MXPA01001592A - Multi-layer biaxially oriented polypropylene film having an improved barrier, a method for the production thereof, and the use thereof - Google Patents

Abstract

The invention relates to a multi-layer, sealable, biaxially oriented polypropylene film having improved barrier properties which is constructed of a base layer, of at least one sealable covering layer and of at least one intermediate layer. The intermediate layer contains a wax with an average molecular weight Mn ranging from 200 to 1200. The invention also relates to a method for producing the film and to the use thereof.

Description

MULTI-LAYER POLYPROPYLENE FILM, ORIENTED IN BIAXIAL FORM THAT HAS BARRIER PROPERTIES IMPROVED, PROCEDURE FOR PRODUCING AND USING THE SAME.

DESCRIPTIVE MEMORY The invention relates to a multilayer biaxially oriented polypropylene film comprising a base layer and at least one top layer that can be heat sealed and at least one intermediate layer in accordance with a layer structure. BZD, whose film comprises wax in its middle layer. The improvement in the barrier properties of films, in particular films that are used in the packaging sector, has become more important recently. For cost and environmental reasons, the packaging industry desires increasingly thin films having similar or improved barrier properties with respect, in particular, to the passage of water vapor. The barrier action of the BOPP films against water vapor (WVBA) and oxygen (OBA) decreases with the film thickness. In the normal thickness range of the BOPP films (from 4 to 100 μm) there is an approximately hyperbolic relationship between the water vapor barrier action (WVBA) and the thickness (d) (WVBA x d = constant). The Constant depends essentially on the composition of the raw material and the stretching conditions. For BOPP films that are used for packing according to the prior art, the constant has a value of approximately: const. = 28 gxmm / m2xd. Here the water vapor permeability has been measured in accordance with DIN 53.122. US-A-4,921, 749 (= EP-A-0247 898) discloses a BOPP film that can be heat sealed which has improved mechanical and optical properties. The heat sealing capacity of the film and the permeability to water vapor and oxygen are likewise improved. All improvements result from the addition of a low molecular weight resin to the base layer. In this case the resin content is between 3 and 30% by weight. The resin has a molecular weight significantly less than 5000, preferably less than 1000, and is, for example, 600. The softening point of the resin is 120 to 140 ° C. US 5,155,160 discloses improvements in barrier properties by the addition of wax to non-oriented polypropylene films. The waxes described are paraffin waxes and polyethylene waxes having molecular weights of 300 to 800. It is said that the barrier action is less than 0.2 g / 645 cm2 -24 hours. There is a continuing demand for further improvement in the vapor barrier action of biaxially oriented packaging films made from polypropylene. All the methods described to date can not reduce the barrier action against water vapor to the desired degree or impair other essential properties of the film in an unacceptable manner. Therefore, the object of the present invention is to provide a biaxially oriented polypropylene film which is distinguished by a good barrier action, in particular against water vapor, and which has good mechanical properties. It must be possible to produce the film with reliable operation and operation at production speeds of up to 400 m / min. Other physical properties of the film required in view of its use as a packaging film should not be adversely affected. The film should have a high brightness, should not have optical defects in the form of fish eyes or bubbles, should have a good resistance to scraping, be produced without fail in high speed packaging machines with low film thicknesses and, for transparent, it must have a low film turbidity. In addition, heat sealing properties should not be adversely affected. This object is achieved according to the invention by means of a multilayer polypropylene film of the generic type mentioned at the beginning, wherein the intermediate layer comprises a wax having an average molecular weight (number average) of 200 to 1200. In a preferred embodiment, the film consists of a base layer B, the intermediate layers Z applied on both sides of the same, and the upper layers D applied to the intermediate layers, ie a symmetrical structure of five layers DZBZD. In a also preferred embodiment, the The film consists of a base layer B, an intermediate layer Z applied to only one side thereof, and two upper layers D applied to the base layer and to the intermediate layer in accordance with DBZD. If desired, these basic structures comprising three, four or five layers may contain additional intermediate layers. The base layer of the film generally comprises at least 70 to 100% by weight, preferably from 75% to 98% by weight, in particular from 80 to 95% by weight, based in each case on the base layer, a propylene polymer described later. This propylene polymer comprises at least 90% by weight, preferably from 94 to 100% by weight, in particular from 98 to 100% by weight of propylene. The corresponding content of comonomer at most 10% by weight or from 0 to 6% by weight or from 0 to 2% by weight respectively, generally consists, if present, of ethylene. The data in percent by weight in each case refer to the propylene homopolymer. Of the aforementioned propylene polymers, isotactic propylene homopolymers are preferred for the base layer. In general, the propylene homopolymer has a melting point of 140 to 170 ° C, preferably from 150 to 165 ° C and a melt flow index (measurement DIN 53 735 at a load of 21.6 N and 230 ° C ) from 1.5 to 20 g / 10 minutes, preferably from 2 to 15 g / 10 minutes. The soluble content in n-heptane the polymer is generally from 1 to 6% by weight, based on the polymer. In a preferred embodiment of the invention, the propylene homopolymer used is quite isotactic. For such highly sotactic propylene homopolymers of this type, the sotactic index of the n-heptane insoluble content chain of polypropylene, determined by 13 C-NMR spectroscopy, is at least 95%, preferably from 96 up to 99%. In a further preferred embodiment of the film according to the invention, the propylene homopolymer of the base layer has been degraded by peroxide. A measure of the degree of degradation of the polymer is the so-called degradation factor A, which indicates the relative change in the melt flow index of the polypropylene, measured in accordance with DIN 53 735, based on the starting polymer. ? - M I2 MFI? MFI-F melt flow index of the propylene polymer before the addition of the organic peroxide. MFI2 = melt flow index of the propylene polymer degraded by peroxide.

In general, the degradation factor A of the propylene polymer used is in the range of 1.5 to 15, preferably from 1.5 to 10. Particularly preferred organic peroxides are dialkyl peroxides, in which the term alkyl radical is taken to refer to conventional saturated straight or branched chain lower alkyl radicals having up to six carbon atoms. Particular preference is given to 2,5-dimethyl-2,5-di (t-butylperoxy) hexane peroxide and di-t-butyl peroxide. In general, the base layer comprises conventional stabilizers and neutralizers in effective amounts in each case, and further, if desired, an antistatic and / or hydrocarbon resin. All the amounts in% by weight, indicated below, refer to the weight of the base layer. The stabilizers that can be used are the conventional stabilization compounds for polymers of ethylene, propylene and other olefins. Its aggregate amount is between 0.05 and 2% by weight. Particularly suitable are phenolic-type stabilizers, phosphitic stabilizers, alkaline / alkaline earth metal stearates and alkaline / alkaline earth metal carbonates. Preference is given to phenolic stabilizers in an amount from 0.1 to 0.6% by weight, in particular from 0.15 to 0.3% by weight, and having a molecular weight greater than 500 g / mol. Tetrakis-3- (3,5-di-) are particularly convenient. pentaerythrityl tert-butyl-4-hydroxyphenyl) propionate and 1, 3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl) benzene. The neutralizers are preferably dihydrotalcite, calcium stearate and / or calcium carbonate having an average particle size at most of 0.7 μm, an absolute particle size of less than 10 μm and a specific surface area of at least 40 m2 / g. In general, the film comprises from 0.02 to 2% by weight, preferably from 0.03 to 1% by weight, of neutralizer. Preferred antistatic agents are the aliphatic, straight-chain and saturated tertiary amines containing an aliphatic radical having from 10 to 20 carbon atoms, which is substituted by? -hydroxy-C1-C4 alkyl groups, of which particularly suitable are N, N-bis (2-hydroxyethyl) alkylamines having from 10 to 20 carbon atoms, preferably 12 to 18 carbon atoms in the alkyl radical. The monoesters of glycerol and aliphatic fatty acids are also suitable as antistatic agents, preference being given to fatty acid radicals having 10 to 20 carbon atoms. Glycerol monostearate is particularly preferred. The resin-modified embodiments contain the resin in an amount of 1 to 20% by weight, preferably from 1 to 12% by weight, in particular from 1 to 10% by weight, based on the weight of the base layer.

The hydrocarbon-type resins are low molecular weight polymers whose average molecular weight (weight average) is generally in the range of 300 to 8000, preferably 400 to 5000, more preferred 500 to 2000. The average molecular weight of the Resins are therefore significantly lower than that of the propylene polymers that form the main component of the individual film layers and generally have an average molecular weight greater than 100,000. Preferred resins are hydrocarbon resins which have been, if desired, partially or preferably completely hydrogenated. Suitable hydrocarbon resins are basically synthetic resins or resins of natural origin, which are generally partially or completely hydrogenated. The use of resins that have a softening point of > 80 ° C (measured in accordance with DIN 1995-U4 or ASTM-28), has proved to be particularly advantageous, preference being given to those having a softening point from 100 to 180 ° C, in particular from 120 to 160 ° C. Of the numerous resins, preference is given to hydrocarbon resins in the form of petroleum resins, styrene resins, cyclopentadiene resins and terpene resins (these resins are described in Ullrnanns Encyklopadie der techn. Chemie [Ullmann's Encyclopaedia of Indutrial Chemistry], 4th Edition, Volume 12, pages 525 to 555).

Petroleum resins are hydrocarbon resins prepared by polymerization of intensely degraded 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, sodium, 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 petroleum gas. These resins are prepared by keeping the materials containing cyclopentadiene at elevated temperature for a prolonged time. Depending on the reaction temperature, dimers, trimers or oligomers can be obtained. Terpene resins are polymers of terpenes, that is, hydrocarbons of the formula C10H-16, which are present in virtually all essential oils or resins that contain oil of vegetable origin, and terpene resins modified with phenol. Specific examples of terpenes that could be mentioned are pinene, a-pinene, dipentene, limonene, myrcene, camphene and similar terpenes. The hydrocarbon resins can also be referred to as modified hydrocarbon resins. The modification is effected, generally, by the reaction of the starting materials before the polymerization, by introducing specific monomers, or by reaction of the polymerized product, in particular by hydrogenation or by partial hydrogenation. The hydrocarbon resins used are also styrene homopolymers, styrene copolymers, cyclopentadiene homopolymers, cyclopentadiene copolymers and / or terpene polymers having in each case a softening point above 120 ° C (in the case of unsaturated polymers) , the hydrogenated product is preferred). Particular preference is given to the use of cyclopentadiene polymers having a softening point of at least 125 ° C, or copolymers of α-methylstyrene and vinyltoluene having a softening point of 110 to 160 ° C in the layer base. In a white or opaque or white / opaque film embodiment according to the invention, the base layer additionally comprises pigments and / or vacuole initiator particles. Such films have a transparency to light, measured in accordance with ASTM-D 1033-77, when much of 50%, preferably at most 70%. The pigments cover the particles that essentially do not result in the formation of vacuoles when the film is stretched. The staining action of the pigments is caused by the particles themselves. The term "pigments" is generally associated with a particle size in the range of 0.1 to a maximum of 1 μm and encompasses both the so-called "white pigments", which give a white color to the films, and the "pigments with color". The base layer generally comprises pigments in an amount of 1 to 25% by weight, preferably 2 to 15% by weight, based in each case on the base layer. Conventional pigments are materials such as, for example, aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates such as aluminum silicate (kaolin clay) and magnesium silicate (talc), dioxide of silicon and titanium dioxide, of which white pigments, such as calcium carbonate, silicon dioxide, titanium dioxide and barium sulfate are preferably used. The opaque embodiments of the films comprise vacuole initiator particles, which are not compatible with the polymer matrix and result in the formation of vacuole-like cavities when the films are stretched, depending on the size, nature and number of vacuoles the size of the films. solid particles and the stretching conditions, such as the stretch ratio and the stretching temperature. The vacuoles give the films a pearly opaque appearance caused by the scattering of light in the vacuole / polymer matrix. In general, the average particle diameter of the vacuole initiator particles is from 1 to 6 μm, preferably from 1.5 to 5 μm. The base layer generally comprises vacuole initiator particles in an amount of 1 to 25% by weight. The conventional vacuole initiator particles of the base layer are inorganic and / or organic materials, not compatible with polypropylene, such as aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates such as aluminum silicate (kaolin clay) and magnesium silicate (talc), silicon dioxide and titanium dioxide, of which calcium carbonate, silicon dioxide and titanium dioxide are preferably used. Suitable organic fillers are conventional polymers which are incompatible with the basecoat polymer, in particular those such as HDPE, polyesters, polystyrenes, polyamides and halogenated organic polymers, preference being given to polyesters, such as, for example, terephthalate polybutylene and polyethylene terephthalate. For purposes of the present invention, "incompatible materials or incompatible polymers" means that the material or polymer is present in the film in the form of a separate particle or as a separate phase. White / opaque films provided with vacuole initiator particles and with pigments, generally comprise the vacuole initiator particles in an amount of 1 to 10% by weight, preferably from 1 to 5% by weight, and pigments in an amount of 1 to 7% by weight, preferably from 1 to 5% by weight, weight. The density of the opaque or white films can vary within wide limits and depends on the nature and quantity of the fillers. The density is generally in the range of 0.4 to 1.1 g / cm3. The pigmented films have a density in the order of 0.9 g / cm3 or greater, preferably in the range of 0.9 to 1.1 g / cm3.

Films comprising only vacuole-initiating particles have a density less than 0.9 g / cm 3. For packaging films having a content of vacuole-initiator particles of 2 to 5% by weight, the density is in the range of 0.6 to 0.85 g / cm3. For films having a content of vacuole-initiating particles of 5 to 14% by weight, the density is in the range of 0.4 to 0.8 g / cm3. Films comprising pigments and vacuole-initiating particles have a density in the range of 0.5 to 0.85 g / cm 3, depending on the ratio of pigment content to content of vacuole-initiating particles. The polypropylene film according to the invention also comprises at least one intermediate layer of polymers made from olefins having from 2 to 10 carbon atoms applied to the base layer. Examples of olefinic polymers of this type are 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 of two or more of said homopolymers, copolymers and terpolymers or a combination of two or more of said homopolymers, copolymers and mixed terpolymers, if desired with one or more of said homopolymers, copolymers and terpolymers, in which particular preference is given to propylene homopolymer or random ethylene-propylene copolymers having an ethylene content of 1 to 10% by weight, preferably from 2.5 to 8% by weight, or random copolymers of propylene-1-butylene having a butylene content of 2 to 25% by weight, preferably 4 to 20% by weight, based in each case on the total weight of the copolymer or random ethylene-propylene-1-butylene terpolymers having an ethylene content of 1 to 10% by weight, preferably from 2 to 6% by weight, and a butylene content of 2 to 20% by weight, preferably 4 to 20% by weight, based in each case on the total weight of the terpolymer, or a combination of an ethylene terpolymer -propylene-1-butylene and a propylene-1-butylene copolymer having an ethylene content of 0.1 to 7% by weight, and a propylene content of 50 to 90% by weight, and a butylene content of 10 to 40% by weight, taking as a basis in each case the total weight of the polymer combination. The propylene homopolymer used in the intermediate layer comprises predominantly (at least 98%) propylene and has a melting point of 140 ° C or higher, preferably 150 to 170 ° C, preference being given to isotactic homopolypropylene which have a n-heptane soluble content of 6% by weight or less, based on isotactic homopolypropylene. The homopolymer generally has a melt flow index of 1.5 g / 10 minutes up to 20 g / 10 minutes, preferably from 2.0 g / 10 minutes up to 15 g / 10 minutes. In a preferred embodiment of the invention, the propylene homopolymer used in the intermediate layer is highly isotactic. For such highly isotactic propylene homopolymers of this type, the isotactic character index of the n-heptane insoluble content chain of polypropylene, determined by 13 C-NMR spectroscopy, is at least 95%, preferably from 96 to 99% . The copolymers and / or terpolymers described above used in the intermediate layer generally have a melt flow index of 1.5 to 30 g / 10 minutes, preferably 3 to 15 g / 10 minutes. The melting point is in the range of 120 to 140 ° C. The combination of copolymers and terpolymers described above has a melt flow index of 5 to 9 g / 10 minutes and a melting point of 120 to 150 ° C.

All the aforementioned melt flow rates are measured at 230 ° C and at a force of 21.6 N (DIN 53 735). If desired, all the intermediate layer polymers described above could have been degraded by peroxide in the same manner as described above for the base layer, using basically the same peroxides. The degradation factor for the polymers of the intermediate layer is generally in the range of 1.5 to 15, preferably from 1.5 to 10. In accordance with the invention, the film comprises in its intermediate layer a wax to improve the barrier action against the water vapor. It has been found that the desired barrier action is particularly effective if the amount of wax is not less than a certain minimum amount, based on the total weight of the film, this minimum amount being dependent, inter alia, on the thickness of the film. movie. Films having a total thickness of up to 25 μm should conveniently comprise at least 0.5% by weight of wax, based on the weight of the film. Films that have a total thickness of >25 to 60 μm should comprise, advantageously, at least 0.2% by weight of wax, based on the weight of the film. Films that have a total thickness of > 60 μm should comprise, advantageously, at least 0.15 by weight of wax, based on the weight of the film.

This convenient amount of wax that is selected as a function of the film thickness is, according to the invention, added to the intermediate layer or layers. Surprisingly, it is possible in this way to greatly reduce the absolute amount of wax in the film in order to achieve good barrier values. Therefore, it is avoided to damage other film properties. The minimum amounts of wax mentioned above, based on the weight of the film, can be established in the most effective range either through the corresponding concentration of wax in the intermediate layer or by a variation of the thickness of the intermediate layer for a certain concentration of wax. Therefore, both the wax concentration in the intermediate layer and the thickness of the intermediate layer can be varied over wide ranges, although these two parameters can not be completely selected independently of one another. It should therefore be ensured that the minimum desirable quantities of wax are obtained, based on the film. The intermediate layer generally comprises from 3 to 40% by weight, preferably from 5 to 30% by weight of wax, based on the weight of the intermediate layer, in which, as previously emphasized, the amount of wax should to be advantageously selected in such a way that the film - as described above - comprises in total the minimum amount of wax as a function of its total thickness.

The thickness of the intermediate layer is generally in the range of 0.2 to 10 μm, preferably in the range of 0.4 to 5 μm, in particular in the range of 0.5 to 3 μm, the thickness of the intermediate layer being selected in accordance with the criteria explained above. For the purposes of the present invention, the waxes comprise polyethylene waxes and / or paraffins (paraffin macrocrystalline and microcrystalline) having an average molecular weight (number average) of 200 to 1200. Polyethylene waxes are low weight polymers molecular that are essentially constituted from ethylene units and that are partially crystalline or completely crystalline. The polymer chains containing the ethylene units are stretched molecules, which may be branched, with predominance of relatively short side chains. In general, polyethylene waxes are prepared by direct polymerization of ethylene, using regulators if desired or by depolymerization of relatively high molecular weight polyethylene. According to the invention, polyethylene waxes have an average molecular weight Mn (number average) of 200 to 1200, preferably 400 to 600, and more preferably have a molecular weight distribution (polydispersity) Mw / Mn less of 2, preferably from 1 to 1.5. The melting point is generally in the range of 70 to 150 ° C, preferably 80 to 100 ° C.

The paraffins include macrocrystalline paraffins (paraffin waxes) and microcrystalline paraffins (microcrystals) having an average molecular weight (number average) of 200 to 1200. Macrocrystalline paraffins are obtained from fractions distilled under vacuum during the conversion of the same in lubricating oils. Microcrystalline paraffins originate from the residues of vacuum distillation and the sediments of paraffinic petroleum oils (deposition paraffins). The macrocrystalline paraffins consist predominantly of n-paraffins which additionally contain isoparaffins, naphthenes and alkylaromatic compounds, depending on the degree of refinement. Microcrystalline paraffins consist of a mixture of hydrocarbons which are predominantly solid at room temperature. Contrary to the case of macrocrystalline paraffins, isoparaffins and naphthenic paraffins predominate. The microcrystalline paraffins are distinguished by the presence of isoparaffins and highly branched naphthenes that inhibit crystallization. For the purposes of the invention, paraffins having a melting point of 60 to 100 ° C, preferably 60 to 85 ° C are particularly preferred. It has been found that waxes only develop the desired barrier enhancing action in the intermediate layer if the average molecular weight (number average) is in the range of 200 to 1200. Although waxes having a higher molecular weight improve the Sliding properties of the film, you are not having an effect on the film's barrier action. In addition to the wax, which is essential for the invention, the intermediate layer may comprise additional conventional additives, such as, for example, the neutralizers, stabilizers and antistatic agents described above for the basecoat and conventional lubricants, in effective amounts in each case. The lubricants are higher aliphatic acid amides, higher aliphatic acid esters and metal soaps, as well as silicone oils. The addition of higher aliphatic acid amides and silicone oils is particularly appropriate. The aliphatic acid amides are amides of a water-insoluble monocarboxylic acid having from 8 to 24 carbon atoms, preferably from 10 to 18 carbon atoms. Erucamide, stearamide and oleamide are preferred. Silicone oils are polydialkylsiloxanes appropriate, preferably polydimethylsiloxane, polymethylphenylsiloxane, olefin-modified silicone, polyether modified silicone, such as, for example., Polyethylene glycol and polypropylene, and modified silicone epoxiamino- and alcohol. The viscosity of the appropriate silicone oils is in the range of 5,000 to 1,000,000 mm 2 / s. The polydimethylsiloxane having a viscosity of 10,000 to 100,000 mm 2 / s is preferred. For the opaque embodiments of the invention, it should be mentioned that the intermediate layer must not comprise any of the vacuole-initiating filler materials, in order to prevent the formation of vacuoles in the intermediate layer during the stretching of the film. It has been found that the advantages of the invention are impaired in the case of an intermediate layer containing vacuoles, that is, the waxes do not develop their action in the intended shape and degree in the intermediate layer containing vacuoles. In particular, a barrier action against increased water vapor can not be ensured. It is therefore essential for the opaque embodiments of the invention that the intermediate layer does not comprise vacuoles. However, if desired, the intermediate layer of the opaque mode of the film may additionally comprise pigments which, essentially, do not generate vacuoles. The pigments used are the particles described above as pigments for the base layer, with TiO2 being particularly preferred as a pigment for the intermediate layer. The intermediate layer generally comprises from 1 to 20%, preferably from 2 to 10%, of pigments, based in each case on the weight of the intermediate layer. The polypropylene film according to the invention also includes top layers, applied on both sides, comprising polymers of olefins having from 2 to 10 carbon atoms. Examples of olefinic polymers of this type are a homopolymer of propylene 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 of two or more of said homopolymers, copolymers and terpolymers or a combination of two or more of said homopolymers, copolymers and terpolymers mixed if desired with one or more of said homopolymers copolymers and terpolymers, in which is particular preference to propylene homopolymer or random ethylene-propylene having an ethylene content of 1 to 10% by weight, preferably from 2.5 to 8% by weight, or random copolymers of propylene-1-butylene having a butylene content of 2 to 25% by weight, preferably 4 to 20% by weight, based on the total weight of the ethylene-propylene-1 random copolymer or terpolymers in each case -butylene having an ethylene content of 1 to 10% by weight, preferably 2 to 6% by weight, and a butylene content of 2 to 20% by weight, preferably 4 to 20% by weight, based on each case the total weight of the terpolymer, or a combination of an ethylene-propylene-1-butylene terpolymer and a copolymer of propylene-1-butylene having an ethylene content of 0.1 to 7% by weight, and a propylene content of 50 to 90% by weight, and a butylene content of 10 to 40% by weight, based in each case on the total weight of the combination of polymers. The propylene homopolymer used in the top layer of the embodiments that can not be heat sealed from the film comprise predominantly (at least 98%) propylene and has a melting point of 140 ° C or higher, preferably from 150 to 170 ° C, preference being given to isotactic homopolypropylene having a n-heptane soluble content of 6% by weight or less, based on isotactic homopolypropylene. The homopolymer generally has a melt flow index of 1.5 g / 10 minutes up to 20 g / 10 minutes, preferably from 2.0 g / 10 minutes up to 15 g / 10 minutes. The above-described copolymers or terpolymers used in the top layer of heat-sealed embodiments of the film generally have a melt flow rate of 1.5 to 30 g / 10 minutes, preferably 3 to 15 g / 10. minutes The melting point is in the range of 120 to 140 ° C. The combination of copolymers and terpolymers described above has a melt flow index of 5 to 9 g / 10 minutes and a melting point of 120 to 150 ° C. All the melt flow rates indicated above are measured at 230 ° C and at a force of 21.6 N (DIN 53 735).

If desired, all of the polymers of the above-described top layer can be degraded by peroxide in the same manner as described above for the base layer, the same peroxides being basically used. The degradation factor for the top layer polymers is generally in the range of 1.5 to 15, preferably from 1.5 to 10. In a mat mode, the top layer additionally comprises a high density polyethylene (HDPE), which is mixed or combined with the polymers of the upper layer described above. The composition and details of the upper layers of the mat are described, for example, in German patent application P 43 13 430.0, which is expressly incorporated in the present invention by way of reference. The upper layers can, as described above for the base layer and the intermediate layer, comprise stabilizers, neutralizers, lubricants, antiblocking agents and / or antistatic agents in the corresponding amounts. In a preferred embodiment, the top layers comprise anti-blocking agents described below. Suitable anti-blocking agents are inorganic additives, such as silicon dioxide, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate and the like, and / or non-compatible organic polymers, such as polyamides, polyesters, polycarbonates and the like, Preference being given to the benzoguanamine-formaldehyde, silicon dioxide and calcium carbonate polymers. The effective amount of agent antiblocker, preferably SiO2, is in the range of 0.1 to 2% by weight, preferably from 0.1 to 0.8% by weight, based in each case on the weight of the upper layer. The average particle size is between 1 to 5 μm, in particular between 2 to 5 μm, particles having a spherical shape, such as those described in EP-A-0 236 945 and DE-A-38, being particularly suitable. 01 535. The thickness of the upper layer or layers is generally greater than 0.2 μm and is preferably in the range of 0.4 to 2 μm, in particular from 0.5 to 1.5 μm. The total thickness of the polypropylene film according to the invention may vary within wide limits and depends on the intended use. For transparent embodiments, it is preferably from 4 to 80 μm, preferably from 5 to 50 μm, in particular from 10 to 30 μm. The opaque / white modalities generally have a thickness of 10 to 150 μm, preferably from 15 to 100 μm, in particular from 10 to 80 μm, the base layer constituting from about 40 to 95% of the total thickness of the film. The invention also relates to a process for the production of the polypropylene film according to the invention by a coextrusion process, which is known per se. As is conventional in the coextrusion process, the polymer or polymer mixture of the individual layers is first compressed and liquefied in an extruder, being possible for any additive previously added that is present in the polymer or polymer mixture. The molten materials are then forced simultaneously through a die of flat (slotted) film, and the extruded multi-layer film is collected in one or more pick-up rollers, during which it cools and solidifies. The film obtained in this way is then stretched longitudinally and transversely in the extrusion direction, which results in the alignment of the molecular chains. The longitudinal stretching is advantageously carried out with the help of two rollers running at different speeds corresponding to the desired stretching ratio, and the transverse stretching is achieved with the help of a tensioning frame. The longitudinal stretching ratios are in the range of 5.0 to 9, preferably from 5.5 to 8.5. The transverse stretch ratios are in the range of 5.0 to 9.0, preferably from 6.5 to 9.0. The biaxial stretching of the film is followed by thermosetting (heat treatment) thereof, during which the film is maintained at a temperature of 60 to 160 ° C for about 0.1 to 20 seconds. The film is subsequently wound in a conventional manner by a winding unit. It has been found to be particularly favorable to maintain the pick-up roller or rollers, by means of which the extruded film is cooled and solidifies, at a temperature of 10 to 100 ° C, preferably from 20 to 70 ° C, by means of a heating and cooling circuit. The temperatures at which the longitudinal and transverse stretching is performed can vary over a relatively wide range and depend on the desired properties of the film. In general, the longitudinal stretch is performed between 80 and 150 ° C and the transverse stretch is preferably carried out between 120 and 170 ° C. After biaxial stretching, one or both surfaces of the film is / are preferably treated by corona or by flame with one of the known methods. The treatment intensity is generally in the range of 36 to 50 mN / m, preferably from 38 to 45 mN / m. In the case of corona treatment, an advantageous method is to pass the film between two conductive elements that serve as electrodes, with a high voltage, with an alternating voltage usually being applied (from approximately 5 to 20 kV and from approximately to 30 kHz), between the electrodes so that spraying or corona discharges may occur. Due to crown or spray discharge, the air above the surface of the film ionizes and reacts with the molecules on the surface of the film, resulting in the formation of polar inclusions in the essentially non-polar polymer matrix.

For the flame treatment with a polarized flame (see US-A-4, 622, 237), a direct electrical voltage is applied between a burner (negative pole) and a cooling roller. The level of the applied voltage is between 400 and 3000 V, preferably in the range of 500 to 2000 V. The applied voltage gives the ionized atoms an increased acceleration, and these collide with the surface of the polymer with a higher kinetic energy. The chemical bonds within the polymer molecule dissociate more easily, and the formation of free radicals proceeds more quickly. The thermal load on the polymer is much lower than in the case of the standard flame treatment, and films can be obtained in which the heat sealing properties of the treated side are even better than those of the untreated side. The multilayer film according to the invention is distinguished by its good barrier action against water vapor. It has been found that the incorporation of wax in the intermediate layer is advantageous in comparison with a synergistic combination of resin and wax in the base layer. First, excellent improvement in barrier action can be achieved with a comparatively lower absolute amount of wax. Second, the film is produced in a fairly inexpensive way. The barrier values can be adjusted in a particularly flexible manner by the concentration and the thickness of the intermediate layer. These facilitate the particularly high flexibility that consumers want.

Surprisingly, the exclusive formulation of the intermediate layer with wax is sufficient to achieve a good barrier action. It has been found that the additional amounts of resin in the intermediate layer do not affect an additional improvement in the barrier action. This is even more surprising since it has been discovered simultaneously with the investigations for this application, that the combination of wax and resin in the base layer is combined in a synergistic manner. These results suggest the assumption that the mechanisms of action of the wax in the intermediate layer are different than in the base layer of an oriented film, although even today the foundation for the action of improving the barrier action is not really understood. of the wax. Surprisingly, additives such as hydrocarbon resin or waxes need not be additionally added to either the base layer or the top layer in order to guarantee the desired barrier properties. In addition, it has been discovered that other desirable service properties of the film are not adversely affected by the wax in the intermediate layer. In addition to the improved barrier action, the film is also distinguished by good transparency, high gloss and good heat sealing properties. The invention is of paramount importance in the case of films containing vacuoles. In these types of films, the relatively small amounts of wax according to the invention can, Although the base layer that contains vacuoles, develop a surprising barrier action. Conventional vacuole-containing films, in which wax has been added to the base layer, require a considerably greater amount of wax compared to clear films. Presumably, the vacuoles generate an internal surface towards which the wax migrates. The invention is explained in more detail in the following examples.

EXAMPLE 1 A transparent five-layer film having a symmetrical structure DZBZD with a total thickness of 20 μm is produced by coextruding the corresponding raw material mixes and the subsequent stepwise orientation in the longitudinal and transverse directions. Each of the upper layers D has a total thickness of 0.6 μm, and each of the intermediate layers Z has a thickness of 1.5 μm. The calculated wax content, based on the total weight of the film, is 1% by weight. The wax content based on the total weight of the film is calculated from the wax content in the raw material mixture of the intermediate layer and in the thickness of the intermediate layer and the total thickness of the film.

Mixture of raw material of the base layer B 99.85% by weight of highly isotactic propylene homopolymer, with a melting point of 166 ° C and a melt flow index of 3.4 g / 10 minutes, in which the insoluble portion in n-heptane it has a chain sotactic character index of 98%. 0.15% by weight of N, N-bis-ethoxyalkylamine (antistatic).

Mixture of raw material from the intermediate layers Z 93.0% by weight of isotactic polypropylene from Solvay under the brand name ® PHP 405. 7.0% by weight of polyethylene wax, with average molecular weight Mn of 500 and a molecular weight distribution Mw / Mn of 1.08.

Mixture of raw material of the upper layers D 74% by weight, approximately, of ethylene-propylene random copolymer, with C2 content of 4.5% by weight. 25% by weight, approximately, of ethylene-propylene-butylene random terpolymer, with an ethylene content of 3% by weight and a butylene content of 7% by weight (the remainder, propylene). 0.33% by weight of SiO2 as an antiblocking agent with an average particle size of 2 μm. 1.20% by weight of polydimethylsiloxane, with viscosity of ,000 mm2 / s.

The production conditions in the individual steps of the procedure were: Extrusion: Temperatures: Basic layer: 260 ° C. Upper layers: 240 ° C Intermediate layer: 260 ° C Collector roller temperature: 20 ° C Longitudinal stretching: Temperature: 110 ° C. Longitudinal stretch ratio: 5.5 Transverse stretch: Temperature: 160 ° C Transverse stretch ratio: 9 Parameters: Temperature: 140 ° C. Convergence: 20%.

The transverse stretch ratio l = 9 is an effective value. This effective value is calculated from the final width W of the film, reduced twice the width w of the hem band, divided by the width C of the longitudinally stretched film, likewise reduced by twice the width w of the hem band.

EXAMPLE 2 A film was produced as described in example 1. In this example the raw material mixture of the intermediate layer comprises 10% by weight of the same wax, corresponding to a calculated total content, based on the total weight of the film , of approximately 1.5% by weight. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 3 A film was produced as described in example 1. In this example the raw material mixture of the intermediate layer comprises 13.3% by weight of the same wax, corresponding to a calculated total content, based on the total weight of the film , approximately 2% by weight. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 4 A film was produced as described in example 1. In this example the raw material mixture of the intermediate layer comprises 13.3% by weight of the same wax. In this case, the thickness of each intermediate layer was 3 μm. Correspondingly, the total calculated content of wax, based on the total weight of the film, was approximately 4% by weight. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 5 A film was produced as described in example 1. In this example the raw material mixture of the intermediate layer comprises 20% by weight of the same wax, corresponding to a calculated total content, based on the total weight of the film , of about 3.0% by weight. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 6 A film was produced as described in example 4. In this example the raw material mixture of the intermediate layer comprises % by weight of the same wax, corresponding to a total calculated content, based on the total weight of the film, of approximately 6.0% by weight. The rest of the composition and the production conditions were not changed compared to Example 4.

EXAMPLE 7 A film was produced as described in Example 1. In this example the raw material mixture of the intermediate layer comprises 27% by weight of the same wax, corresponding to a calculated total content, based on the total weight of the film , about 4.0% by weight. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 8 A film was produced as described in example 1. In this example, the raw material mixture of the intermediate layer additionally comprises 10.0% by weight of a hydrocarbon resin. The rest of the composition and the production conditions were not changed compared to Example 1.

EXAMPLE 9 A film was produced as described in example 1. In this example, the raw material mixture of the intermediate layer additionally comprises 5% by weight of a hydrocarbon resin. The rest of the composition and the production conditions were not changed compared to Example 1.

COMPARATIVE EXAMPLE 1 A film was produced as described in example 1. In contrast to example 1, in this example the film does not contain intermediate layer and does not comprise polyethylene wax neither in the base layer nor in the top layer. The rest of the composition and the production conditions were not changed compared to Example 1.

COMPARATIVE EXAMPLE 2 A film was produced as described in comparative example 1. In this example, the raw material mixture of the base layer comprises hydrocarbon resin, corresponding to a calculated total content, based on the total weight of the film, of 5.0% in weigh.

The rest of the composition and the production conditions were not changed compared to comparative example 1.

COMPARATIVE EXAMPLE 3 A film was produced as described in comparative example 1. In this example the raw material mixture of the base layer comprises hydrocarbon resin, corresponding to a calculated total content, based on the total weight of the film, of 8.0% in weigh. The rest of the composition and the production conditions were not changed compared to comparative example 1.

COMPARATIVE EXAMPLE 4 A film was produced as described in comparative example 1. In this example, the raw material mixture of the base layer comprises hydrocarbon resin, corresponding to a calculated total content, based on the total weight of the film, of 10.0% in weigh. The rest of the composition and the production conditions were not changed compared to comparative example 1.

COMPARATIVE EXAMPLE 5 A film was produced as described in comparative example 1. In this example the raw material mixture of the base layer comprises the same polyethylene wax as described in example 1, corresponding to a total calculated content of wax, taking as base the total weight of the film, 1.0% by weight. The film did not contain hydrocarbon resin. The rest of the composition and the production conditions were not changed compared to comparative example 1.

COMPARATIVE EXAMPLE 6 A film was produced as described in comparative example 5. In this example the raw material mixture of the base layer comprises the same polyethylene wax, corresponding to a calculated total wax content, based on the total weight of the film , of 2.0% by weight. The rest of the composition and the production conditions were not changed compared to comparative example 5.

COMPARATIVE EXAMPLE 7 A film was produced as described in comparative example 5. In this example the raw material mixture of the base layer it comprises the same polyethylene wax, corresponding to a total calculated content of wax, based on the total weight of the film, of 3.0% by weight. The rest of the composition and the production conditions were not changed compared to comparative example 5.

COMPARATIVE EXAMPLE 8 A film was produced as described in comparative example 5. In this example the raw material mixture of the base layer comprises the same polyethylene wax, corresponding to a calculated total wax content, based on the total weight of the film , 4.0% by weight. The rest of the composition and the production conditions were not changed compared to comparative example 5. Raw materials and films were characterized using the following measurement methods: The melt flow rate The melt flow rate was measured in accordance with DIN 53 735, at a load of 21.6 N and at 230 ° C.

The melting point Measurement by differential scanning calorimetry (DSC), maximum of the melting curve, heating rate 20 ° C / min.

Permeability to water vapor and oxygen The permeability to water vapor and oxygen was determined in accordance with DIN 53 122, part 2.

Surface tension The surface tension was determined by means of the so-called ink method (DIN 53 364).

Molecular weight determination The average molecular weights Mw and Mn and the dispersion of the average molecular weight Mw / Mn were determined according to DIN 55 672, part 1, by means of gel permeation chromatography. Instead of THF, orthodichlorobenzene is used as the eluent. Because the olefinic polymers investigated are not soluble at room temperature, the whole measurement was carried out at elevated temperature (around 135 ° C).

Isotactic content The isotactic content of the homopolymer can be characterized up to an approximation by the insoluble fraction of the raw material in n-heptane. Usually an extraction is carried out in a Soxhlet apparatus with boiling n-heptane, it being convenient to fill the Soxhlet apparatus with a compressed disk instead of granules. The thickness of the compressed disk should not be greater than 500 microns. In order to quantitatively determine the n-heptane insoluble content of the homopolymer, it is of vital importance to ensure a sufficient extraction time, from 8 to 24 hours. The operative definition of the isotactic content PP¡So, in percentage, is given by the ratio of the weights of the insoluble fraction in n-heptane, dry with the weight of the sample: PP¡so = 100 x (fraction insoluble in n-heptane / weight of the sample) An analysis of the dry n-heptane extract shows that as a rule it does not consist of pure atactic propylene homopolymer. In the extraction, aliphatic and olefinic oligomers are also included, especially isotactic oligomers, as well as possible additives, such as hydrogenated hydrocarbon resins and wax. isotactic character index of the chain The isotactic content PP, S0 defined above, determined as the insoluble content in n-heptane, is not sufficient to characterize the isotactic character of the polymer chain. This shows that it is appropriate to determine the isotactic index II of the homopolymer chain by means of high resolution 13C-NMR spectroscopy, in which the selected NMR sample should not be the original raw material, but rather its insoluble fraction in n-heptane. In order to characterize the isotactic character of the polymer chains, the isotactic index II of triads of 13C-NMR spectroscopy (Triads) is normally used in practice.

Determination of isotactic index II of chain related to triad (triads) The index II of isotactic character of chain (triads) of the n-heptane insoluble content of the homopolymer and of the film is determined from the 13C-NMR spectrum of the same. The intensities of the triad signals resulting from the methyl groups with different environments are compared. With respect to the evaluation of the 13 C-NMR spectrum, a distinction must be made between two cases: A) The raw material examined is a propylene homopolymer without random content of C2. B) The raw material examined is a propylene homopolymer with a low random content of C2; referred to hereafter as C2-C3 copolymer.

Case A: The isotactic chain index of the homopolymer is determined from its 13 C-NMR spectrum. The intensities are compared of the signals that result from methyl groups with different environments. In the 13C-NMR spectrum of a homopolymer, three groups of signs, called triads. 1. The "mm triad" presents a chemical shift of approximately 21 to 22 ppm, which is assigned to the methyl groups having adjacent methyl groups directly to the left and to the right. 2. The "triad mr" occurs at a chemical shift of approximately 20.2 to 21 ppm, which is assigned to methyl groups that have adjacent methyl groups directly to the left and to the right. 3. The "triad rr" presents a chemical shift of about 19.3 to 20 ppm, which is assigned to the methyl groups that do not have directly adjacent methyl groups.

The intensities of the groups of signals assigned are determined as the integral of the signals. The sotactic character index of the string is defined as follows: Jmm + O.b Jmr Triadas II =. 100 --U Jmm + Jmr + Jrr in which Jmm, Jmr and Jrr are the integrals of the groups of signals assigned.

Case B: In the 3 C-NMR spectrum of an ethylene-propylene copolymer, the chemical shift of the methyl groups of interest is in the range of 19 to 22 ppm. The spectrum of the methyl groups can be divided into three blocks. In these blocks, the CH3 groups appear in sequences of triads, whose assignment to local environments is explained in more detail below: Block 1: Groups CH3 in the PPP sequence (triad mm) C C C I I I -C -C-C-C-C-C- Block 2: CH3 groups in the PPP sequence (Triads mr or rm) C C C-C-C-C-C-C- and the CH3 groups in the EPP sequence (m-chain): C C I I -C-C-C-C-C-C- Block 3: CH3 groups in the PPP sequence (triads rr) C-C-C-C-C-C-C C CH3 groups in an EPP sequence (r chain): C -C-C-C-C-C-C- C Groups CH3 in an EPE sequence: C-C-C-C-C-C- During the determination of the index II (Triads) of isotactic chain character referred to the triads, of the insoluble content in n- heptane of an ethylene-propylene copolymer, only the PPP triads were considered, that is, only those propylene units that are between two neighboring propylene units (see also EP-B-0 115 940, page 3, lines 48 and 49) . The definition of the index of isotactic character of the triad of an ethylene-propylene copolymer comes to be: II (Triads) = 100x (Jmm / Jppp) Calculation of the isotactic chain index of an ethylene-propylene copolymer 1. Give Jmm by means of the peak integral of block 1. 2. Calculate the integral (Jtotai) of the peaks of all the peaks of the groups methyl in blocks 1, 2 and 3. 3. By means of simple considerations, it can be shown that Jppp = Jtotai - JEPP - JEPE- Preparation and measurement of the sample 60 to 100 mg of polypropylene are weighed in a 10 mm tube for NMR and hexachlorobutadiene and tetrachloroethane are added in a mixing ratio of 1.5: 1, until a filling height of approximately 45 mm is reached . The suspension is stored approximately at 140 ° C until (usually approximately one hour) a homogeneous solution is formed. In order to accelerate the dissolution process, the sample is shaken from time to time with a glass gendarme. The 13 C-NMR spectrum is recorded at elevated temperature (usually at 365 ° K) under normal measurement conditions (semiquantitative).

REFERENCES W. O. Crain, Jr., A. Zambelli and J. D. Roberts, Macromolecules, 4, 330 (1971). Zambelli, G. Gatti, C. Sacchi, W. O. Crain, Jr., and J. D. Roberts, Macromolecules, 4, 471 (1971). C. J. Carman and C. E. Wiikes, Rubber Chem. Technol., 44, 781 (1971).

TABLE 1 > VO TABLE 2 or

Claims (19)

NOVELTY OF THE INVENTION CLAIMS

1. - A biaxially oriented multilayer polypropylene film comprising a base layer and at least one top layer that can be heat sealed and at least one intermediate layer in accordance with a BZD layer structure, whose film comprises wax in its intermediate layer, characterized in that the intermediate layer contains a wax having an average molecular weight Mn of 200 to 1200.

2. A polypropylene film according to claim 1, further characterized in that the intermediate layer comprises wax in an amount from 3 to 40% by weight, preferably from 5 to 30% by weight, based on the weight of intermediate layer

3. A polypropylene film according to claim 1 and / or 2, further characterized in that the wax is a polyethylene wax having an Mw / Mn of 1 to 2.

4. A polypropylene film according to one or more of claims 1 to 3, further characterized in that the it was a macrocrystalline paraffin (paraffin wax) or a microcrystalline wax (microcranium).

5. - A polypropylene film according to one or more of claims 1 to 4, further characterized in that the intermediate layer has a thickness of 0.2 to 10 μm, preferably 0.4 to 5 μm.

6. A polypropylene film according to one or more of claims 1 to 5, further characterized in that the intermediate layer comprises a highly isotactic propylene homopolymer having an index of isotactic chain character of the n-heptane insoluble content, determined by means of 13 C-NMR spectroscopy, of at least 95%, preferably of at least 96 to 99%.

7. A polypropylene film according to one or more of claims 1 to 6, further characterized in that it has an upper layer of olefinic polymers, which can be heat sealed, on both sides.

8. A polypropylene film according to one or more of claims 1 to 7, further characterized in that the intermediate layers of olefin polymers, preferably propylene homopolymer, containing wax, are applied on both sides between the base layer and the intermediate layer or layers.

9. A polypropylene film according to one or more of claims 1 to 8, further characterized in that it has a mat-like top layer.

10. A polypropylene film according to one or more of claims 1 to 9, further characterized in that the base layer it comprises a highly isotactic propylene homopolymer having an isotactic chain index of the n-heptane insoluble content, determined by means of 13 C-NMR spectroscopy, of at least 95%, preferably from at least 96 to 99 %.

11. A polypropylene film according to one or more of claims 1 to 10, further characterized in that the base layer comprises a hydrocarbon resin in an amount of 1 to 20%, based on the weight of the base layer.

12. A polypropylene film according to one or more of claims 1 to 11, further characterized in that the base layer comprises an antistatic agent, preferably a tertiary aliphatic amine.

13. A polypropylene film according to one or more of claims 1 to 12, further characterized in that the film is transparent and has a thickness of 4 to 80 μm.

14. A polypropylene film according to one or more of claims 1 to 12, further characterized in that the film is opaque and / or white and has a light transparency of at most 70%.

15. A polypropylene film according to claim 14, further characterized in that the film has an intermediate layer free of vacuoles.

16. A polypropylene film according to one or more of claims 1 to 15, further characterized in that the layer or layers The above include lubricants, preferably polydimethylsiloxane, and antiblock agents, preferably SiO2.

17. A polypropylene film according to one or more of claims 1 to 16, further characterized in that all the layers comprise neutralizer and stabilizer.

18. A process polypropylene film according to claim 1, characterized in that the orientation in the longitudinal direction is carried out with a longitudinal stretch ratio of 5: 1 to 9: 1 and the orientation in the transverse direction is carried out with a cross-stretch ratio of 5: 1 to 10: 1.

19. The use of a polypropylene film according to one or more of claims 1 to 17 as a packaging film, preferably as a film for wrapping cigarette packs.