EP1567332A2

EP1567332A2 - Biaxially oriented film comprising a layer consisting of ethylene vinyl alcohol copolymer (evoh)

Info

Publication number: EP1567332A2
Application number: EP03795852A
Authority: EP
Inventors: Detlef Busch; Christian Peters; Joachim Jung
Original assignee: Treofan Germany GmbH and Co KG
Current assignee: Treofan Germany GmbH and Co KG
Priority date: 2002-11-29
Filing date: 2003-11-28
Publication date: 2005-08-31
Also published as: US20060172102A1; ZA200503055B; AU2003298148B2; MXPA05005706A; AU2003298148A1; US7563399B2; WO2004050353A2; WO2004050353A3

Abstract

The invention relates to a method for producing a multi-layer biaxially oriented film comprising a layer C consisting of an ethylene-vinyl-alcohol copolymer (EVOH layer) and a respective layer B of a modified polyolefin on either side of said layer, with a layer A of a partially crystalline thermoplastic polyolefin on the surface of each modified polyolefin layer. The film is drawn in the longitudinal direction and then in the transversal direction by means of a tenter frame. The invention is characterised in that the EVOH layer C, the adhesion promoter layers B and layer A are coextruded with an equal width and that the clips of the frame grip all five layers simultaneously and collectively.

Description

Biaxially oriented film with a layer of ethylene vinyl alcohol copolymer (EVOH)

The invention relates to a multilayer film with at least one inner layer made of an ethylene-vinyl alcohol copolymer (EVOH).

Biaxially oriented polypropylene films are known in the prior art. These films are used in a wide variety of areas, such as food packaging, cigarette wrapping, laminating film and technical applications. The polypropylene film receives many important usage properties through the orientation in and perpendicular to the machine direction, the so-called biaxial orientation. These so-called boPP films are characterized, among other things, by good transparency, a high gloss and a barrier against water vapor.

The barrier properties of the biaxially oriented polypropylene films against oxygen, however, are in need of improvement. For many applications, the films are therefore coated in an additional processing step after production. Acrylic coatings, PVDC, PVOH and others are used here. Another possibility is the metallization of boPP films.

It is also often proposed to combine the polypropylene layer with another barrier layer. However, the choice of materials that can be combined with polypropylene is limited. For this reason, only a few film composites have hitherto been successfully developed which have barrier layers made of polymers which differ from polypropylene. For example, polyester layers, polyamide layers or layers of ethylene-vinyl alcohol copolymer are used. However, these materials have completely different rheological properties than the usual polypropylenes and therefore cannot be stretched together without special measures. Also the Adhesion of these layers to one another is problematic. Solutions to these problems have already been proposed in the prior art.

For example, a multilayer stretched polypropylene film with a layer of ethylene-vinyl alcohol (EVOH) copolymer is known. US 4,561,920 describes a laminate of an EVOH copolymer layer and a polymer adhesive layer on at least one surface of the EVOH layer and a polypropylene layer. The polymer adhesive layer is composed of maleic anhydride modified polyolefin. According to this teaching, it is essential to the invention that the EVOH copolymer has a melt flow index of at least 8 g / 10 min (190 ° C. and 2.16 g) so that the composite can be biaxially stretched. Furthermore, it is described that in order to achieve good oxygen barriers, the biaxially stretched composite must be subjected to thermal fixation. For this purpose, the stretched film is finally passed over a series of heated rollers in order to promote recrystallization of the EVOH and thus to improve the barrier values. It is stated that the oxygen barrier at 20 ° C. and 0% atmospheric humidity is approx. 12 cc / m ² * day.

EP 0758675 B1 describes a laminate of an EVOH layer and a polypropylene film. The two layers are bonded to one another by means of an adhesive layer. The laminate can be produced by means of coextrusion, in which the layers of EVOH, polypropylene and adhesion promoter are coextruded together and simultaneously through an annular die.

The oxygen barrier of these known films is still in need of improvement. In particular, good oxygen barriers with increased air humidity should also be ensured. Furthermore, it would be desirable to find a process that is more flexible with regard to the raw materials that can be used.

The object of the present invention was therefore to provide a film To make available, which can be produced on the conventional stenter systems via the process of sequential biaxial orientation and which has a favorable property profile. Above all, the good usage properties, such as gloss, transparency, etc., of the known biaxially oriented polypropylene films should be retained. In addition, good barrier properties to oxygen and flavoring agents are particularly desirable.

This object is achieved by a method for producing a multilayer biaxially oriented film which comprises at least five layers with the arrangement AB / C / B / A, the inner layer C being composed of an ethylene-vinyl alcohol copolymer (EVOH) and On both sides of each surface of the ethylene vinyl alcohol (EVOH) layer C, an adhesion promoter layer B made of modified polyolefin is applied and on the surfaces of the respective adhesion promoter layer a layer A made of a partially crystalline thermoplastic polymer is applied, the individual layers of the film Corresponding melts are coextruded through a flat die, the multilayer film thus obtained is removed for consolidation on one or more rollers, the film is then stretched in the longitudinal direction and then stretched in the transverse direction by means of a tenter frame, the ethylene-vinyl-alcohol copolymer layer C and the

Adhesion promoter layers B and layers A are co-extruded with the same width and the clips are gripped together and simultaneously during transverse stretching all five layers. The dependent subclaims indicate preferred embodiments of the invention.

In the context of the present invention, it was found that the transverse stretching forces are introduced into the film by means of the method according to the invention, in which the clips grip layers A, B and C together, in such a way that a considerably better stretching of the EVOH layer together with the others Layers of the coextruded composite is possible. It is believed that the procedure induced a higher stretch Crystallization in layers C and A leads because all layers absorb the transverse stretching forces directly and not, as in the free-edge extrusion according to the prior art, the stretching forces are only absorbed by the base layer and transferred to the other layers. As a result, a better oxygen barrier is achieved than in comparable known structures without the need for additional thermal recrystallization. At the same time, layers that differ more rheologically can be stretched together using the method according to the invention. It is thus possible, for example, to use EVOH polymers with an MFI of less than 8 g / 10 min for the inner EVOH layer without the known fish-eye effect occurring. The method according to the invention thus has two decisive advantages. The films produced by this process have a higher oxygen barrier and the materials for the individual layers can be selected more flexibly, ie also taking other aspects into account and not only in terms of stretchability.

The composition and structure of the individual layers are described below.

Inner layer C:

The inner layer C made of EVOH copolymer (hereinafter referred to as EVOH layer) contains at least 50% by weight, preferably 70 to 100% by weight, in particular 80 to <100% by weight, in each case based on the layer, one below described ethylene vinyl alcohol (EVOH) copolymer. Layer C is referred to as the inner layer, since further layers are applied to both surfaces of layer C.

EVOH copolymers are known per se in the prior art and are produced by saponification or hydrolysis of ethylene-vinyl acetate copolymers. EVOH copolymers are particularly suitable, which one Have a degree of hydrolysis (degree of saponification) of 96 to 99%. Furthermore, the ethylene content should be in the range from 25 to 75 mol%, preferably in the range from 30 to 60 mol%, in particular in the range from 35 to 50 mol%. The melting point is generally in the range from 150 to 190 ° C. The melt flow index at 190 ° and 2.16 g can be in the range from below 8 g / 10 min, preferably in the range from 1 to 7 g / 10 min, in particular 2 to 6 g / 10 min. Surprisingly, according to the method according to the invention, it is possible to stretch the film composite even when using such an EVOH copolymer and to achieve very good barrier values. In a further embodiment, the melt flow index of the EVOH copolymer can also be higher, for example> 8 g / 10min, preferably 10 to 20 g / 10min.

The thickness of the EVOH layer is generally 1 to 10 μm, preferably 2 to 8 μm, in particular 3 to 6 μm. It was found that the adhesive strengths of the layers depend critically on the thickness of the EVOH layer. It is particularly advantageous that this thickness of 10 μm is not exceeded. If the layers are too thick, depending on the EVOH selected, the coextruded layers will already be delaminated after the extrusion on the cooling roll. It is then no longer possible to stretch this layer structure. Surprisingly, the process according to the invention makes it possible to stretch the film sequentially with comparatively high stretching factors, even if the layer thickness of the EVOH layer is over 2 μm.

Bonding agent layer B:

It is essential to the invention that layer A and EVOH layer C are connected to one another via an adhesion promoter layer B. The adhesive layer is therefore applied between the inner ethylene-vinyl alcohol (EVOH) layer and the layer of partially crystalline polyolefin A (force layer), ie it is applied to each surface of the ethylene-vinyl alcohol (EVOH) layer. The adhesive layer B ensures that the ethylene vinyl alcohol (EVOH) layer C and layer A is so firmly bonded to one another that both layers C and A are stretched together when they are gripped simultaneously and jointly by the clips in the transverse stretching frame, and that the ethylene-vinyl alcohol (EVOH) layer is oriented in this way that the adhesion of the individual layers to one another is maintained. The adhesive layer is made up of modified polyolefins. In general, the adhesive layer contains at least 90% by weight, preferably 95 to 100% by weight, in particular 99 to <100% by weight, of the modified polyolefin, in each case based on the weight of the adhesive layer.

The modified polyolefins are based on ethylene polymers or propylene polymers, of which propylene homopolymers, propylene copolymers or propylene terpolymers are preferred. Propylene copolymers or terpolymers predominantly contain propylene units, preferably at least 80-98% by weight, and ethylene and / or butylene units in appropriate amounts as comonomers. These polymers are preferably mixed with maleic anhydride, optionally also with other carboxylic acid units or their esters, e.g. Acrylic acid or its derivatives, modified.

Such modified polypropylenes and polyolefins are known per se in the prior art and are sold, for example, by Mitsui Chemicals under the trade name Admer ^® or by Mitsubishi Chemicals under Modic ^® or by Chemplex under Plexar ^® , and as Epolene ^® by Eastman. The modified polypropylenes are produced from the unmodified polypropylenes and maleic anhydride by reacting maleic anhydride with polypropylenes of suitable viscosity at elevated temperature. A method is described for example in US 3,480,580. The modification is also referred to as the grafting reaction and the modified polypropylenes are accordingly referred to as grafted polymers which are “crafted” or grafted with maleic anhydride.

Are preferred for the purposes of the present invention Propylene homopolymer or propylene copolymers modified with maleic anhydride (eg Q-series from Mitsui Chemicals), their melt index in the range of 1 to 10 g / 10min at 230 ° C (ASTM D 1238) and their Vicat softening point between 110 and 155 ° C lies.

The thickness of the adhesive layer B is generally 0.4 to 4 μm, preferably 0.5 to 3 μm, in particular 0.8 to 2 μm.

Layer A:

To orient a film made of thermoplastic polymer, it is fundamentally necessary that the stretching forces introduced into the film via rollers or a stretching frame or other suitable means act on all layers of the film in order to lead to an orientation of each layer. It has been found in the context of the present invention that a film structure which comprises both a polypropylene layer and an EVOH layer can be stretched particularly advantageously in the longitudinal and transverse directions by the process according to the invention. As already explained, it is important that all layers A, B and C are gripped by the clips in the transverse stretching, as a result of which the stretching forces are absorbed directly by all layers A and C.

According to the conventional free-edge processes, the central base layer is extruded wider than the other layers, so that the clips do not grip the additional layers. Accordingly, when applied to the film structure here, the clips would only grip the central layer made of EVOH, which alone would have to absorb the stretching forces and transfer them to the polypropylene layers. It has been shown that a film can also be produced by this process if the individual layers have excellent adhesion to one another and if comparatively moderate stretching factors are used. This helps you to choose the main components for each Layers very limited, for example to the selection of an EVOH with an MFI of at least 8 g / 10 min or an EVOH with an ethylene content of at least 40 mol%.

From the above explanations it follows that layer C can also be selected from a much larger variety than in the previously known methods. Layer C must have a sufficiently high adhesive strength with respect to adhesive layer B and be suitable for absorbing stretching forces, i.e. a semi-crystalline polyolefin. Otherwise, only care must be taken to ensure that the softening point is not too low in relation to the transverse stretching temperature in the frame, so that sticking of the clips with this layer C in the transverse stretching frame is prevented. All materials that meet these requirements are basically suitable as polyolefin for layer C.

Semi-crystalline polyolefins whose crystallinity is at least 10 to 70%, preferably 30 to 70% and whose melting point is at least 140 ° C. are suitable for layer C. A propylene polymer is preferably used, the ethylene content of which is between 0 and 5% by weight, based on the polymer. Isotactic propylene homopolymers with a melting point of 150 to 170 ° C., preferably 155 to 165 ° C., and a melt flow index (measurement DIN 53 735 at 21.6 N load and 230 ° C.) of 1.0 to 15 g / are particularly suitable. 10 min, preferably from 1.5 to 8 g / 10 min. The n-heptane-soluble proportion of the isotactic propylene homopolymer is generally 1 to 10% by weight, preferably 2 to 5% by weight, based on the starting polymer. The crystallinity of the propylene homopolymer is preferably 40 to 70%, in particular 50 to 70%. The molecular weight distribution of the homopolymer can vary. The ratio of the weight average M _w to the number average M _n is generally between 1 and 15, preferably 2 to 10, very particularly preferably 2 to 6. Such a narrow molecular weight distribution of the propylene homopolymer is achieved for example, by its peroxidic degradation or by production of the polypropylene using suitable metallocene catalysts.

It is preferred that the layer thickness of layer C is 5 to 15 μm, preferably 6 to 10 μm. It was found that with layer thicknesses of less than 5 μm, stretching becomes more difficult and the bond can only be oriented biaxially poorly. With layer thicknesses of over 15 μm, the total thickness of the film becomes unfavorable, although for some applications the total thickness of the film is not subject to an upper limit.

In a further embodiment, layer C can be an opaque layer, as is present as an opaque base layer in known opaque boPP foils. In these embodiments, the layer C is opaque by adding fillers. In general, layer C in this embodiment contains at least 70% by weight, preferably 75 to 99% by weight, in particular 80 to 98% by weight, in each case based on the weight of layer C, one of the above for layer C. described partially crystalline polyolefins or propylene polymers, wherein the propylene homopolymers described are also preferred.

The opaque layer C additionally contains fillers in an amount of at most 30% by weight, preferably 1 to 25% by weight, in particular 2 to 20% by weight, based on the weight of layer C. Fillers are for the purposes of the present Invention Pigments and / or vacuole-initiating particles and are known per se in the prior art.

Pigments are incompatible 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. “Pigments” generally have an average particle diameter of 0.01 to a maximum of 1 μm, and include both so-called “white pigments”, which color the films in white, and “colored pigments”, which give the films a colored or black color. Common pigments are materials 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, among which white pigments such as calcium carbonate, silicon dioxide, titanium dioxide and barium sulfate are preferably used.

"Vacuum-initiating fillers" are solid particles which are incompatible with the polymer matrix and lead to the formation of vacuole-like cavities when stretched in the polypropylene layer, the size, type and number of vacuoles depending on the size and quantity of the solid particles and the stretching conditions, such as stretching ratio and stretching temperature are dependent. The vacuoles reduce the density and give the layer a characteristic pearlescent, opaque appearance, which is caused by light scattering at the "vacuole / polymer matrix" interfaces. As a rule, the vacuole-initiating fillers have a minimum size of 1 μm in order to achieve an effective, i.e. lead opacifying amount of vacuoles. In general, the average particle diameter of the particles is 1 to 6 μm, preferably 1.5 to 5 μm.

Usual vacuole-initiating fillers are inorganic and / or organic materials which are incompatible 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) and

Silicon dioxide, among which calcium carbonate and silicon dioxide are preferably used. Suitable organic fillers are the customarily used polymers which are incompatible with the polymer of the base layer, in particular those such as HDPE, copolymers of cyclic olefins such as norbornene or tetracyclododecene with ethylene or propene, polyester,

Polystyrenes, polyamides, halogenated organic polymers, with polyesters such as polybutylene terephthalates being preferred. "Incompatible materials or incompatible polymers" in the sense of the present invention means that the material or the polymer in the film as a separate

Particles or as a separate phase. The opaque layer C generally contains pigments in an amount of 0.5 to 10% by weight, preferably 1 to 8% by weight, in particular 1 to 5% by weight. Vacuum-initiating fillers are generally present in an amount of 0.5 to 30% by weight, preferably 1 to 15% by weight, in particular 1 to 10% by weight. The information relates to the weight of layer C.

Depending on the composition of the opaque layer C, the density of the opaque layer C and thus the film can vary in a range from 0.4 to 1.1 g / cm ³ . Vacuoles contribute to lowering the density, whereas pigments, such as TiO _2, increase the density of the opaque layer due to their higher specific weight. In opaque embodiments, the density of the opaque layer is preferably 0.5 to 0.95 g / cm ³ .

It has been found that opaque layers, in particular also those with a vacuole-containing structure, are suitable as a layer for transmitting the stretching forces and thus for producing a stretched composite. For such embodiments, too, the layer thickness of the opaque layer C is preferably in the range from 5 to 15 μm.

In a preferred embodiment, in addition to the structure consisting of EVOH layer C, adhesive layers B and layers C, the film according to the invention has at least one, preferably both sides, cover layers which is / are applied to the surface (s) of layers C. With this, six- and seven-layer film structures are realized. These polyolefinic cover layers are then the outer layers of the multilayer film structure and certain functionalities such as sealability, gloss, friction and other properties of the film, which depend on the outer layers. The cover layers are generally composed of polymers from olefins having 2 to 10 carbon atoms. The cover layers generally contain 95 to 100% by weight of a polyolefin, preferably 98 to <100% by weight of polyolefin, based on the weight of the particular layer Top layer.

Examples of suitable olefinic polymers of the outer layers are polyethylenes, polypropylenes, polybutylenes, or copolymers of olefins with two to eight carbon atoms, among which copolymers or terpolymers of ethylene-propylene and / or butylene units or mixtures of the polymers mentioned are preferred. These olefinic polymers preferably contain no functional groups and are only made up of olefinic monomers. Preferred copolymers are

Statistical ethylene-propylene copolymers, preferably with an ethylene content of 1 to 10% by weight, in particular 2.5 to 8% by weight, or

Statistical propylene-butylene-1 copolymers, preferably with a butylene content of 2 to 25% by weight, preferably 4 to 20% by weight, or

Statistical ethylene-propylene-butylene-1 terpolymers, preferably with an ethylene content of 1 to 10% by weight and a butylene-1 content of 2 to 20% by weight, or

• a mixture or a blend of ethylene-propylene-butylene-1-terpolymer and propylene-butylene-1-copolymer with one

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.

The data in% by weight relate in each case to the weight of the copolymer or terpolymer. The above-described copolymers and / or terpolymers used in the top layers and which are composed only of olefins 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 in the range from 120 to 140 ° C. The blend of copolymers and terpolymers described above has a melt flow index of 5 to 9 g / 10 min and one Melting point from 120 to 150 ° C. All melt flow indices specified above are measured at 230 ° C. and a force of 21.6 N (DIN 53 735).

Suitable polyethylenes for the cover layers are HDPE, MDPE, LDPE as are usually used in biaxially oriented packaging films.

The thickness of the respective cover layer is generally greater than 0.1 μm and is preferably in the range from 0.5 to 10 μm, in particular 1 to 5 μm.

The cover layers and / or layer C can additionally contain conventional additives such as neutralizing agents, stabilizers, antistatic agents, antiblocking agents and / or lubricants in effective amounts. The following data in% by weight relate to the weight of the respective top layer.

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 or crosslinked polymers such as crosslinked polymethyl methacrylate or crosslinked silicone oils. Silicon dioxide and calcium carbonate are preferred. The average particle size is between 1 and 6 μm, in particular 2 and 5 μm. The effective amount of antiblocking agent is in the range of 0.1 to 5% by weight, preferably 0.5 to 3% by weight, in particular 0.8 to 2% by weight.

Preferred antistatic agents are alkali alkanesulfonates, 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 with 10 to 20 carbon atoms, with 10 to 20 carbon atoms Hydroxy- (-C-C ₄ ) alkyl groups are substituted, N, N-bis (2-hydroxyethyl) alkylamines having 10 to 20 carbon atoms, preferably 12 to 18 carbon atoms, being particularly suitable in the alkyl radical. The effective amount of antistatic is in the range of 0.05 to 0.5% by weight.

Lubricants 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 0.01 to 3% by weight, preferably 0.02 to 1% by weight. The addition of higher aliphatic acid amides in the range from 0.01 to 0.25% by weight in the base layer is particularly suitable. A particularly suitable aliphatic acid amide are erucic acid amide and stearylamide. The addition of polydimethylsiloxanes in the range from 0.02 to 2.0% by weight is preferred, in particular polydimethylsiloxanes with a viscosity of 5000 to 1,000,000 mm ² / s.

The 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 and phosphitic stabilizers are particularly suitable. Phenolic stabilizers with a molecular weight of more than 500 g / mol are preferred, in particular pentaerythrityl tetrakis 3- (3,5-di-tertiary-butyl-4-hydroxyphenyl) propionate or 1,3,5-trimethyl-2, 4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl) benzene. Phenolic stabilizers are used alone in an amount of 0.1 to 0.6% by weight, in particular 0.1 to 0.3% by weight, phenolic and phosphitic stabilizers in a ratio of 1: 4 to 2: 1 and in one Total amount of 0.1 to 0.4 wt .-%, in particular 0.1 to 0.25 wt .-% used.

Neutralizing agents are preferably dihydrotalcite, 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 m ² / g. The total thickness of the film according to the invention can vary within wide limits and depends on the intended use. It is preferably 4 to 100 μm, in particular 5 to 80 μm, preferably 6 to 60 μm.

In one possible embodiment, the surface (s) of the layer (s) C or the additional cover layer (s) are / are corona, plasma or flame treated. This treatment increases the adhesion to printing inks, adhesives, cold seal layers, metal layers, etc. in a manner known per se.

The individual process steps of the process according to the invention are explained in more detail below:

In this process, the melts corresponding to the individual layers of the film are in principle co-extruded through a flat die, the film obtained in this way is drawn off on one or more rollers for consolidation, the film is then stretched (oriented), the stretched film is fixed and, if necessary, on the surface layer intended for treatment is plasma-corona or flame-treated. The biaxial stretching (orientation) is carried out sequentially, stretching first lengthwise (in the machine direction) and then transversely (perpendicular to the machine direction).

As is customary in the extrusion process, the polymer or the polymer mixture of the individual layers is compressed and liquefied in an extruder, it being possible for the additives which may have been added to be present in the polymer or in the polymer mixture. According to the prior art, the melts of the various layers are placed on top of one another as melt streams and brought together and then pressed together and simultaneously through a flat die (slot die). The The individual layers can be brought together in different areas of the nozzle, so that the melts are superimposed on one another at different times.

According to the prior art, multilayer films are usually extruded according to the so-called free-edge principle, i.e. The width of the cover layers is reduced compared to the width of the base layer, which means that the two edge areas of the film web remain free of cover layer material. According to the free edge method, the holding clips of the transverse stretching frame grip the film at this free edge and thus transfer the stretching forces directly to the central base layer of a film. The method according to the invention is very advantageous over this prior art.

It is essential for the process according to the invention that, in contrast to the conventional free-edge method described, the layer C is extruded with the same width or approximately the same width as the adhesive layers B underneath and the EVOH layer A. This is essential for the clips to take hold of layers C and adhesion promoter layers B and the EVOH layer at the same time. It has been shown that the simultaneous action of the clip forces on the composite A B / C / B / A enables a substantially more uniform and higher stretching of all layers, and thus the EVOH layer can be stretched together with layer C.

In addition, it was found that compliance with certain extrusion conditions is particularly advantageous. It has been found that the adhesion of the individual layers to one another, in particular the adhesion of the EVOH layer to the adhesive layer C, depends on the residence time of the melts placed one on top of the other. It contributes to good adhesion if the individual, already superimposed layers of the film remain in the superimposed, molten state for a certain period of time before exiting the nozzle, so that a more intensive bond between the individual layers is achieved. It is therefore particularly advantageous in a further embodiment for the method according to the invention to ensure a residence time of the melted layers in the nozzle, in particular a residence time of the melt of the adhesion promoter layers on the EVOH melt of at least 6 seconds. The longer this dwell time of the melts lying on top of one another, in particular the adhesion promoter melts on the EVOH melt, is extended in the nozzle, the better the adhesion of the layers. This residence time is preferably 8 to 180 seconds, in particular 8 to 100 seconds. It has been observed that it is easier if the melted layers, in particular the adhesion promoter layers on the EVOH layer, are too short in the nozzle during the subsequent biaxial orientation of the coextruded composite Delamination can occur, especially during transverse stretching, and there may not be sufficient adhesive strength. The lack of adhesion leads to the ethylene vinyl alcohol (EVOH) layer not being oriented, which causes cracks in the layer which are macroscopically perceived as massive optical defects. The films also have no oxygen barrier.

The residence time of the melts in the nozzle can basically be controlled via the nozzle geometry and the extruder output. A nozzle extended in the main flow direction (across the nozzle lip) extends the dwell time. A lower extruder output extends in connection with the correspondingly adjusted deduction and. Running speeds of the take-off rollers also the dwell time.

Of course, it is necessary to heat the nozzle as usual so that the melts lying on top of each other are kept at the necessary temperature. The nozzle temperature is usually 200 to 300 ° C., preferably 210-250 ° C.

The multi-layer melt guided in this way is formed into a flat film in the nozzle and is placed on one or immediately after it emerges from the nozzle several take-off rollers at a temperature of 10 to 100 ° C, preferably 10 to 60 ° C, wherein it cools to a multilayer film and solidifies.

The pre-film thus obtained is then stretched longitudinally and transversely to the direction of extrusion. The longitudinal stretching is preferably carried out at a temperature of 110 to 165 ° C., preferably 120 to 160 ° C., in particular 140 to 160 ° C., expediently with the aid of two rollers running at different speeds in accordance with the desired stretching ratio. The longitudinal stretching ratios are in the range from 2 to 8, preferably 3 to 6, in particular 4 to 6. Surprisingly, stretching factors of more than 4.5 can be used in the process according to the invention, which are common in the stretching of boPP films.

The transverse stretching is preferably carried out at a temperature of 130 to 180.degree. C., preferably 140 to 180.degree. C. with the aid of an appropriate clip frame. As already mentioned, it is essential that the composite of the layers A / B / C / BA be taken together and at the same time by the clips of the quest frame. The transverse stretching ratios are in the range from 3 to 10, preferably 5 to 9.

It can be high according to the inventive method

Area stretch ratios (longitudinal stretch factor * transverse stretch factor) for these

Composites of over 20, preferably 24 to 50, in particular 25 to 40, can be realized using the sequential method.

The stretching of the film is optionally followed by a customary fixation for reducing the tendency to shrink. For this purpose, the film is converged through the frame outlet at a controlled temperature. This has nothing to do with the targeted thermal aftertreatment for recrystallization, in which the film first cools down after transverse stretching and then again is heated to an elevated temperature via heated rollers. Finally, the film is wound up in the usual way with a winding device.

A thermal aftertreatment to recrystallize the EVOH layer to improve the barrier is not necessary in the process according to the invention, but can nevertheless be useful for other reasons. Such thermal aftertreatment at elevated temperature is generally dispensed with.

After the biaxial stretching, one or both surfaces of the film are / are preferably plasma, corona or flame treated by one of the known methods. The treatment intensity is generally in the range from 35 to 50 mN / m, preferably 37 to 45 mN / m.

The corona treatment is expediently carried out in such a way that the film is passed between two conductor elements serving as electrodes, such a high voltage, usually alternating voltage (approximately 5 to 20 kV and 5 to 30 kHz) 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 non-polar polymer matrix.

Unless otherwise stated in the description, the following measurement methods were used to characterize the raw materials and the films:

melt Flow Index

The melt flow index was measured according to DIN 53 735 at 21.6 N load and 230 ° C. melting point

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

Haze The haze of the film was measured according to ASTM-D 1003-52.

shine

The gloss was determined in accordance with DIN 67 530. The was measured

Reflector value 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 strikes 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.

surface tension

The surface tension was measured using the so-called ink method (DIN

53 364).

Permeability to water vapor and oxygen

The water vapor permeability is determined in accordance with DIN 53 122 part 2. The oxygen barrier effect is determined in accordance with DIN 53 380 Part 3 at an air humidity of approx. 50%.

The invention will now be explained in more detail on the basis of exemplary embodiments:

example 1

At a nozzle temperature of 240 ° C, a five-layer film consisting of a base layer C made of ethylene-vinyl alcohol (EVOH) with both sides

Adhesion promoter layers B and polyolefin layers A co-extruded together. All layers were extruded with the same width (no free edge). The melts were then drawn off on a take-off roller and gradually oriented in the longitudinal and transverse directions. The thickness of the layers A was approximately 8 μm, the thickness of the adhesive layers B was approximately 0.8 μm and the thickness of the ethylene vinyl alcohol (EVOH) layer C was 5 μm, corresponding to a total film thickness of approximately 23 μm.

Base layer C: 100% by weight EVOH (EVAL ES104B) with 44 mol% ethylene content) and with a melting point Tm of 156 ° C and a melt flow index of 6.5g / 10min [at 230 ° C; 21, 6N]

Bonding agent layers B: 100% by weight modified maleic anhydride with a polypropylene

Melting point Tm of 160 ° C and a melt flow index of 7 g / 10min to 8g / 10min [at 230 ° C, 21, 6N] (type ADMER QF)

Layers A:

100% by weight isotactic propylene homopolymer with one

Melting point of 162 ° C and a crystallinity of 60% and a melt flow index of 6.0g / 10min

The manufacturing conditions in the individual process steps were:

Extrusion temperatures: base layer C: 220 ° C

Adhesion promoter layer B: 190 ° C

Layers A: 240 ° C

Take-off roller temperature: 30 ° C Longitudinal stretch: Temperature: 155 ° C

Longitudinal stretch ratio: 4.0-5.0 Lateral stretching: temperature: 170 ° C

Lateral stretch ratio: 6.8

Fixation: temperature: 168 ° C

Convergence: 10%

The transverse stretching ratio XQ = 6.8 is an effective value.

This effective value is calculated from the final film width B, reduced by twice the width of the hem stripe b, divided by the width of the elongated film

Foil C, also reduced by twice the hem width b. The oxygen barrier was 17 cm ³ / m ² * day * bar. The water vapor barrier 10.7 g / m ² * d.

Example 2

A film was produced as described in Example 1. In contrast to Example 1, 100% by weight Soranol AT 4403 was used as layer EVOH polymer in layer C. The EVOH had an ethylene content of 44 mol% and a melt index of 3-4 g / 10 min (210 ° C and 2.16 kg) and a melting point of 164 ° C. A maleic anhydride-modified Tymor 220 polypropylene from Morton with a melt flow index of 6 g / 10 min (230 ° C., 16 kg) and a melting point of 163 ° C. was used as the adhesion promoter. The process conditions as well as the layer thicknesses and extrusion widths of the individual layers were not changed apart from the transverse stretching factor. The transverse stretching was 8.5 in this example. In this way, a film with an oxygen barrier of approximately 5 cm ³ / m ² * day * bar was obtained.

Example 3

A film was produced as described in Example 1. In contrast to Example 1, another EVOH was used in the central layer. This EVOH had an ethylene content of 32 mol%, a melting point of approx. 140 ° C. and an MFI of 4.5 g / 10 min. The remaining composition of the layers, the layer thicknesses and the process conditions according to Example 1 were not changed. The film thus obtained had an oxygen barrier of 10.5 cm ³ / m ^{2 *} day * bar.

Comparative Example 1 A film was produced as described in Example 1. In contrast to Example 1, the EVOH layer was extruded about 5% wider than the other layers, so that in the transverse stretching only the EVOH layer was gripped by the clips. The film showed strong tears and optical defects (fish eyes). The oxygen barrier was over 300 cm ³ / m ² * day * bar. The process was not suitable for stretching the film composite.

Claims

claims

1. A process for producing a multilayer biaxially oriented film which comprises at least five layers with the arrangement A / B / C / B / A, the inner layer C being composed of an ethylene-vinyl alcohol copolymer (EVOH layer) and on both sides an adhesion promoter layer B made of modified polyolefin is applied to each surface of the EVOH layer C and a layer A made of a partially crystalline thermoplastic polyolefin is applied to the surfaces of the respective adhesion promoter layer, characterized in that the melts corresponding to the individual layers of the film are provided by a flat die are coextruded, the multilayer film thus obtained is removed for solidification on one or more rollers, the film is then stretched in the longitudinal direction and then in the transverse direction by means of a tenter frame, characterized in that the EVOH layer C and the adhesion promoter layers B and the layer A with the same Br be co-extruded and grasp the claws of the frame when stretching all five layers together and simultaneously!

2. The method according to claim 1, characterized in that modified polyolefin of layers B is a modified with maleic anhydride

Is polypropylene or polyethylene.

3. The method according to claim 2, characterized in that the maleic anhydride modified polypropylene has a melt flow index of 1 to 10 g / 10min and has a softening point between 110 and 155 ° C.

4. The method according to one or more of claims 1-3, characterized in that the adhesive layers each have a thickness of 0.4 to 4 microns.

5. The method according to one or more of claims 1-4, characterized in that the EVOH copolymer has an ethylene content of 30-60 mol% of ethylene and has a melting point in the range of 140-190 ° C.

6. The method according to claim 5, characterized in that the EVOH copolymer has a melt flow index of 1-7g / 10min.

7. The method according to one or more of claims 1 to 6, characterized in that the ethylene-vinyl alcohol (EVOH) layer has a thickness of at most 10 microns

8. The method according to one or more of claims 1 to 7, characterized in that the polyolefin of the layers A has a melting point of 150-170 ° C.

9. The method according to claim 8, characterized in that the polyolefin of the layers A is an isotactic propylene homopolymer with a melting point of 155-165 ° c.

10. The method according to one or more of claims 1 to 9, characterized in that at least one of the two layers A contains vacuole-initiating fillers and / or pigments in addition to the partially crystalline polyolefin and is opaque.

11. The method according to one or more of claims 1 to 10, characterized in that the layers A are each 5 to 15 microns thick

12. The method according to one or more of claims 1 to 11, characterized in that a cover layer is applied to at least one surface of the layers A and this cover layer (s) is / are sealable.

13. The method according to one or more of claims 1-12, characterized in that the melts corresponding to the adhesion promoter layers and the melt corresponding to the EVOH layer lie on one another for at least 6 seconds in the molten state prior to exiting the nozzle.

14. The method according to one or more of claims 1-13, characterized in that the orientation in the longitudinal direction with a longitudinal stretching ratio of 3: 1 to 8: 1 and in the transverse direction with a transverse stretching ratio of 3: 1 to 10: 1.

15. Film produced by a method of claims 1 to 14.

16. Film obtainable by a process according to one or more of claims 1-14, characterized in that the oxygen barrier under

10 cm ³ / m ² * day * bar, where d is the film thickness.

17. Packaging from a film according to claim 15 or 16.

18. lidding film made of a film according to claim 15 or 16.

19. A film according to claim 15 or 16, characterized in that the film is metallized on one surface.

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