Method for the fabrication of polypropylene films

The present invention relates to a method for producing biaxially oriented polypropylene films which comprise at least one layer, and to the use thereof.

Biaxially oriented (boPP films) polypropylene films are used for many different packaging applications due to their good properties. These good usage properties include high mechanical strength, good dimensional stability and a brilliant appearance. In addition to being used as packaging films, boPP films are used in considerable quantities in technical applications. The metallization and the transfer metallization, the lamination and the use as an electrical insulation film in the capacitor production are to be mentioned.

Various methods for producing biaxially oriented polypropylene films are known in the prior art. In the so-called stenter process, the boPP film is produced by extrusion, shaping in a slot die and stretching in the longitudinal and transverse directions.

In particular, this process is carried out in such a way that the propylene polymers are compressed, heated and melted in an extruder, then the melts corresponding to the individual layers of the film are extruded through a flat die, and the film thus obtained is drawn off on one or more rollers for solidification , the film is oriented and then heat set. Finally, the machine roll produced in this way is assembled for the customer-ready cutting roll.

In this production of biaxially oriented films, a large amount of film waste arises due to the process, for example due to the so-called seam strip. Hem strips are film edges that are gripped by the clips of the transverse stretching frame during transverse stretching, but are not stretched together. The undrawn edge is much thicker than the film after orientation and must therefore be cut off. Depending on the type of film and the width of the machine, the loss of material due to the hem strip can amount to up to 25% by weight. For economic reasons, it is necessary to process the hem strip together with the remaining film waste, which arises during assembly, when tearing off or when the machine is started and extended, to regranulate and add it back to the original raw material.

In some areas of application with particularly high quality requirements, such a recycling of film waste with the original raw material is not possible. This applies, for example, to the production of electrical insulation films that are used in the production of capacitors. For these applications, special film properties such as a low dielectric loss factor, a high electrical volume resistance, a high DC and AC voltage resistance and the lowest possible number of defects are required. These properties are achieved, among other things, through the use of a particularly pure polypropylene with a low ash and chlorine content. In addition, the polymers must not have any ionogenic components. It has been shown that these high quality requirements are no longer met when film waste is added to the high-purity raw material. This means that the film waste in the production of electrical films becomes a waste that can only be used in inferior applications, for example in the production of packaging films or injection molding. As a result, the economic losses from film waste in the production of electrical films are particularly pronounced.

The object of the present invention was therefore to provide a method for producing films from high-quality, ie pure polypropylene, which avoids the disadvantages described above. In particular, the process is intended to reduce the economic losses caused by non-recyclable film waste.

This object is achieved by a process of the type mentioned at the outset, in which a propylene polymer B is heated in an extruder and the melt of the propylene polymer B is extruded through a flat die and the melt emerging from the flat die is drawn off on one or more rollers and solidified to form a pre-film is and this pre-film is stretched in the longitudinal and transverse directions, characterized in that<ul id="ul0001" list-style="none" compact="compact"><li>1. a propylene polymer or a propylene polymer mixture A is heated and melted in a second extruder and</li><li>2nd the melt of the propylene polymer or the propylene polymer mixture A is guided to the two sides of the propylene polymer melt B such that both melts are extruded together and simultaneously through the flat die and the propylene polymer or the propylene polymer mixture A forms the edge region of the film during the production process, and</li><li>3rd the edge areas from the propylene polymer or the propylene polymer mixture A are cut off after the longitudinal and transverse stretching of the film.</li></ul>

According to the invention, the propylene polymers or the corresponding mixtures differ with regard to their residual ash and chlorine content, which is why they are referred to as polymer B and A. The residual ash and chlorine content of propylene polymer A are generally at least twice as large, preferably at least three times as large, in particular at least five times as large as the residual ash content of propylene polymer B. This means that propylene polymer B is a higher quality and therefore more expensive polymer than propylene polymer A.

Propylene polymer B is a high purity polymer. For the purposes of the present invention, this means that the residual ash content of propylene polymer B is less than or equal to 70 ppm, preferably ≦ 50 ppm, in particular ≦ 40 ppm, and the chlorine content is less than or equal to 50 ppm, preferably ≦ 20 ppm.

The propylene polymer B generally contains 90 to 100% by weight, preferably 95 to 100% by weight, in particular 98 to 100% by weight, of propylene units, based on the weight of the polymer. In general, the polypropylene B has a melting point of 130 ° C or higher, preferably 140 to 170^OC, and a melt flow index of 0.5 g / 10 min to 10 g / 10 min, preferably 0.8 g / 10 min to 5 g / 10 min, measured at 230 ° C. and a force of 21.6 N (DIN 53 735).

Particularly suitable propylene polymers B have an average molecular weight M_w in the range from 150,000 to 400,000, preferably from 180,000 to 350,000. The molecular weight distribution can vary within wide limits, M._w/ M_n is generally 2 to 15, preferably 2 to 6, in particular 3 to 6.

Among the propylene polymers B described above is isotactic propylene homopolymer with an n-heptane-soluble fraction of 1 to 15% by weight, preferably 1 to 10% by weight, and with a chain isotactic index of the n-heptane-insoluble fraction of ≧ 85%, preferably ≧ 90 %, particularly preferred. Furthermore, copolymers of ethylene and propylene with an ethylene content of 10% by weight or less are copolymers of propylene with C.₄-C₆-Olefins with an 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 suitable. The weight percentages refer to the respective propylene polymer B.

In order to improve certain properties of the polypropylene film according to the invention, stabilizers and / or neutralizing agents and optionally nucleating agents are generally added to the propylene polymer B. In view of the desired electrical properties of the film, no antistatic agent and no lubricant should be added to the propylene polymer B in a preferred embodiment, since these additives impair the electrical properties of the film. 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.

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 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 molar mass 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.

Neutralizing 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 0.5% by weight.

Nucleating agents can be organic substances, preferably dibenzylidene sorbitol or chemically modified derivatives of dibenzylidene sorbitol or sodium bis (4-tert-butylphenyl) phosphate. Other nucleating agents that can be used include metal salts of benzoic acid, preferably sodium benzoate, and quinacridone and quinacridone derivatives. Inorganic nucleating agents such as talc, silicon dioxide or bentonite are also suitable. What is important here is an extremely fine distribution of the nucleating agent, ie the average particle size is at most 1 μm, preferably at most 0.7 μm.

According to the invention, propylene polymer A is different from propylene polymer B in terms of its residual ash content. As explained above, it is not a high-purity polymer. The residual ash content of propylene polymer A is generally in the range from 50 to 800 ppm, preferably 100 to 600 ppm, in particular 200 to 500 ppm. The chlorine content is 20 to 100 ppm, preferably 30 to 70 ppm.

The propylene polymer A generally contains 90 to 100% by weight, preferably 95 to 100% by weight, in particular 98 to 100% by weight, of propylene units, based on the weight of the polymer. In general, propylene polymer A has a melting point of 130 ° C or higher, preferably 140 to 170 ° C, and a melt flow index of 0.3 g / 10 min to 15 g / 10 min, measured at 230 ° C and a force of 21 , 6 N (DIN 53 735).

Particularly suitable propylene polymers A have an average molecular weight M_w in the range from 150,000 to 400,000, preferably from 180,000 to 350,000. The molecular weight distribution of the preferred propylene polymers A can vary within wide limits. M_w/ M_n is generally 2 to 15, preferably 2 to 6, in particular 3 to 5.

The preferred narrow molecular weight distributions can be achieved, for example, by peroxidic degradation of the propylene polymer A. A measure of the degree of degradation is the so-called degradation factor A, which indicates the relative change in the melt flow index according to DIN 53 735 of polypropylene, based on the starting polymer.<maths id="math0001" num=""><math display="block"><mrow><mtext mathvariant="italic">A</mtext><mtext> = </mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">MFI</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">MFI</mtext></mrow><mrow><mtext>1</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP0740993A2_D0001.tif" /></maths><dl id="dl0001" compact="compact"><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>

The degradation factor of propylene polymer A is 3 to 15, preferably 6 to 10. The peroxidic degradation is advantageously carried out by adding organic peroxides. Dialkyl 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 is preferred.

Among the propylene polymers A described above, isotactic propylene homopolymer with an n-heptane-soluble fraction of 1 to 15% by weight, preferably 1 to 10% by weight, and with a chain isotaxy index of the n-heptane-insoluble fraction of ≧ 85%, preferably ≧ 90 %, particularly preferred. Furthermore, copolymers of ethylene and propylene with an ethylene content of 10% by weight or less are copolymers of propylene with C.₄-C₈-Olefins with an 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 suitable. The percentages by weight refer to the respective propylene polymer.

A mixture of the propylene homopolymers and / or copolymers and / or terpolymers and other polyolefins mentioned is also suitable. The mixture contains at least 50% by weight, in particular at least 75% by weight, of the propylene polymer A described above. Suitable other polyolefins in the propylene polymer mixture are, for example, polyethylenes, the proportion of these polyolefins being in the range from 5 to 60% by weight, preferably 10 to 40% by weight, based on the propylene polymer mixture A.

If appropriate, additives can be added to the propylene polymer A or the propylene polymer mixture A in an effective amount in each case, preferred additives are neutralizing agents and / or stabilizers. Optionally, antistatic agents and / or lubricants can also be included. However, it is preferred not to add any such migratable additives, since these can contaminate the frame and other parts of the production system due to evaporation. These contaminants detach from the frame and can drip down onto the film.

The type and amount of the neutralizing agents and the stabilizers have been described above for the propylene polymer B. This information applies to propylene polymer A in the same way.

It has been found that the edge regions, ie the seam strips, made of propylene polymer A are particularly firmly connected to the actual film made of propylene polymer B if the melting points of the two polymers are approximately the same size, ie they should advantageously be no more than 15 ° C, preferably by no more than 10 ° C, differ or be approximately the same size. Furthermore, the melt flow indices (MFI) of the two raw materials must generally be coordinated with one another so that the seam strips and the film are particularly firmly connected to one another. The MFI of propylene polymer A should preferably be a maximum of three times the MFI of propylene polymer B. If appropriate, the melt indices of the two polymers can be of the same size, or the MFI of propylene polymer A can be somewhat smaller than that of propylene polymer B.

It has been found that, with the nozzle setting remaining the same, the thickness of the film edge can be specifically varied or set via the MFI of polymer A without the thickness of the film itself changing. The larger the MFI of polymer A in relation to the MFI of polymer B, the thicker the edge area and vice versa. In this way, the seam strip thickness can be optimized independently of the film to be produced. This is particularly advantageous in the production of very thin films. Here, according to the conventional method, the nozzle in the edge region must be opened comparatively very wide in order to ensure the sufficient thickness of the seam strip. There is a risk that the nozzle lips will be irreversibly bent. According to the method according to the invention, such extreme nozzle settings are no longer necessary in the production of very thin films.

Single-layer or multilayer films can be produced by the process according to the invention. Multilayer polypropylene films comprise at least one, optionally double-sided, top layer (s), which generally 75 to 100% by weight, preferably 90 to 99.5% by weight, based in each case on the weight of the top layer, of polymers of olefins with 2 to 10 Contains carbon atoms.

Examples of such 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 homo-, co- and    Terpolymers, optionally mixed with one or more of the homo-, 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 wt .-%, 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 wt .-%, 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.

The propylene homopolymer used in the cover layer contains for the most part (at least 90%) propylene and 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 1.0 g / 10 min to 20 g / 10 min, preferably 2.0 g / 10 min to 15 g / 10 min.

The copolymers, terpolymers and blends of copolymers and terpolymers described above used in the top layer 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 150 ° C. The melt flow index given above is measured at 230 ° C. and a force of 21.6 N (DIN 53 735).

Optionally, all of the top layer polymers described above can be peroxidically degraded in the same manner as described above for the base layer. The degradation factor for the top layer polymers is generally in a range from 3 to 15, preferably 6 to 10.

Multilayer embodiments of films which are produced by the process according to the invention comprise at least one base layer, which essentially consists of the high-purity propylene polymer B, and the cover layer described above. The base layer can also have cover layers on both sides and optionally additional intermediate layers. For the purposes of the present invention, the base layer is the layer which makes up more than 50 to 100%, preferably 70 to 95%, of the total film thickness. The top layer is the layer that forms the outermost layer of the film.

The total thickness of the films which can be produced by the process according to the invention can vary within wide limits and depends on the intended use. The preferred embodiments of the film have total thicknesses of 2 to 100 μm, with 2 to 50 μm, in particular 2 to 20 μm, being preferred. The thickness of the intermediate layer (s) that may be present is 0.5 to 15 μm, with intermediate layer thicknesses of 1 to 10 μm, in particular 2 to 8 μm, being preferred. 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 0.2 to 5 μm, preferably 0.3 to 2 μm, 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 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 analogously to the total thickness.

In the process according to the invention, the propylene polymer B is first compressed, heated and melted in a main extruder (EXTR. 1). In a second extruder (EXTR. 2) the propylene polymer or the propylene polymer mixture A is also compressed, heated and melted. The melt of the propylene polymer or the propylene polymer mixture A is fed to the two sides of the propylene polymer melt B in such a way that the two melts are extruded together and simultaneously through the flat die and the propylene polymer or the propylene polymer mixture A forms the edge region of the film during the production process. The polymer melt A can be guided to the edge of the melt B, for example, as shown in FIG. 3, with a coextrusion adapter rotated by 90 °.

For comparison, FIG. 2 shows the conventional use of the standard adapter for producing a coextruded three-layer film. The polymer melts are fed analogously via two extruders (EXTR. 1 and EXTR. 2). The melt flows are superimposed as shown (16). The three-layer melt is formed into a multilayer film (ABA) in the nozzle.

In contrast, FIGS. 3 and 3A illustrate the use of the coextrusion adapter 6 in an arrangement rotated by 90 ° (90 ° in relation to FIG. 2) according to the inventive method. The compression, heating and melting of the polymers and the supply of the melt streams A, B are carried out analogously. However, the melt flows A, B are placed side by side as shown (16). In the slot die 8, the melts placed next to one another are formed into a film 10, the edge regions (12) of which are composed of polymer A (from the melt flow of extruder 2). The actual film (14) consists of the polymer B (from the melt flow of the extruder 1).

A mono-nozzle in which the melt A is injected laterally (so-called eye nozzle) is also suitable for the method according to the invention. The basic structure of an eye nozzle is shown in FIGS. 4A and B. The slot nozzle 8 has a bore 20 on the left and right of the main channel 18. The melt B is fed through the main channel 18 into the central part of the nozzle. The melt A flows through the bores 20 into the two edge regions of the nozzle. In this way, the melts A and B are formed side by side to form a film, the edge regions of which consist of the polymer A.

The film extruded in this way is drawn off on one or more rollers for consolidation. It has proven particularly advantageous to keep the take-off roller or rollers by which the pressed film is solidified at a temperature of at least 70 ° C., preferably 80 to 120 ° C.

The pre-film obtained in this way is stretched longitudinally and transversely to the direction of extrusion, which leads to a biaxial orientation of the molecular chains. The biaxial orientation is carried out in succession, with stretching preferably first being longitudinal (in the machine direction) and then transverse (perpendicular to the machine direction). Stretching is preferably in the longitudinal direction by 4: 1 to 9: 1, in particular 5.5: 1 to 8.5: 1, and in the transverse direction preferably by 6: 1 to 11: 1. 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. The clips grip the edge area of ​​the film, so that essentially only polymer B is stretched into a thin film (14) and the edges (12) of polymer A remain undrawn and thick. A corresponding schematic illustration is shown in FIG. 1.

The temperatures at which the longitudinal and transverse stretching are carried out can vary within a wide range and depend on the particular composition of the layers and on the desired properties of the film. In general, the longitudinal stretching is carried out at 80 to 160 ° C., preferably 100 to 160 ° C., and the transverse stretching at 120 to 170 ° C., preferably 130 to 160 ° C.

The biaxial stretching of the film is followed by its heat setting (heat treatment), the film being held at a temperature of 100 to 160 ° C., preferably 110 to 130 ° C., for about 0.1 to 10 s.

It is preferred, as mentioned above, after the biaxial stretching, to treat one or both surfaces of the film by corona or flame treatment using one of the known methods. The treatment intensities are in the usual range, 35 to 50 mN / m, preferably 36 to 45 mN / m.

For 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.

After the surface treatment or after heat setting, the film is hemmed with conventional cutting devices and the film itself is wound up with a winding device known per se. In general, the width of the edge to be seamed is up to 300 mm, preferably 100 to 200 mm, and consists of at least 30% by weight, preferably 50 to 100% by weight, in particular 70 to 90% by weight, of the Propylene polymers A. In conventional production processes, the edge of the film has a thickness of up to 200 μm, generally from 20 to 100 μm, preferably from 20 to 50 μm. Depending on the circumstances, edge thicknesses above or below may also be suitable. The entire cut (hemmed) edge areas are expediently chopped, granulated and can then be used as regranulate in the production of packaging film. The regranulate, which is to be reused later, can be specifically adjusted to this reuse by a suitable choice of the edge strip polymer A. This was not possible with the conventional methods.

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

example 1

A transparent film with a thickness of 6 μm was produced by coextrusion and subsequent stepwise orientation in the longitudinal and transverse directions. The film produced has one layer. The adapter technology was used to produce the adjacent melts "seam strip-foil seam strips". The usual adapter arrangement is rotated by 90 °.

A original polymer:

<dl id="dl0002" compact="compact"><dt>100 % By weight</dt><dd>isotactic polypropylene from Borealis with the brand name ®VB 2142 E, melt index = 2.2 g / 10 min</dd></dl>

B seam strip polymer:

<dl id="dl0003" compact="compact"><dt>100 % By weight</dt><dd>isotactic polypropylene from Hoechst with the brand name ®Hostalen PPN 1060, melt index = 3.0 g / 10 min</dd></dl>

The manufacturing conditions in the individual process steps are: <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" align="left">Extrusion:</entry><entry namest="col2" nameend="col2" align="left">Temperature:</entry><entry namest="col3" nameend="col3" align="left">A layer</entry><entry namest="col4" nameend="col4" align="center">280 ° C</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" /><entry namest="col3" nameend="col3" align="left">B layers</entry><entry namest="col4" nameend="col4" align="center">280 ° C</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="left">Take-off roller temperatures</entry><entry namest="col4" nameend="col4" align="center">90 ° C</entry></row><row><entry namest="col1" nameend="col1" align="left">Longitudinal stretch:</entry><entry namest="col2" nameend="col2" align="left">temperature</entry><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="center">150 ° C</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="left">Longitudinal ratio</entry><entry namest="col4" nameend="col4" align="center">5,5</entry></row><row><entry namest="col1" nameend="col1" align="left">Transverse stretching:</entry><entry namest="col2" nameend="col2" align="left">temperature</entry><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="center">160 ° C</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="left">Cross-section ratio</entry><entry namest="col4" nameend="col4" align="center">9,5</entry></row><row><entry namest="col1" nameend="col1" align="left">Fixation:</entry><entry namest="col2" nameend="col2" align="left">temperature</entry><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="center">140 ° C</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="left">convergence</entry><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="center">15 %</entry></row></tbody></tgroup></table></tables>

Example 2

In comparison to Example 1, the melts were arranged and shaped via a so-called eye nozzle (see FIGS. 4A, 4B).

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

Melt flow index:

The melt flow index was measured in accordance with 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.

Crystallinity:

The 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:

The samples were examined using DSC (differential scanning calorimetry). The heating rate was 20 K / min. In order to eliminate the thermal history in the sample, the sample was first measured in the DSC device above the glass transition 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:

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

Residual ash content:

To measure the residual ash content, the proportion of non-combustible mineral fillers is determined quantitatively. The residual ash content (loss on ignition) is calculated from the initial weight of the sample and the ignition residue. The measurement result is given in ppm. A representative sample of approx. 1 kg is taken from the material to be tested (granulate, regrind, etc.). The material must be clean and completely dry; it may be predrying at approx. 80 ° C in the circulating air oven required. Three empty porcelain crucibles are annealed for at least 1 hour at a temperature of 650 ° C in the crucible furnace and weighed to an accuracy of 0.1 mg after cooling in the desiccator to room temperature. The annealing is repeated until constant weight is reached between two consecutive weighings. Then 50 g (± 0.1 g) of material is weighed into each crucible and placed in the muffle furnace at 650 ° C. The temperature in the furnace is now raised to 1,000 ° C and annealed at this temperature for at least 1 h. After the crucibles have cooled in the desiccator, they are weighed to an accuracy of 0.1 mg. The ash content is measured in ppm (parts per million) = mg / m³ specified. All three crucibles are evaluated according to the following formula and the two least differing values ​​are combined to form an average:<maths id="math0002" num=""><math display="block"><mrow><mtext mathvariant="italic">ppm</mtext><mtext> = </mtext><mfrac><mrow><mtext mathvariant="italic">Scale</mtext><mtext> (</mtext><mtext mathvariant="italic">G</mtext><mtext>)</mtext></mrow><mrow><mtext mathvariant="italic">Weight</mtext><mtext> (</mtext><mtext mathvariant="italic">G</mtext><mtext>)</mtext></mrow></mfrac><mtext></mtext><mtext mathvariant="italic">x</mtext><mtext> 1 000 000</mtext></mrow></math><img file="EP0740993A2_D0002.tif" /></maths>

Chlorine content:

The chlorine content in polyolefins is measured quantitatively by means of X-ray fluorescence analysis (RFA) in accordance with DIN 51 001, part 1. A granulate / powder tablet is pressed, which is measured with the XRF against a calibration curve. The calibration curve was set up using 10 calibration samples, in which the chlorine content was determined using an independent method (wet technology).

Molecular weight determination:

To determine the average molecular weight M_w three-detector gel permeation chromatography is used. The substance is dissolved in an eluent such as THF and passed through a separation column. The separation column is 90 cm long and filled with a porous carrier material whose pore size is 5 µm. The detection is carried out by means of UV absorption spectroscopy at different wavelengths, as well as by means of the refractive index and light scattering capacity of the fractions. The calibration is carried out using a standard compound with a known molecular weight. The comparison of the UV absorption of the standard substance with the absorption of the sample enables the molecular weights to be assigned (DIN 55 672 Part 1).