μm < d < 10.0 μm (2) Aa = 3.4; Bd = 2.4.

13. The process for producing a biaxially oriented polypropylene film having more than one layer according to claim 12, further characterized in that the recirculated material is fed to the extrusion die at a concentration of 10 to 50% by weight, with based on the total weight of the film.

14. - The use of the film according to one or more of claims 1 to 10, for packaging food products and other consumable articles.