EP1863637A1

EP1863637A1 - Label film for a blow moulding method

Info

Publication number: EP1863637A1
Application number: EP06707585A
Authority: EP
Inventors: Karl-Heinz Kochem; Bertram Schmitz; Mathias Roth; Wilfrid Tews
Original assignee: Treofan Germany GmbH and Co KG
Current assignee: Treofan Germany GmbH and Co KG
Priority date: 2005-03-19
Filing date: 2006-03-16
Publication date: 2007-12-12
Also published as: WO2006099990A1; AU2006226596B2; HK1118256A1; US8968632B2; AU2006226596A1; MX2007011517A; CA2601989A1; CN101142082A; CN101142082B; US20130037983A1; CA2601989C; US20090041965A1

Abstract

The invention relates to a biaxial oriented film with a microporous layer, comprising a propylene polymer and at least one ß-nucleating agent the microporosity of which is generated by conversion of ß-crystalline polypropylene on drawing the film and the use thereof for the labelling of containers in blow moulding. The Gurley value for the film is 10,000 to 300,000 Gurley.

Description

Label film for blow molding

The present invention relates to a microporous layer biaxially oriented film containing propylene polymers and at least one β-nucleating agent, the microporosity of which is produced by converting β-crystalline polypropylene when the film is stretched and its use as an in-mold label in blow molding.

Label films cover an extensive and technically complex field. A distinction is made between different labeling techniques, which are fundamentally different with regard to the process conditions and inevitably place different technical requirements on the label materials. All labeling processes have in common that, as the end result, visually appealing labeled containers must result, in which good adhesion to the labeled container must be ensured.

The labeling, various techniques are very used for applying the label ^■. A distinction is made between self-adhesive labels, wrap-around labels, shrink-on labels, in-mold labels, patch labeling, etc. The use of a thermoplastic film as a label is possible in all these different labeling methods.

In-mold labeling also differentiates between different techniques that use different process conditions. All in-mold labeling is common that the label participates in the actual molding process of the container and is applied during its. However, very different shaping methods are used, such as injection molding, blow molding, deep drawing.

In the injection molding process, a label is inserted into the injection mold and back-injected by a molten plastic. Due to the high temperatures and pressures, the label connects to the injection molding and becomes an integral, non-removable component of the molding. By this method, for example, cups and lids of ice cream or margarine cups are made.

In this case, individual labels are removed from a stack or cut from a roll and inserted into the injection mold. The mold is designed so that the melt stream is injected behind the label and the front of the film rests against the wall of the injection mold. When spraying, the hot melt combines with the label. After spraying, the tool opens, the molded part with label is ejected and cools down. As a result, the label must adhere wrinkle-free and visually flawless on the container.

When spraying the injection pressure is in a range of 300 to 600 bar. The plastics used have a melt flow index of around 40 g / 10 min. The injection temperatures depend on the plastic used. In some cases, the mold is additionally cooled to avoid sticking of the mold to the mold.

When deep drawing unoriented thick plastic plates, usually cast PP or PS, heated to a thickness of about 200 .mu.m and pulled or pressed by vacuum or punch tools in a corresponding mold. Again, the single label is inserted into the mold and connects to the actual container during the molding process. Significantly lower temperatures are used so that label adhesion to the container can be a critical factor. The good adhesion must be ensured even at these low processing temperatures. The processing speeds of this method are lower than in injection molding.

Even when blow molding containers or hollow bodies, direct in-mold labeling is possible. In this process, a molten tube is extruded vertically downwards through an annular die. A vertically split mold moves together and encloses the hose, which is squeezed at the bottom. At the top of a mandrel is introduced through which the opening of the molding is formed. Air is supplied to the hot melt tube via the mandrel, so that it expands and abuts against the inner walls of the mold. Here, the label must connect with the viscous plastic of the molten tube. Subsequently, the mold is opened and the supernatant cut off at the shaped opening. The molded and labeled container is ejected and cools down.

In these blow molding, the pressure during inflation of the molten tube is about 4-15 bar and the temperatures are much lower than in injection molding. The plastic materials have a lower MFI than in injection molding to form a dimensionally stable molten tube and therefore behave differently during the cooling process than the low viscosity materials for injection molding.

In these blow molding processes, too, more and more biaxially oriented films made of thermoplastic materials are used for labeling containers during molding. For this purpose, the films must have a selected property profile in order to ensure that the label film and the blown molding body conform to each other smoothly and bubble-free and join together. For this purpose, various solutions have been proposed in the prior art.

It is known, for example, in the prior art that in the case of in-mold blow molding, air pockets which, as large bubbles, impair optics and adhesion can be reduced by a particular surface roughness of the film.

For this purpose, the film on the side facing the container must have a roughness in the micron range, which allows displacement of the air during labeling.

Such roughness, for example, by a special recipe of

Cover layer of multilayer films or by structuring the surface generated.

Thus, US Pat. No. 5,254,302 describes a BOPP film whose reverse side has been modified by embossing a defined surface structure. After embossing, the film is coated on this side with a hot-melt adhesive system such that the surface structure is retained. The adhesive system ensures the adhesion of the label film to the molding and the structured surface prevents the formation of bubbles.

US Pat. No. 4,986,866 describes a multi-layered paper-like label film having a sealable cover layer which is mechanically coined by means of rollers before the drawing process. Again, this surface structure is to ensure the venting and allow bubble-free adhesion of the label.

DE 199 49 898 describes the use of a polypropylene film with an average roughness of at least 3.5 μm for the labeling in the blow molding process. This roughness is produced by a polypropylene blend in the topcoat, which blend consists of polypropylene and incompatible or partially compatible thermoplastic polymers.

In addition to these bubbles, the formation of a so-called orange peel occurs as a further independent unwanted effect in blow-molding labeling. This effect has nothing to do with the big bubbles caused by lack of ventilation. Orange peel does not appear in the form of isolated more or less large bubbles, but rather the entire label surface is uneven with some regularity, so that the appearance of the surface texture of an orange is very similar, therefore often referred to as orange peel. Sometimes this disorder is also referred to as dermis or "leathery effect". Various solutions have been proposed to reduce the orange peel effect. One direction of development is based on the assumption that the orange peel results from shrinkage of the blow molded container on cooling. On the other hand, the moldings of the in-mold shrink during cooling Injection molding very strong, yet this method is much less prone to disturbing orange peel effects.

EP 0 559 484 describes a film for in-mold labeling, wherein no distinction is made between injection molding in-mold and blow-molding in-mold. The film has a top layer of polyethylene and fillers applied to a vacuole-containing base layer. The polyethylene layer is the

Container facing and on the opposite outside can more

Layers are applied. According to this teaching, the occurrence of a dermis effect can be masked by further pigmented outer layers.

EP 0 546 741 describes a film having a vakuolenhaltigen cover layer, which is applied to a non-vacuole-containing base layer. The vakuolenhaltige cover layer faces the container in the in-mold process. According to this teaching, the orange peel is formed by shrinkage of the vacuum-containing label film during injection molding and can be avoided by avoiding too much vacuole formation and reducing the filler content of the film.

In contrast, WO00 / 12288 teaches that less orange peel occurs due to controlled shrinkage of the label and suggests the improvement of the

Orange peel effect during blow molding due to certain shrinkage properties of the film. Thereafter, the in-mold label film should have a shrinkage of at least 4% in both directions at 130 ⁰ C and 10min. Through this

Shrinkage occurs during blow-molding labeling less orange peel. At the same time, however, this teaching confirms that too low a density increases again

Orange peel formation leads. It is therefore additionally recommended to keep the density of the film in a range of 0.65 to 0.85 g / cm ³ .

In practice, it has been found that all blow molding processes are much more susceptible to orange peel effects in labeling than the in-mold labeling methods of injection molding. All of the known teachings do not satisfactorily solve the problem of the formation of orange skin in the use of biaxially oriented films in in-mold blow molding, or they have other serious disadvantages. Although the proposed measures show some reliable results in injection molding applications, in the blow molding process the appearance of the label on the container is still poor and severely affected by orange peel.

EP 0 865 909 describes the use of "microvoided" films for labels. The film contains a β-nucleating agent, by which an increased proportion of β-crystalline polypropylene is produced in the prefilm during cooling of the melt film. When stretching the pre-foil "microvoids" are generated. It is described that the film has good printability.

EP 1 501 886 describes the use of a biaxially oriented polypropylene microporous film containing β-nucleating agents. The microporosity is generated by converting β-crystalline polypropylene on stretching the film. Due to the high porosity, the film can advantageously be used for labeling containers during blow molding. The methods of making the film are very slow in practice to ensure the required high porosities. In addition, high porosities mechanically weaken the film in such a way that it frequently breaks down in the stretching frame. This makes the film more expensive and makes economic use difficult despite technical advantages.

The object of the present invention was to provide a label film which should have a good adhesion and no orange peel in the in-mold labeling in the blow-molding process and which can be produced with sufficient production speed and production reliability.

The object on which the invention is based is achieved by a biaxially oriented film with a porous layer containing polypropylene and β-nucleating agent. contains medium and whose microporosity is produced by conversion of ß-crystalline polypropylene during stretching of the film and whose Gurley value is> 10,000 s. The microporous layer is an outer layer of the film. The object is further achieved by the use of this film for labeling containers during blow molding.

It has been found that, surprisingly, a film having a microporous layer whose Gurley value is greater than 10,000s can be used excellently in blow-molding labeling and no orange peel occurs under a wide variety of process conditions when this microporosity is indirectly produced by β-nucleating agents. According to the teaching of EP 1 501 886, a high porosity of the film is required in order to ensure a good venting via the high gas permeability of the porous layer, which is in contact with the container. In the context of the present invention, it has surprisingly been found that the high porosities, although favorable, are not required. As such, the net-like structure of the porous layer contributes to deaeration even with comparatively low gas permeabilities (high Gurley values) and prevents blistering during labeling surprisingly effectively when this layer faces the container during labeling. This makes it possible to increase the production speeds significantly. The film is mechanically much more stable overall, which reduces the number of tears in the production. The invention thus has considerable economic advantages and the film according to the invention can be used without blistering during blow-mold labeling.

The microporous structure of the porous layer is significantly different from that of conventional vacuole-containing films. FIGS. 2a and 2b show the typical structure of a vacuole-containing layer in cross-section (2a) and in a plan view (2b). As a result of the incompatibility of the vacuole-initiating particles, tearing occurs between the surface of the particle and the polymer matrix during stretching and a closed, air-filled cavity is formed, in which the incompatible particle is located. These cavities are called vacuole or The vacuoles are distributed over the entire layer and reduce the density of the films or of the layer, but these films still have a good barrier, for example to water vapor, since the vacuoles are closed and the structure as a whole is not permeable Opaque films with a vacuole-containing layer result in the blow-molding process, the unwanted orange peel.

In contrast, the porous layer is gas-permeable and has an open-pore network structure. This structure is not caused by incompatible fillers or particles, but by a technically very different process. The microporous layer contains polypropylene and β-nucleating agents. This mixture of polypropylene with ß-nucleating agent is first melted as usual in the film production in an extruder and extruded through a slot die as a melt film on a cooling roll. The β-nucleating agent promotes the crystallization of β-crystalline polypropylene during cooling of the melt film, resulting in an unstretched prefilm with a high content of β-crystalline polypropylene. When stretching this prefilm, the temperature and stretching conditions can be chosen so that the ß crystallites convert into the thermally stable alpha phase of the polypropylene. Since the density of the β-crystallites is lower, this conversion is accompanied by volume shrinkage, thereby resulting in the characteristic porous structure, similar to a ruptured network.

Surprisingly, it has been found that a film with a porous layer can be used equally well in the blow molding process as a label film, even if the porosity is significantly lower and the structure has less open-pored regions. As can be seen from FIGS. 1a (top view) and 1b (cross section), highly porous foils according to EP 1 501 886 have such an open-pore network structure that open pores are uniformly distributed over the entire surface. The films according to the invention according to FIGS. 3a and 3b have a similar fibril-like structure, but there are substantially fewer pores on the surface, so that the surface also has regions which are almost free are closed. Surprisingly, it is sufficient that the pores are present only in partial areas of the surface in order to avoid blistering during blow-mold labeling.

The composition of the microporous layer, hereinafter also referred to as layer, will now be described in detail in detail. The microporous layer contains propylene homopolymer and / or a propylene block copolymer, optionally additionally other polyolefins, and at least one .beta.-nucleating agent, and optionally additionally conventional additives, for example stabilizers, neutralizing agents, lubricants, antistatic agents, pigments in respective effective amounts. In general, additional incompatible Vakuolenintierende fillers such as calcium carbonate or polyester, such as PET or PBT, omitted, so that the layer contains less than 5 wt .-%, preferably 0 to at most 1 wt .-% of Vakuoleninitierenden fillers. Such small amounts can get into the layer, for example via the incorporation of film regenerate.

In general, the layer contains at least 70% by weight, preferably from 80 to 99.95% by weight, in particular from 90 to 97% by weight, of a propylene homopolymer and / or propylene block copolymer and from 0.001 to 5% by weight, preferably 0, 1 to 3 wt .-% of at least one ß-nucleating agent, each based on the weight of the layer.

Suitable propylene homopolymers contain 80 to 100 wt .-%, preferably 90 to 100 wt .-% of propylene units and have a melting point of 140 ⁰ C or higher, preferably 150 to 17O ⁰ C, and generally a

Melt flow index of 0.5 to 10 g / 10 min, preferably 2 to 8 g / 10 min, at 230 ⁰ C and a force of 2.16 kg (DIN 53735). Isotactic propylene homopolymers having an atactic content of 15% by weight and less are preferred propylene polymers for the layer, with isotactic propylene homopolymer being particularly preferred. Suitable propylene block copolymers contain predominantly, ie more than 50% by weight, preferably 70 to 99% by weight, in particular 90 to 99% by weight, of propylene units. Suitable comonomers in appropriate amount are ethylene, butylene or higher alkene homologues, of which ethylene is preferred. The melt flow index of the block copolymers is within a range of 1 to 15 g / 10 min, preferably 2 to 10 g / 10min (230 ⁰ C; 2.16 kg). The melting point is above 140 ⁰ C, preferably in a range of 150 to 165 ⁰ C.

The stated percentages by weight relate to the respective polymer.

Blends of propylene homopolymer and propylene block copolymer contain these two components in any mixing ratios. Preferably, the ratio of propylene homopolymer to propylene block copolymer ranges from 10 to 90 wt% to 90 to 10 wt%, preferably, 20 to 70 wt% to 70 to 20 wt%. Such mixtures of homopolymer and block copolymer are particularly preferred and improve the appearance of the microporous layer, as well as the stretchability.

Optionally, the porous layer may contain other polyolefins in addition to the propylene homopolymer and / or propylene block copolymer. The proportion of these other polyolefins is generally below 30 wt .-%, preferably in a range of 1 to 20 wt .-%. Other polyolefins include, for example, random copolymers of ethylene and propylene having an ethylene content of 20% by weight or less, random copolymers of propylene with C ₄ -C ₈ olefins having an olefin content of 20% by weight or less, terpolymers of propylene, Ethylene and butylene having an ethylene content of 10% by weight or less and having a butylene content of 15% by weight or less, or polyethylenes such as HDPE, LDPE, VLDPE, MDPE and LLDPE.

As β-nucleating agents for the microporous layer, basically all known additives are suitable which promote the formation of β-crystals upon cooling of a polypropylene melt. Such ß-nucleating agents, as well as their Operation in a polypropylene matrix are well known in the art and will be described in detail below.

From polypropylene, various crystalline phases are known. When a melt is cooled, the α-crystalline PP, whose

Melting point at about 158 - 162 ⁰ C. By a certain temperature control, a small proportion of β-crystalline phase can be generated on cooling, which compared to the monoclinic α-modification with 148-150 ⁰ C a much lower

Has melting point. In the prior art additives are known which lead to an increased proportion of the β-modification in the crystallization of the polypropylene, for example γ-quinacridones, dihydroquinacridines or calcium salts of phthalic acid.

For the purposes of the present invention, highly active β-nucleating agents are preferably used in the porous layer which can produce a β-content of 10-80%, preferably 20-60%, upon cooling of the melt film. For this purpose, for example, a two-component nucleation system of calcium carbonate and organic dicarboxylic acids is suitable, which is described in DE 3610644, which is hereby incorporated by reference. Particularly advantageous are calcium salts of dicarboxylic acids, such as calcium pimelate or calcium suberate as described in DE 4420989, to which also expressly incorporated by reference. The dicarboxamides, in particular N, N-dicyclohexyl-2,6-naphthalenedicarboxamides described in EP-0557721 are also suitable β-nucleating agents.

In addition to the nucleating agents, maintaining a certain temperature range and residence times at these temperatures when cooling the melt film is important for achieving a high level of β-crystalline polypropylene. The cooling of the melt film is preferably carried out at a temperature of 60 to 130 ⁰ C, in particular 80 to 120 ⁰ C. According to the invention, the very slow cooling, which promotes the growth of ß-KristaHite, compared to EP 1 501 886 done faster. The withdrawal speed, ie the The speed at which the melt film passes over the first chill roll should be chosen so that the residence times at the given temperatures allow the growth of the β crystals. In this case, it is no longer necessary to achieve a maximum concentration of β-crystals in the prefilm by maximum residence times on the take-off roll. Rather, the production rate can be increased to such an extent that the concentration of β-crystallites in the prefilm is in the range of 20 to 60%, so that the film thus produced has a Gurley value of 10,000 to 300,000 s after the biaxial stretching. The take-off speed can vary greatly depending on the dimension of the take-off roll and its temperature and is preferably less than 35 m / min, in particular 1 to 20 m / min.

Particularly preferred embodiments contain from 0.001 to 5 wt .-%, preferably 0.05 to 3.0 wt .-%, in particular 0.1 to 1, 0 wt .-% calcium pimelate or calcium suberate in the microporous layer of propylene - homopolymer.

In general, the microporous label film is single-layered and consists only of the microporous layer. It goes without saying, however, that this monolayer film can optionally be provided with a printing or a coating or an additional cover layer before it is used as a label film in blow molding. The thickness of the porous layer is generally in a range of 20 to 150 μm, preferably 30 to 100 μm. According to the invention, the outer surface of the porous layer is not covered with further layers, i. neither printing nor coating, lamination or any other processing which would result in the pores of the porous layer being covered on this side of the film. Thus, the surface of the porous layer forms a surface of the film.

Optionally, the microporous layer on the outside may be provided with a corona, flame or plasma treatment to improve the adhesive properties and wettability. The density of the microporous layer is generally in a range of 0.3 to 0.85 g / cm ³ , preferably 0.4 to 0.7 g / cm ³ 'which corresponds to the density of the film in single-layered embodiments. Surprisingly, it has been found that a particularly low density does not lead to an enhancement of the orange peel effect, as with vacuole-containing, opaque films. With respect to vakuolenhaltiger, opaque films teach pamphlets that a too low density by excessive voiding leads to an enhanced orange peel effect. Surprisingly, this is not the case with porous films. The density can be reduced to extremely low levels and the film can still be applied properly during blow molding, without a disturbing orange peel occurs.

In a further embodiment, the microporous layer can be provided with a further covering layer, the microporous layer, in the case of the use according to the invention of this multilayered embodiment, facing the container and joining with the molded body during blow molding. Accordingly, the additional cover layer forms the outside of the label in the use according to the invention. The additional covering layer can be applied for example by lamination or lamination of the porous layer with another film. Preferably, it is a coextruded topcoat. Optionally, a coating is possible.

Coatings can be applied by conventional methods. Coatings are for example of acrylic acids, acrylates, PVOH or other polymers which are suitable as sealable or printable surface layers. Such coatings are described in detail, for example, in US Pat. No. 6,013,353 (column 6), to which disclosure reference is expressly made.

The optionally coextruded cover layer generally contains at least 70% by weight, preferably 75 to <100% by weight, in particular 90 to 98% by weight, of a polyolefin, preferably of a propylene polymer and optionally further conventional additives such as neutralizing agent, stabilizer, antistatic agents, lubricants, for example fatty acid amides or siloxanes or antiblocking agents in respective effective amounts.

The propylene polymer of the cover layer is, for example, a propylene homopolymer as already described above or for the porous layer

Copolymer of propylene and ethylene or propylene and butylene or propylene and another olefin of 5 to 10 carbon atoms. For the purposes of the invention, terpolymers of ethylene and propylene and butylene or ethylene and propylene and another olefin of 5 to 10 carbon atoms are also suitable for the topcoat. Furthermore, mixtures or blends of two or more of said copolymers and terpolymers can be used.

For the cover layer, ethylene-propylene random copolymers and ethylene-propylene-butylene terpolymers are preferred, in particular ethylene-propylene random copolymers having an ethylene content of 2 to 10% by weight, preferably 5 to 8% by weight, or random Ethylene-propylene-butylene-1 terpolymers having an ethylene content of 1 to 10 wt .-%, preferably 2 to 6 wt .-%, and a Butylen- 1 content of 3 to 20 wt .-%, preferably 8 to 10 Wt .-%, each based on the weight of the co- or terpolymer.

The above-described random copolymers and terpolymers generally have a melt flow index of 1.5 to 30 g / 10 min, preferably from 3 to 15 g / 10 min. The melting point is in the range of 105 ⁰ C to 14O ⁰ C. The above-described blend of copolymers and terpolymers has a melt flow index of 5 to 9 g / 10 min and a melting point of 120 to 150 ⁰ C. All Schmelzflußindices given above are at 230 ⁰ C and a force of 2.16 kg (DIN 53735) measured.

The thickness of this cover layer is generally in a range of 0.1 to 10 .mu.m, preferably 0.5 to 5 .mu.m. Optionally, the surface of this

Topcoat to improve the printability with a corona, flame or

Plasma treatment are provided. The density of the film is not through the porous covering layer, which also contains no vacuoles, increases only insignificantly compared to single-layered embodiments and therefore generally ranges from 0.35 to 0.85 g / cm 3, preferably from 0.4 to 0.65 g / cm 3, for these embodiments ,

Optionally, the topcoat may additionally contain customary additives such as stabilizers, neutralizing agents, antiblocking agents, lubricants, antistatic agents, etc., in customary amounts.

The porous film according to the invention is preferably produced by the extrusion process known per se or by the coextrusion process (flat film process).

In this process, the procedure is that the polypropylene, which is mixed with ß-nucleating agent, melted in an extruder and extruded through a flat die onto a take-off roll, on which the melt solidifies to form the ß-crystallites. In the case of the two-layer embodiment, the corresponding coextrusion takes place together with the cover layer. The cooling temperatures and cooling times are chosen so that a sufficient proportion of β-crystalline polypropylene is formed in the prefilm. This prefilm with ß-crystalline polypropylene is then stretched biaxially so that it comes to the transformation to a transformation of ß-crystallites in alpha polypropylene. The biaxially stretched film is finally heat-set and optionally corona-, plasma- or flame-treated on one or both surfaces.

The biaxial stretching (orientation) will generally be performed sequentially, preferably stretching first longitudinally (in the machine direction) and then transversely (perpendicular to the machine direction).

The take-off roller or the draw-off rollers, in order to promote the formation of a high proportion of beta-crystalline polypropylene at a temperature of 60 to 130 ⁰ C, preferably maintained from 80 to 12O ⁰ C. In the drawing in the longitudinal direction, the temperature is less than 14O ⁰ C, preferably 90 to 125 ⁰ C. The ^draw ratio is in the range 3: 1 to 5: 1 The stretching in the transverse direction is carried out at a temperature of greater than 140 ⁰ C, preferably at 145 to 16O ⁰ C. The transverse stretch ratio is stretched in a range of 3: 1 to 7: 1.

The longitudinal stretching will be carried out expediently with the help of two different speeds corresponding to the desired stretching ratio and the transverse stretching with the aid of a corresponding clip frame.

The biaxial stretching of the film is generally followed by its heat-setting (heat treatment), the film being kept at a temperature of 110 to 150 ^° C. for about 0.5 to 10 seconds. Subsequently, the film is wound in the usual manner with a winding device.

Preferably, as mentioned above, after biaxial stretching, one or both surfaces of the film are corona, plasma or flame treated by one of the known methods. Such a surface treatment is particularly preferred on the opposite surface of the porous layer (outside of the label), if in the context of further processing, a printing and / or metallization is provided.

For the alternative corona treatment, the film is passed between two conductor elements serving as electrodes, with such high voltage, usually alternating voltage (about 10,000 V and 10,000 Hz), applied between the electrodes so that spraying or corona discharges can take place. By the

Spray or corona discharge, the air is ionized above the film surface and reacts with the molecules of the film surface, so that polar deposits are formed in the substantially non-polar polymer matrix. The

Treatment intensities are within the usual range, with 38 to 45 mN / m being preferred. By this method, a porous film having an opaque appearance is obtained. The porous layer has a net-like structure with interconnected pores (see Figures 3a and 3b), which is permeable to gases. According to the invention, these films in a single-layer embodiment have a Gurley value in the range from 10,000 to 300,000 sec. In multi-layered embodiments with a gas-impermeable cover layer, the porous layer has a corresponding structure so that comparable Gurley values for the layer are present.

According to the invention, the porous film is used in the blow molding process. Details of the blow molding process have already been described above in connection with the prior art. Preferably, the porous film is used for labeling polyethylene containers during blow molding. According to the invention, the film is inserted so that the porous layer faces the container. Suitable blow molding processes are also described, for example, in ISDN 3-446-15071-4, to which reference is hereby expressly made.

To characterize the raw materials and the films, the following measuring methods were used:

melt Flow Index

The melt flow index of the propylene polymers was measured according to DIN 53 735 at 2.16 kg load and 230 ^° C. and at 190 ^° C. and 2.16 kg for polyethylenes.

melting points

DSC measurement, maxima of the melting curve, heating rate 20 K / min.

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

beta-crystal content The DSC method is used to determine the .beta.-crystalline portion (for example in the prefilm) in polypropylene.

Characterization by DSC is described in J. o. Appl. Polymer Science, Vol. 74, p .: 2357-2368, 1999 by Varga and carried out as follows: The sample additized with the β-nucleator is first heated to 22O ⁰ C in the DSC at a heating rate of 20 ° C / min and melted (1st heating). Thereafter, it is cooled at a cooling rate of 10 ° C / min to 100 ° C, before being remelted at a heating rate of 10 ° C / min (2nd heating). During the second heating, the ratio of the enthalpies of fusion of the β-crystalline phase ( _Hβ ) to the

Sum of enthalpies of fusion of ß- and α-crystalline phase (H _ß + H _α ) of

Crystallinity Kβ, _D sc determined.

Permeability (Gurley value) The permeability of the films was measured with the Gurley Tester 4110, according to ASTM D 726-58. It determines the time (in seconds) that 100 cm ^{3 of} air will take to permeate through the 1 inch ² (6,452 cm ² ) label area. The pressure difference across the film corresponds to the pressure of a water column of 12.4 cm in height. The time required then corresponds to the Gurley value.

The invention will now be illustrated by the following examples.

Example 1 :

After the extrusion process, a single layer film was extruded from a T die at an extrusion temperature of 245 ⁰ C. The film had the following composition: about 50 wt .-% propylene homopolymer (PP) with an n-heptane-soluble content of 4.5 wt .-% (based on 100% PP) and a melting point of 165 ⁰ C; and a melt flow index of 3.2 g / 10 min at 230 ⁰ C and 2.16 kg load (DIN 53 735) and about 49.9 wt .-% propylene-ethylene block copolymer having an ethylene content of about 5 wt. -% based on the block copolymer and a MFI (23O ⁰ C and 2.16 kg) of 6 g / 10 min

0.1% by weight of Ca-pimelate as β-nucleating agent

The film additionally contained stabilizer and neutralizing agent in conventional amounts.

The molten polymer mixture was withdrawn after extrusion via a first take-off roll and another rolling center and solidified, then stretched longitudinally, transversely stretched and fixed, the following conditions were selected in detail:

Extrusion: extrusion temperature 245 ⁰ C.

Cooling roll: Temperature 125 ⁰ C, residence time sec on the take-off roll 17 longitudinal stretching. Stretching roll T = 95 ⁰ C

Longitudinal stretching by a factor of 4. Cross-section: Heating fields T = 145 ⁰ C

Yield fields T = 140 ⁰ C

Transverse extension by a factor of 5.5

The porous film thus prepared was about 95 μm thick and had a density of 0.50 g / cm ³ and showed a uniform white-opaque appearance. The Gurley value was 95,000 sec.

Comparative Example 1 A film was produced as described in Example 1. In contrast to Example 1, the residence time on the take-off roll was increased to 55 sec. Thus, the production rate in Example 1 was more than three times as high as in this Comparative Example 1. The Gurley value of the film according to Comparative Example 1 was about 1040 sec and the density 0.35 g / cm ³ at a film thickness of about 80 .mu.m.

Comparative Example 2 An opaque three-layer film with a layer structure ABC with a total thickness of 80 μm was produced by coextrusion and by subsequent stepwise orientation in the longitudinal and in the transverse direction. The cover layers had a thickness of 0.6 μm each.

Base layer B (= vacuolated layer):

93 wt .-% propylene homopolymer having a melting point of 165 ° C.

7.0% by weight of CaCO3 of the Millicarb type with an average diameter of 3 μm

Covering layer A.

99.67% by weight of ethylene-propylene random copolymer having a C2 content of 3.5% by weight

0.33 wt .-% SiO 2 as antiblocking agent with a mean diameter of 2 microns cover layer B as the cover layer A.

The production conditions in the individual process steps were:

Extrusion temperatures 28O ⁰ C

Temperature of take-off roll: 30 ⁰ C longitudinal stretch: temperature: 122 ⁰ C

Longitudinal stretching ratio: 6.0

Transverse extension: temperature: 155 ⁰ C

Transverse stretch ratio: 8.0

Fixation: Temperature: 14O ⁰ C Convergence: 15% In this way, an opaque vacuole-containing film was obtained with a density of 0.6 g / cm3. The film was not porous, a Gurley value could therefore not be determined on this film.

Use according to the invention

The films according to the examples and comparative examples were cut to label shape, as usual provided on the blow molding machine and inserted into the mold before the blow molding process, wherein the films were inserted according to Example 1 and Comparative Example 1 so that the microporous layer facing the container , A blow molding machine was equipped with a tool for a bulbous bottle. The blow molding machine was charged with HD-PE blown stock with an MFI of 0.4 g / 10 min. The HDPE was extruded at a mass temperature of about 200 ⁰ C through a ring nozzle hose-shaped. The mold was closed, sealing the bottom of the molten tube. In the upper end of the tube, a lance was retracted and inflated the tube with a pressure of 10 bar in the mold. The mold was then moved apart and the container removed.

The porous label sheets of Example 1 and Comparative Example 1 were integral with the container and all exhibited a perfectly smooth appearance without any signs of orange peel. The opaque vacuolate-containing film according to the comparative example was also bonded to the container and showed the characteristic appearance of the orange peel. The inventive films according to Examples 1 showed no impairment in adhesion or appearance despite significantly increased Gurley values, ie lower gas permeabilities. Thus, the films could be advantageously produced at more than twice the production speed as Comparative Example 1 without causing problems in the use according to the invention.

Claims

claims

A biaxially oriented film having a microporous layer containing propylene polymers and at least one β-nucleating agent and whose microporosity is produced by converting β-crystalline polypropylene when the film is stretched, characterized in that the microporous layer has a Gurley value of 10,000 to 300,000 sec and forms an outer layer of the film.

2. A film according to claim 1, characterized in that the microporous layer has a Gurley value of 30,000 to 150,000 Gurley.

3. A film according to claim 2, characterized in that the density of the film is in a range of 0.3 to 0.85 g / cm ³ .

4. A film according to claim 2 and / or 3, characterized in that the microporous layer contains a propylene homopolymer and / or a propylene block copolymer.

5. A film according to claim 1, characterized in that the microporous layer contains a mixture of propylene homopolymer and propylene block copolymer and the ratio is in a range of 90:10 to 10:90.

6. A film according to one or more of claims 1 to 5, characterized in that the microporous layer contains 0.001 wt% to 5 wt .-% .- based on the weight of the layer, ß-nucleating agent.

7. A film according to one or more of claims 1 to 6, characterized in that the nucleating agent is a calcium salt of pimelic acid or suberic acid or a carboxamide.

8. A film according to one or more of claims 1 to 7, characterized in that the film is produced according to the stenter, and the draw-off roller temperature is in a range of 60 to 130 ⁰ C.

9. A film according to one or more of claims 1 to 8, characterized in that the film is monolayer and has only the microporous layer.

10. A film according to one or more of claims 1 to 8, characterized in that the film is multi-layered and on one surface of the microporous layer, an additional cover layer is applied.

11. A film according to claim 10, characterized in that the cover layer is applied by means of coextrusion, coating or lamination.

12. A biaxially oriented film with microporous layer containing propylene polymers and at least one ß-nucleating agent and whose microporosity is produced by conversion of ß-crystalline polypropylene in stretching the film, characterized in that the microporous layer forms an outer layer of the film and on a Side of the microporous layer, a cover layer is applied, characterized in that the film by biaxial stretching an unstretched prefilm with a content of ß-crystalline polypropylene of 20 to 60% can be produced.

13. Use of a film according to one of claims 1 to 12 for labeling a container during the blow molding process, characterized in that the porous

Layer facing the container.

14. A method for producing a labeled container by blow molding, comprising extruding a thermoplastic polymer through a ring die as a molten tube into a two-part mold into which a film or at least one film section is inserted, and closing the two-part mold of the molten tube at one end squeezes off and on Introducing air such that the melt tube is inflated and adapts to the shape so that it forms a hollow body and at the same time the inserted label is applied, characterized in that the label of a biaxially oriented film according to one of claims 1 to 12 and the microporous layer of this film connects to the container.

15. A process for producing a film having a microporous layer, characterized in that one extrudes a mixture of at least one polypropylene polymer and at least one ß-nucleating agent monolayer or outer layer of a multilayer film and stripped on a take-off roll and cooled and then the thus obtained Pre-film is stretched in the longitudinal and transverse directions, characterized in that the melt is cooled after extrusion such that the content of ß-crystalline polypropylene in the single layer or in the outer layer with ß-nucleating agent before stretching is 20 to 60%.

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