Oriented polyolefin film with amorphous polymer, process of preparation and use thereof.

0001The present invention relates to an oriented polyolefin film which contains at least one vacuole-containing layer. The invention further relates to a method for producing the polyolefin film and its use.

0002Polyolefin films are used for many different applications and can roughly be divided into two groups, the transparent and the non-transparent film types. Transparent films naturally have the least possible haze, while the non-transparent types have such a high haze that a meaningful measurement of this size is not possible. Therefore, the transparency of non-transparent films is determined instead. Depending on the degree of light transmission, a distinction is made between translucent and opaque or white foils.

0003Non-transparent films contain pigments or vacuole-initiating particles or a combination of these in at least one layer, as a result of which the films have a reduced light transmission in comparison with transparent films.

0004Pigments are particles that essentially do not lead to the formation of vacuoles when the film is stretched. The coloring effect of the pigments is caused by the particles themselves. The term "pigment" is generally linked to a particle size of 0.01 to a maximum of 1 micron and includes both so-called "white pigments", which color the films white, and "colored pigments", which give the film a colorful or black color.

0005Opaque films contain vacuole-initiating particles which are incompatible with the polymer matrix and lead to the formation of vacuole-like cavities when the films are stretched, the size, type and number of vacuoles depending on the material and the size of the solid particles and the stretching conditions, such as stretching ratio and stretching temperature . The vacuoles reduce the density and give the films a characteristic pearlescent, opaque appearance, which is caused by light scattering at the vacuole / polymer matrix interfaces. In general, the average particle diameter of the vacuole-initiating particles is 1 to 10 μm.

0006Typical vacuole-initiating particles are inorganic and / or organic materials incompatible with polypropylene, such as oxides, sulfates, carbonates or silicates, and incompatible polymers such as polyesters or polyamides. "Incompatible materials" or "incompatible polymers" means that the material or the polymer is present in the film as a separate particle or as a separate phase.

0007The density of the non-transparent films can vary within wide limits and depends on the type and amount of the fillers. The density is generally in the range of 0.4 to 1.1 g / cm³.

0008Such non-transparent films are described in detail in the following documents:

0009EP-A-0 004 633 describes a heat-sealable, opaque, biaxially oriented plastic film which contains finely divided solid, in particular inorganic, particles with a size of 0.2 to 20 μm and has at least one heat-sealing layer made of a propylene-ethylene copolymer. To achieve the opacity, in addition to the inorganic particles, opaque organic particles are also used, for. B. made of cross-linked plastic, the melting point of the plastic particles being above the temperatures occurring during the production of the film. The film should be improved in its sealability, in its gloss and in its printability compared to the prior art. The described film has the disadvantage that its opacity and density are subject to fluctuations due to the raw material and the process, which can mainly be compensated for by changing the particle concentration. Depending on the particle concentration and the particle size distribution of the particles, however, agglomerations of the particles can occur, as a result of which the film appearance is uneven and impaired. In principle, the opacity of these opaque films with constant particle concentration depends on the film thickness. In addition, the mechanical properties of the film are still in need of improvement. When using inorganic particles, the film shows undesirable chalking during assembly and further processing.

0010EP-A-0 083 495 describes an opaque, biaxially oriented film with a satin finish and a surface gloss of greater than 100%, which contains at least one spherical solid particle per vacuole. Furthermore, the film comprises a pore-free, transparent, thermoplastic jacket layer on both surfaces of the core layer, which has a thickness which determines the appearance of the film. As a material for the solid particle z. B. specified nylon. The particles generally have a diameter that is greater than 1 µm. In this film too, the opacity is mainly determined by the amount of solid particles and the process parameters in the biaxial orientation, and the disadvantages described above with regard to fluctuations in opacity occur.

0011The object of the present invention was to provide a non-transparent polyolefin film in which the translucency or the opacity can be set by simple measures and varied to a large extent. Furthermore, the film should not be chalked out and have good mechanical and homogeneous optical properties. In particular, high film stiffness and high gloss are particularly desirable.

0012This object is achieved by a polyolefin film of the type mentioned at the outset, the characteristic feature of which is that the vacuole-containing layer contains an amorphous, vacuole-initiating polymer which, after orientation, is present as a separate particle within the vacuole.

0013The film according to the invention has one or more layers. Single-layer embodiments are constructed like the vacuole-containing layer of the multilayer film described below. Multi-layer embodiments are at least two-layer and always comprise the vacuole-containing layer and at least one further layer, wherein the vacuole-containing layer can form the base, the intermediate or the top layer of the multilayer film. In a preferred embodiment, the vacuole-containing layer forms the base layer of the film with at least one, preferably with cover layer (s) on both sides, it being possible for there to be an intermediate layer or vacuole-containing or vacuole-containing intermediate layer between the vacuole-containing base layer and the cover layer (s) /can. In a further preferred embodiment, the vacuole-containing layer forms an intermediate layer of the multilayer film, which is present between the non-vacuole-containing base layer and the cover layer. Further embodiments with a vacuole-containing intermediate layer are constructed in five layers and have vacuole-containing intermediate layers on both sides. In a further embodiment, the vacuole-containing layer can form a cover layer on the vacuole-containing or non-vacuole-containing base or intermediate layer. For the purposes of the present invention, the base layer is the layer which makes up more than 50% of the total film thickness. The top layer is the layer that forms the outermost layer of the film.

0014Depending on the intended use, the respective embodiment of the non-transparent film can be translucent, opaque or white-opaque. For the purposes of the present invention, non-transparent films are understood to be those films whose light transmission according to ASTM-D 1003-77 is less than 95%. A distinction is made between translucent, opaque and white-opaque types according to their light transmission. Translucent films have a light transmission of 95 to 70%, and opaque or white-opaque types have a light transmission of 69 to 0%, each measured according to ASTM-D 1003-77.

0015The vacuole-containing layer of the film according to the invention contains a polyolefin, preferably a propylene polymer, and an amorphous, vacuole-initiating polymer and, if appropriate, further additives added in effective amounts in each case. In general, the vacuole-containing layer contains at least 50% by weight, preferably 70 to 99% by weight, in particular 80 to 98% by weight, of the propylene polymer, based on the weight of the vacuole-containing layer.

0016The propylene polymer generally contains 90 to 100% by weight, preferably 95 to 100% by weight, in particular 98 to 100% by weight, of propylene and generally has a melting point of 120 ° C. or higher, preferably 150 to 170 ° C and generally a melt flow index of 0.5 g / 10 min to 8 g / 10 min, preferably 2 g / 10 min to 5 g / 10 min, at 230 ° C and a force of 21.6 N (DIN 53 735 ). Isotactic propylene homopolymer with an atactic content of 15% by weight or less, copolymers of ethylene and propylene with an ethylene content of 10% by weight or less, copolymers of propylene with C₄-C₈-α-olefins with an α-olefin content of 10 % By weight or less, 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 are preferred propylene polymers for the vacuole-containing layer, isotactic propylene homopolymer is particularly preferred. The weight percentages refer to the respective polymer.

0017A mixture of the propylene homo- and / or copolymers and / or terpolymers mentioned and other polyolefins, in particular monomers having 2 to 6 carbon atoms, is also suitable, the mixture being at least 50% by weight, in particular at least 75 Wt .-%, propylene polymer contains. Suitable other polyolefins in the polymer mixture are polyethylenes, in particular HDPE, LDPE, LLDPE, the proportion of these polyolefins in each case not exceeding 15% by weight, based on the polymer mixture.

0018According to the invention, the vacuole-containing layer or the film in one-layer embodiments contains an amorphous, vacuole-initiating polymer in general in an amount of at most 40% by weight, preferably 1 to 30% by weight, in particular 2 to 20% by weight, based on the weight of the vacuole-containing layer or film. It has been found that the amorphous polymer, which is a polymeric solid per se and has no particle character as a raw material, surprisingly nevertheless acts as a vacuole-initiating filler. When the film is stretched, microcracks and microvoids, so-called vacuoles, are formed between the polymer matrix of the layer and the amorphous polymer, in the area in which the visible light is refracted. This gives the film a translucent or opaque appearance and a reduced density, which makes it particularly suitable for certain packaging purposes, particularly in the food sector.

0019For the purposes of the present invention, amorphous polymers are understood to mean those polymers which, despite an irregular arrangement of the molecular chains at room temperature, are solids. They are essentially non-crystalline and their degree of crystallinity is generally below 5%, preferably below 2%, or is 0%. Amorphous polymers whose glass transition temperature T_G is in the range from 70 to 300 ° C., preferably 80 to 250 ° C., in particular 100 to 200 ° C., or their Vicat softening temperature T_V (VST / B / 120) is between 70 and 200 ° C, preferably between 80 and 180 ° C. In general, the amorphous polymer has an average molecular weight M_w in the range from 500 to 500,000, preferably 1,000 to 250,000, in particular 3,000 to 200,000.

0020The refractive index of the amorphous polymer is generally in the range from 1.3 to 1.7, preferably 1.4 to 1.6. It is particularly advantageous for the refractive index of the amorphous polymer if it is in a specific ratio to the refractive index of the polyolefin of the vacuole-containing layer. In general, the refractive indices of amorphous polymer and polypropylene differ by a maximum of 0.1 unit, preferably by a maximum of 0.05 unit.

0021The amorphous polymer is surprisingly present in the oriented film in the form of separated particles, which can be clearly seen in scanning electron micrographs if the orientation at least partially resulted in tears, that is to say when a vacuole-like cavity has at least partially formed around the particle of amorphous polymer . The particle size of the particles present in the oriented film is in the range from 0.2 to 10 μm, preferably 0.5 to 7 μm, in particular 1 to 5 μm. Surprisingly, vacuoles are formed around the particles of amorphous polymer even with very small particle sizes of ≦ 1 µm.

0022A large number of generally transparent polymers are suitable as amorphous polymers with the property profile described above. Examples include atactic polystyrene (T._G = 95 to 105 ° C, preferably 100 ° C), poly-α-methylstyrene (T._G = 170 to 180 °, preferably 175 ° C), polycarbonate (T._V = 120 to 160 ° C, preferably 140 ° C), aromatic polycarbonates with increased heat resistance (T._V = 160 to 190 ° C, preferably 172 ° C), polyacrylates, especially polymethyl methacrylate (T._G = 115 to 130 °, preferably 122 ° C), amorphous homo- and copolymers of polycyclic olefins (depending on the composition and molecular weight T_G = 70 to 300 ° C), polyvinyl carbazole (T_G = 180 to 220 ° C, preferably 200 ° C), atactic polyvinylcyclohexane (T_G = 130 to 150 ° C, preferably 140 ° C), polyvinyl chloride (T._G = 65 to 90 ° C, preferably 80 ° C), polyacrylonitrile (T_G = 100 to 110 ° C, preferably 106 ° C), natural and synthetic resins, especially unsaturated hydrocarbon resins (T._G = 70 to 200 ° C), special types of rubber, especially cyclo rubber (T_G = 70 to 120 ° C), uncrosslinked, crosslinked and crosslinked dispersions of amorphous polymers (T._G depending on the polymerization partner and degree of polymerization 70 to 200 ° C). The amorphous polymers are known per se and are described in the prior art. Cycloolefin polymers, which are also known and are described in EP-A-0 407 870, EP-A-0 503 422 and DE-A-40 36 264, to which reference is expressly made here, are particularly suitable.

0023In addition to the amorphous polymers, the vacuole-containing layer can also contain pigments in a further embodiment. For the purposes of the present invention, pigments comprise those particles which essentially do not lead to the formation of vacuoles during stretching. The coloring effect of the pigments is caused by the particles themselves. The term "pigment" is generally linked to a particle size of 0.01 to a maximum of 1 micron and includes both so-called "white pigments", which color the films white, and "colored pigments", which give the film a colorful or black color. In general, the average particle diameter of the pigments is in the range from 0.01 to 1 μm, preferably 0.01 to 0.7 μm, in particular 0.01 to 0.4 μm. The vacuole-containing layer of this embodiment generally contains pigments in an amount of 1 to 25% by weight, in particular 2 to 20% by weight, preferably 5 to 15% by weight, in each case based on the vacuole-containing layer.

0024Common pigments are materials such as 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, among which white pigments such as calcium carbonate, silicon dioxide, titanium dioxide and barium sulfate are preferably used.

0025The titanium dioxide particles consist of at least 95% by weight of rutile and are preferably used with a coating of inorganic oxides, as is usually used as a coating for TiO₂ white pigment in papers or paints to improve lightfastness. The particularly suitable inorganic oxides include the oxides of aluminum, silicon, zinc or magnesium or mixtures of two or more of these compounds. They are made from water-soluble compounds, e.g. B. alkali, especially sodium aluminate, aluminum hydroxide, aluminum sulfate, aluminum nitrate, sodium silicate or silica, precipitated in the aqueous suspension. TiO₂ particles with a coating are, for. B. described in EP-A-0 078 633 and EP-A-0 044 515.

0026The coating may also contain organic compounds with polar and non-polar groups. Preferred organic compounds are alkanols and fatty acids with 8 to 30 carbon atoms in the alkyl group, in particular fatty acids and primary n-alkanols with 12 to 24 carbon atoms, and also polydiorganosiloxanes and / or polyorganohydrogensiloxanes such as polydimethylsiloxane and polymethylhydrogensiloxane.

0027The coating on the TiO₂ particles usually consists of 1 to 12 g, in particular 2 to 6 g, of inorganic oxides, optionally 0.5 to 3 g, in particular 0.7 to 1.5 g, of organic compounds, in each case based on 100 g of TiO₂ particles. It has proven to be particularly advantageous if the TiO₂ particles are coated with Al₂O₃ or with Al₂O₃ and polydimethylsiloxane.

0028The density of the films according to the invention can vary within wide limits and depends, inter alia, on the type and the amount of the amorphous polymers and the pigments which may be added. The density is generally below the calculated density of the individual components of the film, ie the density of the film is reduced. In general, the films have a density of at most 1.5 g / cm 3, preferably the density is in the range from 0.4 to 1.3 g / cm 3, in particular 0.5 to 1.0 g / cm 3.

0029The multilayer embodiment of the film according to the invention comprises at least one further vacuole-containing or non-vacuole-containing layer, which can be the base layer, an intermediate layer or a sealable or non-sealable cover layer of the multilayer film. In principle, the vacuole-containing layer and the other layer (s) can be constructed identically or differently.

0030The other layer generally contains 75 to 100 wt .-%, in particular 90 to 99.5 wt .-%, α-olefinic polymers with 2 to 10 carbon atoms, each based on the weight of the other layer, and optionally additives in each case effective Amounts.

0031Examples of such α-olefinic polymers are    a propylene homopolymer or    a copolymer of    Ethylene and propylene or    Ethylene and butylene-1 or    Propylene and butylene-1 or    a terpolymer of    Ethylene and propylene and butylene-1 or    a mixture of two or more of the homopolymers, copolymers and terpolymers mentioned or    a blend of two or more of the homopolymers, copolymers and terpolymers mentioned, optionally mixed with one or more of the homopolymers mentioned, Copolymers and terpolymers, wherein in particular propylene homopolymer or    statistical ethylene-propylene copolymers with    an ethylene content of 1 to 10% by weight, preferably 2.5 to 8% by weight, or    statistical propylene-butylene-1 copolymers with    a butylene content of 2 to 25% by weight, preferably 4 to 20% by weight,    each based on the total weight of the copolymer, or statistical ethylene-propylene-butylene-1-terpolymers    an ethylene content of 1 to 10% by weight, preferably 2 to 6% by weight, and    a butylene-1 content of 2 to 20% by weight, preferably 4 to 20% by weight,    in each case based on the total weight of the terpolymer, or a blend of an ethylene-propylene-butylene-1 terpolymer and a propylene-butylene-1 copolymer    with an ethylene content of 0.1 to 7% by weight    and a propylene content of 50 to 90% by weight    and a butylene-1 content of 10 to 40% by weight,    based on the total weight of the polymer blend, are preferred.

0032The propylene homopolymer used in the other layer (s) contains 97 to 100% by weight of propylene and generally has a melting point of 140 ° C. or higher, preferably 150 to 170 ° C., isotactic homopolypropylene with an n-heptane-soluble fraction of 6% by weight and less, based on the isotactic homopolypropylene, is preferred. The homopolymer generally has a melt flow index of 1.5 g / 10 min to 20 g / 10 min, preferably 2.0 g / 10 min to 15 g / 10 min. The percentages by weight refer to the polymer.

0033The copolymers and terpolymers described above used in the other layer (s) generally have a melt flow index of 1.5 to 30 g / 10 min, preferably 3 to 15 g / 10 min. The melting point is generally in the range from 120 to 140 ° C. The blend of copolymers and terpolymers described above generally has a melt flow index of 5 to 9 g / 10 min and a melting point of 120 to 150 ° C. All melt flow indices given above are measured at 230 ° C and a force of 21.6 N (DIN 53 735). Other layers of copolymers and / or terpolymers preferably form the cover layers of sealable embodiments of the film.

0034In principle, the other layer can additionally contain the pigments described above for the vacuole-containing layer in appropriate amounts based on the weight of this layer. Embodiments with a vacuole-containing layer contain, analogously to the vacuole-containing layer described above, amorphous polymers as the vacuole-initiating filler.

0035In a further advantageous embodiment, the propylene polymers used in the vacuole-containing layer and / or other layer and / or base and / or intermediate and / or cover layer can be partially degraded by the addition of organic peroxides. 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 according to DIN 53 735 of the polypropylene, based on the starting polymer.<maths id="math0001"><img file="EP0677553A2_D0001.tif" /></maths><dl id="dl0001"><dt>MFI₁ =</dt><dd>Melt flow index of the propylene polymer before the addition of the organic peroxide</dd><dt>MFI₂ =</dt><dd>Melt flow index of the peroxidically degraded propylene polymer</dd></dl> According to the invention, the degradation factor A of the propylene polymer used is in a range from 3 to 15, preferably 6 to 10.

0036Dialkyl peroxides are particularly preferred as organic peroxides, an alkyl radical being understood to mean the customary saturated straight-chain or branched lower alkyl radicals having up to six carbon atoms. In particular, 2,5-dimethyl-2,5-di (t-butylperoxy) hexane or di-t-butyl peroxide are preferred.

0037The total thickness of the film can vary within wide limits and depends on the intended use. The preferred embodiments of the film according to the invention have total thicknesses of 5 to 200 μm, 10 to 100 μm, in particular 20 to 80 μm, being preferred. The thickness of the intermediate layer (s) that may be present is generally 2 to 12 μm, independently of one another, with intermediate layer thicknesses of 3 to 8 μm, in particular 3 to 6 μm, being preferred. The values ​​given relate to an intermediate layer. The thickness of the cover layer (s) is selected independently of other layers and is preferably in the range from 0.1 to 10 μm, in particular 0.3 to 5 μm, preferably 0.5 to 2 μm, with cover layers applied on both sides being the same in terms of thickness and composition or can be different. The thickness of the base layer results accordingly from the difference between the total thickness of the film and the thickness of the applied top and intermediate layer (s) and can therefore vary within wide limits analogously to the total thickness.

0038In order to further improve certain properties of the polyolefin film according to the invention, both the single-layer film and the vacuole-containing layer, the other layer, the base layer, the intermediate layer and / or the top layer / s of the multilayer film can contain additives in a respectively effective amount. optionally low molecular weight hydrocarbon resins compatible with the polymer and / or preferably antistatic agents and / or antiblocking agents and / or lubricants and / or stabilizers and / or neutralizing agents and antiblocking agents. All quantities in the following embodiment in percent by weight (% by weight) relate to the layer or layers to which the additive can be added.

0039A low molecular weight resin is preferably e.g. B. added to improve water vapor permeability (WDD) and to improve film rigidity. It does not create vacuoles, unlike the natural and synthetic resins mentioned above, which are amorphous polymers and incompatible with the polypropylene. These compatible hydrocarbon resins are low molecular weight polymers whose molecular weight is generally in the range from 300 to 8,000, preferably 400 to 5,000, preferably 500 to 2,000. The molecular weight of the resins is thus clearly lower than that of the propylene polymers which form the main component of the individual film layers and generally have a molecular weight of more than 100,000. The proportion of the resin is in a range of 1 to 30% by weight, preferably 2 to 10% by weight. The softening point of the resin is between 60 and 180 ° C (measured according to DIN 1995-U4, corresponds to ASTM E-28), prefera bly over 100 to 160 ° C. Among the numerous low molecular weight resins, the hydrocarbon resins are preferred, namely in the form of petroleum resins (petroleum resins), styrene resins, cyclopentadiene resins and terpene resins (these resins are described in Ullmanns Encyklopadie technical chemistry, 4. Edition, volume 12, pages 525 to 555). Suitable petroleum resins are described in numerous documents, such as EP-A-0 180 087, to which express reference is made here.

0040Preferred antistatic agents are alkali alkane sulfonates, polyether-modified, ie ethoxylated and / or propoxylated polydiorganosiloxanes (polydialkylsiloxanes, polyalkylphenylsiloxanes and the like) and / or the essentially straight-chain and saturated aliphatic, tertiary amines with an aliphatic radical having 10 to 20 carbon atoms, which with ω-hydroxy- (C₁-C₄) -alkyl- Groups are substituted, with N, N-bis (2-hydroxyethyl) alkylamines having 10 to 20 carbon atoms, preferably 12 to 18 carbon atoms, being par ticularly suitable in the alkyl radical. The effective amount of antistatic is in the range of 0.05 to 0.3% by weight.

0041Lubricants are higher aliphatic acid amides, higher aliphatic acid esters, waxes and metal soaps as well as polydimethylsiloxanes. The effective amount of lubricant is in the range of 0.1 to 3% by weight. The addition of higher aliphatic acid amides in the range from 0.15 to 0.25% by weight in the base layer and / or the top layers is particularly suitable. A particularly suitable aliphatic acid amide is erucic acid amide. The addition of polydimethylsiloxanes in the range from 0.3 to 2.0% by weight is preferred, in particular polydimethylsiloxanes with a viscosity of 10,000 to 1,000,000 mm 2 / s.

0042The usual stabilizing compounds for ethylene, propylene and other α-olefin polymers can be used as stabilizers. The amount added is between 0.05 and 2% by weight. Phenolic stabilizers, alkali / alkaline earth stearates and / or alkali / alkaline earth carbonates are particularly suitable. Phenolic stabilizers are preferred in an amount of 0.1 to 0.6% by weight, in particular 0.15 to 0.3% by weight, and with a molecular weight of more than 500 g / mol. Pentaerythrityl-tetrakis-3- ( 3,5-di-tertiary-butyl-4-hydroxyphenyl) propionate or 1,3,5-trimethyl-2,4,6-tris (3,5-di-tertiary-butyl-4-hydroxybenzyl) benzene are particularly advantageous.

0043The antiblocking agents are preferably added to the top layers. Suitable antiblocking agents are inorganic additives such as silicon dioxide, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate and the like and / or incompatible organic polymers such as polyamides, polyesters, polycarbonates and the like, benzoguanamine-formaldehyde polymers, silicon dioxide and calcium carbonate are preferred. The effective amount of antiblocking agent is in the range of 0.1 to 2% by weight, preferably 0.1 to 0.5% by weight. The average particle size is between 1 and 6 μm, in particular 2 and 5 μm, particles with a spherical shape, as described in EP-A-0 236 945 and DE-A-38 01 535, being particularly suitable.

0044Neutralizing agents are preferably calcium stearate and / or calcium carbonate with an average particle size of at most 0.7 µm, an absolute particle size of less than 10 µm and a specific surface area of ​​at least 40 m² / g. Generally, the neutralizing agent is added in an amount of 0.02 to 0.1% by weight.

0045The invention further relates to a process for producing the film according to the invention by the extrusion process known per se. The procedure in this process is that the polymers or the polymer mixture are compressed and heated in an extruder, and then the melts corresponding to the film or the individual layers of the film are extruded through a flat die or are coextruded, the film thus obtained is removed for consolidation on one or more rollers, the film is subsequently oriented, the oriented film is heat-set and optionally corona or flame treated on the surface provided for the treatment.

0046It has proven to be particularly advantageous to keep the take-off roller or rollers, by means of which the pressed-out film is also cooled and solidified, at a temperature of 10 to 90 ° C., preferably 20 to 60 ° C.

0047The pre-film obtained in this way is preferably stretched longitudinally and transversely to the direction of extrusion, which leads to a biaxial orientation of the molecular chains. The biaxial orientation can be carried out simultaneously or in succession, the successive biaxial stretching, in which stretching first being longitudinal (in the machine direction) and then transverse (perpendicular to the machine direction), being particularly favorable. Stretching is preferably 4: 1 to 7: 1 in the longitudinal direction and 6: 1 to 11: 1 in the transverse direction. The longitudinal stretching is expediently carried out with the aid of two rollers running at different speeds in accordance with the desired stretching ratio, and the transverse stretching is carried out with the aid of a corresponding tenter frame.

0048The temperatures at which the longitudinal and transverse stretching are carried out can vary within a wide range. In general, the longitudinal stretching is carried out at 90 to 150 ° C., preferably 100 to 140 ° C., and the transverse stretching at 140 to 190 ° C., preferably 150 to 180 ° C.

0049The biaxial stretching of the film is followed by its heat setting (heat treatment), the film being held at a temperature of 110 to 130 ° C. for about 0.5 to 10 s. The film is then wound up in a conventional manner using a winding device.

0050Optionally, as mentioned above, after the biaxial stretching, one or both surface (s) of the film can be corona or flame treated by one of the known methods, with a direct electrical voltage between for a flame treatment with polarized flame (cf. US-A-4,622,237) a burner (negative pole) and a cooling roller. The level of the applied voltage is between 500 and 3,000 V, preferably in the range from 1,500 to 2,000 V. The applied voltage increases the acceleration of the ionized atoms and hits the polymer surface with greater kinetic energy. The chemical bonds within the polymer molecule are more easily broken and the radical formation takes place more quickly. The thermal load on the polymer is far lower than in the standard flame treatment, and films can be obtained in which the sealing properties of the treated side are even better than those of the untreated side.

0051For the alternative corona treatment, the film is passed between two conductor elements serving as electrodes, such a high voltage, usually alternating voltage (approximately 10,000 V and 10,000 Hz), being applied between the electrodes that spray or corona discharges can take place. The air above the film surface is ionized by the spray or corona discharge and reacts with the molecules of the film surface, so that polar inclusions arise in the essentially nonpolar polymer matrix. The treatment intensities are in the usual range, with 38 to 45 mN / m being preferred.

0052The amorphous polymers are incorporated into the film either as pure granules or as granulated concentrates (masterbatch) by premixing the polyolefin granules or powder with the amorphous polymers and then feeding them to the extruder. The components are mixed further in the extruder and heated to processing temperature. It has been found that the opacity and gloss of the film also depend on the extrusion conditions (temperature, shear). Surprisingly, under otherwise identical conditions with regard to raw material and stretching process, the opacity and the gloss can be varied solely via the conditions in the extruder. This opens up completely new ways of adjusting the characteristics of a translucent or opaque film. It is essential for the process according to the invention that the extrusion temperature is above the glass transition temperature / Vicat softening temperature of the amorphous polymer. In general, the extrusion temperature is at least 10 ° C, preferably 15 to 180 ° C, in particular 20 to 150 ° C, above the T_G or the T_V of the amorphous polymer.

0053It is assumed that the amorphous polymer liquefies under the extrusion conditions customary for film production and then surprisingly separates into more or less large particle-like particles during the extrusion depending on the viscosity of the polyolefin of the vacuole-containing layer and the viscosity of the amorphous polymer at the chosen extrusion temperature and not agglomerated. These particles formed during the extrusion then act during the orientation of the film similarly to the known particulate vacuole-initiating fillers. These conclusions are suggested by scanning electron micrographs of the oriented film, which show that the film has vacuoles within which there are particulate particles of the amorphous polymer. This means that the amorphous polymer, which is simply added as a solid, is present in the film after extrusion and orientation in the form of finely divided particles which are located within the vacuoles.

0054The invention will now be explained in more detail with reference to exemplary embodiments.

example 1

0055An opaque three-layer film with a symmetrical structure and a total thickness of 40 μm was produced by coextrusion and subsequent stepwise orientation in the longitudinal and transverse directions. The cover layers each had a thickness of 0.6 μm.

0056A base layer (= vacuole-containing layer): 94.85% by weight of highly isotactic polypropylene from Solvay with the brand name ®Eltex PHP 405 5.0% by weight of aromatic polycarbonate with increased heat resistance and with a Vicat softening temperature T._V = 160 ° C (®Apec HT, natural, Bayer) 0.15% by weight of N, N-bis-ethoxyalkylamine B top layers: 98.77% by weight of ethylene-propylene random copolymer with a C₂ content of 4.5% by weight 0.33 wt .-% SiO₂ as an antiblocking agent with an average particle size of 2 microns 0.90% by weight of polydimethylsiloxane with a viscosity of 30,000 mm² / s The manufacturing conditions in the individual process steps were: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="4" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" morerows="2" align="left">Extrusion:</entry><entry namest="col2" nameend="col2" morerows="1" align="left">Temperatures</entry><entry namest="col3" nameend="col3" align="left">A layer:</entry><entry namest="col4" nameend="col4" align="right">280 ° C</entry></row><row><entry namest="col3" nameend="col3" align="left">B layers:</entry><entry namest="col4" nameend="col4" align="right">280 ° C</entry></row><row><entry namest="col2" nameend="col3" align="left">Take-off roller temperature:</entry><entry namest="col4" nameend="col4" align="right">30th ° C</entry></row><row><entry namest="col1" nameend="col1" morerows="1" align="left">Longitudinal stretch:</entry><entry namest="col2" nameend="col3" align="left">Temperature:</entry><entry namest="col4" nameend="col4" align="right">130 ° C</entry></row><row><entry namest="col2" nameend="col3" align="left">Longitudinal stretch ratio:</entry><entry namest="col4" nameend="col4" align="right">6,5</entry></row><row><entry namest="col1" nameend="col1" morerows="1" align="left">Transverse stretching:</entry><entry namest="col2" nameend="col3" align="left">Temperature:</entry><entry namest="col4" nameend="col4" align="right">170 ° C</entry></row><row><entry namest="col2" nameend="col3" align="left">Lateral stretch ratio:</entry><entry namest="col4" nameend="col4" align="right">8,5</entry></row><row><entry namest="col1" nameend="col1" morerows="1" rowsep="1" align="left">Fixation:</entry><entry namest="col2" nameend="col3" align="left">Temperature:</entry><entry namest="col4" nameend="col4" align="right">140 ° C</entry></row><row rowsep="1"><entry namest="col2" nameend="col3" align="left">Convergence:</entry><entry namest="col4" nameend="col4" align="right">15 %</entry></row></tbody></tgroup></table></tables>

0057The film produced in this way is translucent.

Example 2

0058Compared to Example 1, the amorphous polymer was changed in the base layer. Now the Polycarbonate ®Makrolon CD 2005 with a T was used_V from 141 ° C. The film produced had an opaque appearance.

Example 3

0059Compared to Example 2, the amorphous polymer was changed in the base layer. An atactic polystyrene with a T was now used_G from 100 ° C. The longitudinal stretching temperature was lowered by 20 ° C. The film produced had an opaque appearance.

Example 4

0060Compared to Example 3, the amorphous polymer was changed in the base layer. The cyclic rubber ®Alpex 450 (Hoechst AG, T_G = 80 ° C). The longitudinal stretching temperature was as in Example 3. The film produced had a translucent, opaque appearance.

Example 5

0061Compared to Example 3, the amorphous polymer was changed in the base layer. A cycloolefin copolymer with a T was now used_G of 174 ° C. and an average molecular weight of 34,000. The longitudinal stretching temperature was as in Example 1. The film produced had an opaque-white appearance and a paper-like character.

Example 6

0062In comparison to Example 5, only the extrusion temperature was reduced by 30 ° C. The film produced in this way had an opaque-white appearance.

0063The following measurement methods were used to characterize the raw materials and the foils:

Melt flow index

0064The melt flow index was measured in accordance with DIN 53 735 at 21.6 N load and 230 ° C.

Melting point

0065DSC measurement, maximum of the melting curve, heating rate 20 ° C / min.

density

0066The density is determined according to DIN 53 479, method A.

shine

0067The gloss was determined in accordance with DIN 67 530. The reflector value was measured as an optical parameter for the surface of a film. Based on the standards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at 60 ° or 85 °. A light beam hits the flat test surface at the set angle of incidence and is reflected or scattered by it. The light rays striking the photoelectronic receiver are displayed as a proportional electrical quantity. The measured value is dimensionless and must be specified with the angle of incidence.

Opacity and whiteness

0068The opacity and the degree of whiteness are determined with the aid of the electric reflectance photometer "ELREPHO" from Zeiss, Oberkochem (DE), standard illuminant C, 2 ° normal observer. The opacity is determined according to DIN 53 146. The whiteness is called<maths id="math0002"><math display="inline"><mrow><mtext>WG = RY + 3RZ - 3RX</mtext></mrow></math><img file="EP0677553A2_D0002.tif" /></maths> Are defined. WG = whiteness; RY, RZ, RX = corresponding reflection factors when using the Y, Z and X color measurement filter. A barium sulfate compact (DIN 5033, part 9) is used as the white standard. A detailed description is e.g. B. in Hansl Loos "color measurement", Verlag Beruf und Schule, Itzehoe (1989).

Translucency

0069The light transmittance is measured in accordance with ASTM-D 1003-77.

Average molecular weight and molecular weight dispersity

0070The average molecular weights (M_w, M_n) and the average molecular weight dispersity (M_w/ M_n) were determined based on DIN 55 672, Part 1, by means of gel permeation chromatography. Instead of THF, orthodichlorobenzene was used as the eluent. Since the olefinic polymers to be examined are not soluble at room temperature, the entire measurement is carried out at elevated temperature (∼135 ° C).

Crystallinity

0071The crystallinity was determined using X-ray methods. The corrected diffracted X-ray intensities were set proportional to the proportions of the amorphous and crystalline phases.

Glass temperature

0072The samples were examined using DSC (Difference Scanning Calometry). The heating rate was 20 K / min. In order to eliminate the thermal history in the sample, the sample was first in the DSC device over the glass temperature T_G heated, cooled rapidly and then heated again (second heating). The temperature for the glass transition as half the step height was taken from the thermogram for the second heating.

Vicat softening temperature

0073The Vicat softening temperature VST / B / 120 was measured according to 150 306, DIN 53 460.