HK1118256B - Label film for a blow moulding method - Google Patents

Description

Label film for blow molding

Technical Field

The present invention relates to a biaxially oriented film having a microporous layer comprising a propylene polymer and at least one beta-nucleating agent and whose microporosity is produced by the transformation of beta-crystalline polypropylene upon stretching of the film, and the use of the film as an in-mold label in a blow molding process.

Background

Label films cover a wide and technically complex field. A distinction is made between various labelling techniques, which are fundamentally different in terms of process conditions and which necessarily impose different technical requirements on the material of the label. All labeling methods have in common that, as a finished product, a visually attractive labeled container must be obtained, on which good adhesion to the labeled container must be ensured.

In the labeling method, a very different technique of applying the label is applied. The label is divided into self-adhesive label, winding label, shrink label, in-mold label, patch label, etc. In all of these different labeling methods, films made of thermoplastics can be used as labels.

The in-mold labeling aspect also distinguishes between various techniques, wherein different process conditions are applied. Common to all in-mold labeling methods is that the label participates in the actual forming process of the container and is applied in the process. In this case, however, very different shaping methods are used, such as injection molding, blow molding, deep drawing.

In the injection molding process, the label is placed in an injection mold and injection molded from molten plastic on the back side. By the high temperatures and pressures, the label itself engages with the injection-molded part and becomes an integral, inseparable component of the injection-molded part. Cups and lids for e.g. ice cream or artificial cream cups are produced according to the method.

In this case, individual labels are taken out of a stack or cut from a wound body and placed in an injection mold. Here, the mold is designed such that the melt stream is injected on the back side of the label and the front side of the film is applied to the wall of the injection mold. The hot melt engages the label upon injection. After injection, the mold is opened, the labeled injection molded part is ejected and cooled. As a result, the label must adhere to the container without wrinkles and visually without blemishes.

In the injection, the injection pressure was 300-600 bar. The melt flow index of the plastic used was about 40g/10 min. The injection temperature depends on the plastic used. In some cases, the mold is additionally cooled in order to avoid adhesion of the injection-molded part to the mold.

In deep drawing, a thick plastic plate, mostly cast PP or PS, is heated up unoriented with a thickness of about 200 μm and is drawn or pressed into a corresponding mold with the aid of vacuum or pressing tools. Here again, the individual labels are placed in the mould and joined to the actual container during the moulding process. The use of significantly lower temperatures makes the adhesion of the label to the container a potentially critical factor. Good adhesion must also be ensured at these low processing temperatures. The processing speed of the method is lower than that of the injection molding method.

Direct in-mold labeling is also possible in the blow molding of containers or hollow bodies. In this process, a melt hose is extruded vertically downward through an annular die. The vertically separated dies collide and envelop the hose, where it is compressed at the lower end. At the upper end, a hole-forming pin (Blasdorn) for blow molding is introduced, through which the opening of the molded article is formed. Air is fed via the blow pin to the hot melt tube, so that it expands and adheres to the inner wall of the mold. In which case the label must engage the viscous plastic of the melt hose. Subsequently, the mold is opened and the molded elevated portion of the opening is cut off. The molded and labeled container was ejected and cooled.

In this blow molding process, the pressure at which the melt hose is blown up is about 4 to 15 bar and the temperature is significantly lower than in injection molding. In order to form a dimensionally stable melt hose, the plastic material has a lower MFI than in injection molding and therefore behaves differently during cooling than low-viscosity materials for injection molding.

In addition, in this blow molding process, biaxially oriented thermoplastic films are increasingly used for labeling containers during molding. In order to ensure that the label film and the blow-molded body are smooth and adhere to one another and engage one another without air bubbles, the film must have a selected property profile for this purpose. For this purpose, various solutions have been proposed in the prior art.

For example, it is known in the prior art that the inclusion of air, which in the form of large bubbles impairs optical properties and adhesion, can be reduced in the case of in-mold blowing by means of a specific film surface roughness.

For this purpose, the side of the film facing the container must have a roughness of the order of μm, which allows the extrusion of air during labeling. Such roughness is produced, for example, by a specific formulation of the cover layers in the multilayer film or by structuring of the surface.

For example, in US 5,254,302, BOPP films are described, the back of which is modified by embossing with a specific surface structure. After embossing, the film is coated on the side with the hot-melt adhesive system in such a way that the surface structure still remains. The adhesive system ensures the adhesion of the label film on the molded body and the structured surface prevents the formation of air bubbles.

US patent 4,986,866 describes a multilayer paper-like label film with a sealable cover layer, which has to be mechanically embossed by means of rollers before the stretching process. Here too, the surface structure should ensure that air is removed and that bubble-free adhesion of the label is possible.

DE 19949898 describes the use of polypropylene films having an average roughness of at least 3.5 μm for labelling in the blow moulding process. The roughness is produced by a polypropylene mixture in the cover layer, where the mixture consists of polypropylene and incompatible or partially compatible thermoplastic polymers.

In addition to these bubbles, another independent undesirable effect that occurs in blow-molding labeling is: so-called orange peel formation. This effect is independent of large bubbles due to insufficient air scavenging. The orange peel does not appear as more or less large isolated bubbles, but the entire label surface is not flat with a certain regularity, so that the appearance of the surface structure is very similar to an orange, and it is therefore often referred to as orange peel. Sometimes this defect is also referred to as a leather skin or "leather effect". Various solutions have been proposed to reduce the effectiveness of the orange peel. One direction of development is based on the conjecture that orange peel results from shrinkage of blow-molded containers during cooling. On the other hand, injection-molded parts shrink very strongly on cooling, but this method is much less susceptible to damaging orange peel effects.

EP 0559484 describes a film for in-mold labeling, where no distinction is made between in-mold injection molding and in-mold blow molding. The film has a cover layer composed of polyethylene and a filler, which is applied to a base layer containing the cavitation bubbles. The polyethylene layer faces the container and on the opposite outer side further layers may be applied. According to this teaching, the occurrence of the leather skin effect can be masked by an additional pigmented outer layer.

EP 0546741 describes a film with a covering layer containing cavitation bubbles, which is applied to a base layer that does not contain cavitation bubbles. The covering layer containing the cavitation bubbles faces the container in the in-mold process. According to this teaching, orange peel is produced by shrinkage of the label film containing cavitation bubbles in injection molding and can be avoided by avoiding too strong cavitation bubble formation and reducing the filler content of the film.

In contrast, WO 00/12288 teaches that less orange peel occurs by controlled simultaneous shrinkage of the label and recommends improved orange peel effect in blow molding by specific film shrink properties. Thus, the in-mold label film should have a shrinkage at 130 ℃ over 10min of at least 4% in both directions. Because of this shrinkage, less orange peel is formed when blow labeling. However, the teaching also confirms that too low a density in turn leads to increased orange peel formation. Therefore, it is additionally recommended to maintain the density of the film at 0.65 to 0.85g/cm³。

In practice, all blow molding processes have been shown to be substantially more susceptible to orange peel effects during labeling than the in-mold labeling process in injection molding.

All of the known teachings do not satisfactorily address the problem of orange peel formation when using biaxially oriented films in-mold blowing, or they have other serious drawbacks. The proposed measures, although partly showing reliable results in injection moulding applications, in the blow moulding process the appearance of the label on the container is as defective as before and strongly influenced by the orange peel.

EP 0865909 describes the use of "microvoided" films for labels. The film contains a beta-nucleating agent due to which an increased proportion of beta-crystalline polypropylene is produced in the pre-formed film when the melt film is cooled. When the pre-formed film is stretched, "microvoids" are created. The film is said to have good printability.

EP1501886 describes the use of biaxially oriented polypropylene microporous films containing a beta-nucleating agent. The microporosity is created by converting beta-crystalline polypropylene when the membrane is stretched. Due to the high porosity, the film can be advantageously used for labeling containers in blow molding. In practice, the process of preparing the membrane is very slow in order to ensure the required high porosity. In addition, high porosity weakens the film in mechanical strength, so that tearing often occurs in tenters. The membrane thereby becomes more expensive and, despite technical advantages, makes economic use difficult.

Disclosure of Invention

The object of the present invention is to provide a label film which has good adhesion without orange peel when being labeled in a blow molding process and which can be produced at sufficient production speed and production reliability.

The object on which the present invention is based is solved by a biaxially oriented film having a porous layer which comprises polypropylene and a beta-nucleating agent and whose microporosity can be produced by converting beta-crystalline polypropylene when the film is stretched and whose Gurley value is not less than 10000 s. The microporous layer is the layer of the membrane that is on the outside. Furthermore, the object is achieved by the use of the film for labeling containers in blow molding.

Accordingly, the present invention provides the following:

item 1: biaxially oriented film having a microporous layer, said microporous layer comprising a propylene polymer and at least one beta-nucleating agent and having a microporosity resulting from the transformation of beta-crystalline polypropylene upon stretching of said film, characterized in that said microporous layer has a Gurley value of 10000-300000sec and forms an outer layer of said film.

Item 2: the membrane of item 1, characterized in that the microporous layer has a Gurley value of 30000-.

Item 3: the film of item 2, characterized in that the film has a density of 0.3 to 0.85g/cm³。

Item 4: the membrane of item 2 or 3, characterized in that the microporous layer comprises a propylene homopolymer and/or a propylene block copolymer.

Item 5: the membrane of item 1, characterized in that the microporous layer comprises a mixture of propylene homopolymer and propylene block copolymer and the ratio is 90: 10 to 10: 90.

Item 6: the membrane of any of claims 1-3, characterized in that the microporous layer comprises 0.001 wt% to 5 wt% of a beta-nucleating agent, based on the weight of the layer.

Item 7: a film according to any of claims 1 to 3, characterized in that said nucleating agent is a calcium salt of pimelic acid or a calcium salt of suberic acid or a carboxamide.

Item 8: the film according to any one of items 1 to 3, characterized in that the film is produced according to a tenter method and the take-off roll temperature is 60 to 130 ℃.

Item 9: the membrane of any of claims 1-3, characterized in that the membrane is a monolayer and has only the microporous layer.

Item 10: the membrane of any of claims 1-3, characterized in that the membrane is multi-layered and an additional cover layer is applied on one surface of the microporous layer.

Item 11: the film of item 10, characterized in that the cover layer is applied by coextrusion, coating or lamination.

Item 12: use of a film according to one of items 1 to 11 for labeling containers in a blow molding process, characterized in that the porous layer faces the containers.

Item 13: process for the preparation of labeled containers by means of the blow molding process, wherein a thermoplastic polymer is extruded through an annular die as a melt hose into a two-part mold, into which a film or at least one film segment is inserted, the melt hose is pressed at one end by closing of the two-part mold, air is introduced at the opposite end to inflate the melt hose and match it to the mold, so that a hollow body is formed and simultaneously an inserted label is applied thereto, characterized in that the label consists of a film according to one of items 1 to 11 and the microporous layer of the film is joined to the container.

It has been found that membranes with a microporous layer having a Gurley value above 10000s can surprisingly be used very well for blow-moulding labelling and that no orange peel occurs at all under a wide range of process conditions if the microporosity is produced indirectly by a beta-nucleating agent. According to the teaching of EP1501886, a high film porosity is necessary in order to ensure good air removal by the high air permeability of the porous layer in contact with the container. Within the scope of the present invention, it has surprisingly been found that a high porosity, although beneficial, is not necessary. The net-like structure of the porous layer itself contributes to air removal even at a small level of air permeability (high Gurley value) and is surprisingly effective in preventing the formation of air bubbles in the label if the layer faces the container at the time of labeling. The production speed can thereby be significantly increased. The film as a whole is significantly more mechanically stable, thereby reducing the amount of tearing in preparation. The invention therefore has considerable economic advantages and the films of the invention can be used for blow-moulding labelling without bubble formation.

The microporous structure of the porous layer is significantly different from that of conventional membranes containing cavitation bubbles. Fig. 2a and 2b show a cross-sectional view (2a) and a top view (2b) of a typical structure of a layer containing cavitation bubbles. Due to the incompatibility of the particles causing cavitation, tearing occurs between the particle surface and the polymer matrix upon stretching and a closed air-filled cavity is formed, in which the incompatible particles are present. These cavities are referred to as vacuoles or "voids". The voids are distributed throughout the layer and reduce the density of the film, specifically the layer. However, these films still always have a good barrier to, for example, water vapor, since the cavitation bubbles are closed and the structure as a whole is impermeable. Opaque films having a layer containing cavitation bubbles produce an undesirable orange peel during the blow molding process.

In contrast, the porous layer is permeable to gases and has an open pore network structure. The structure is not produced by incompatible fillers or particles, but according to a technically completely different method. The microporous layer comprises polypropylene and a beta-nucleating agent. The polypropylene and beta-nucleating agent mixture is initially melted in an extruder as is usual in film preparation and extruded through a wide slot die as a melt film onto a chill roll. Upon cooling the melt film, the beta-nucleating agent promotes crystallization of the beta-crystalline polypropylene, so that an unstretched preform film high in beta-crystalline polypropylene content is formed. The temperature and stretching conditions may be selected to convert the beta-crystallites into the more thermally stable alpha-phase of the polypropylene when the pre-formed film is stretched. Due to the low density of the β -crystallites, this transition is accompanied by a volume shrinkage and thus a characteristic porous structure, similar to a torn network.

It has surprisingly been found that films with a porous layer can be applied equally well as label films in the blow-moulding process, even when the porosity is significantly lower and the structure has less open area. As can be seen from fig. 1a (top view) and 1b (cross-sectional view), the highly porous membrane according to EP1501886 has an open-pore network structure with openings distributed uniformly over the entire surface. The inventive film according to fig. 3a and 3b has a similar fibrillar structure, but there are significantly fewer pores on the surface, so that the surface also has almost closed regions. Surprisingly, it is sufficient if the holes are present only in partial regions of the surface in order to avoid bubble formation in the blow-molding of labels.

The composition of the microporous layer (hereinafter also referred to as layer) will be described in detail from here on. The microporous layer comprises a propylene homopolymer and/or a propylene block copolymer, optionally additionally comprises other polyolefins, and comprises at least one β -nucleating agent, and optionally additionally comprises conventional additives, such as stabilizers, neutralizing agents, lubricants, antistatic agents, pigments, each in effective amounts. Usually, the use of additional incompatible void initiating fillers, such as calcium carbonate, or polyesters, such as PET or PBT, is abandoned so that the layer contains less than 5 wt.%, preferably 0 up to 1 wt.%, of these void initiating fillers. Such low amounts may be incorporated into the layer, for example, by the introduction of membrane regenerants.

Generally, the layer comprises at least 70 wt.%, preferably from 80 to 99.95 wt.%, in particular from 90 to 97 wt.% of a propylene homopolymer and/or a propylene block copolymer, and from 0.001 to 5 wt.%, preferably from 0.1 to 3 wt.%, of at least one β -nucleating agent, in each case based on the weight of the layer.

Suitable propylene homopolymers comprise from 80 to 100% by weight, preferably from 90 to 100% by weight, of propylene units and have a melting point of 140 ℃ or more, preferably 150 ℃ and 170 ℃ and a melt flow index at 230 ℃ and 2.16kg force (DIN53735) of generally from 0.5 to 10g/10min, preferably from 2 to 8g/10 min. Isotactic propylene homopolymers having an atactic content of 15 wt.% and less represent the preferred propylene polymers for the layers, with isotactic propylene homopolymers being particularly preferred here.

Suitable propylene block copolymers comprise predominantly propylene units, i.e. comprise more than 50 wt%, preferably from 70 to 99 wt%, in particular from 90 to 99 wt%, of propylene units. Suitable comonomers in suitable amounts are ethylene, butene or higher alkene homologues, of which ethylene is preferred. The block copolymers have a melt flow index of from 1 to 15g/10min, preferably from 2 to 10g/10min (230 ℃ C.; 2.16 kg). The melting point is 140 ℃ or higher, preferably 150 ℃ to 165 ℃.

The weight percentages given are based on the corresponding polymer.

The mixture of the propylene homopolymer and the propylene block copolymer contains these two components in an arbitrary mixing ratio. Preferably, the ratio of propylene homopolymer to propylene block copolymer is from 10: 90 wt% to 90: 10 wt%, preferably from 20: 70 wt% to 70: 20 wt%. Such a mixture of homopolymer and block copolymer is particularly preferred and improves the optical properties as well as the stretchability of the microporous layer.

Where appropriate, the porous layer may comprise further polyolefins in addition to the propylene homopolymer and/or the propylene block copolymer. The proportion of these other polyolefins is generally below 30% by weight, preferably from 1 to 20% by weight. Other polyolefins are, for example, statistical copolymers of ethylene and propylene having an ethylene content of 20% by weight or less, propylene and C having an olefin content of 20% by weight or less₄-C₈Statistical copolymers of olefins, terpolymers of propylene, ethylene and butene having an ethylene content of 10% by weight or less and a butene content of 15% by weight or less, or polyethylenes such as HDPE, LDPE, VLDPE, MDPE and LLDPE.

As beta-nucleating agent for the microporous layer, essentially all known additives which promote the formation of beta-crystals when the polypropylene melt is cooled are suitable. Such β -nucleating agents and their mode of action in polypropylene matrices are known per se in the prior art and will be described in detail below.

Various crystalline phases of polypropylene are known. Upon cooling the melt, typically mainly α -crystalline PP is formed, which has a melting point of about 158-162 ℃. By specific temperature control, a low proportion of a β -crystalline phase can be produced on cooling, which has a significantly lower melting point at 148-150 ℃ relative to the monoclinic α -variant. Additives which lead to an increased proportion of β -modifications on crystallization of the polypropylene are known from the prior art, for example the calcium salts of γ -quinacridone, quinacridone or phthalic acid.

For the purposes of the present invention, preference is given to using highly active β -nucleating agents in the porous layer which, on cooling of the melt film, can give a β -ratio of 10 to 80%, preferably 20 to 60%. For this purpose, two-component nucleating systems, for example formed from calcium carbonate and organic dicarboxylic acids, are suitable, which are described, for example, in DE 3610644, which is hereby expressly incorporated by reference. Particularly advantageous are the calcium salts of dicarboxylic acids, such as calcium pimelate or calcium suberate, as described in DE 4420989, which is likewise expressly incorporated by reference. Dicarboxylic amides (Dicarboxamides) described in EP-0557721, in particular N, N-dicyclohexyl-2, 6-naphthalenedicarboxylic amide, are also suitable beta-nucleating agents.

In addition to nucleating agents, to obtain high proportions of beta-crystalline polypropylene, it is important to maintain specific temperature ranges and residence times at these temperatures as the melt film cools. The cooling of the melt film is preferably carried out at a temperature of from 60 to 130 ℃ and in particular from 80 to 120 ℃. According to the invention, a very slow cooling promoting the growth of β -crystallites can be performed relatively quickly compared to EP 1501886. The draw-off (Abzug) speed, the speed at which the melt film travels through the first chill roll, should be selected such that the residence time at a given temperature allows the growth of beta-crystallites. In this case, it is no longer necessary to achieve the maximum β crystal concentration in the prefabricated film by the maximum residence time on the draw-off roller. On the contrary, the production speed can be increased so much that the β -crystallite concentration in the prefabricated film is 20-60 wt%, at which concentration the film thus produced has a Gurley value of 10000-300000s after biaxial stretching. The withdrawal speed can vary strongly depending on the size of the withdrawal rolls and their temperature, preferably less than 35m/min, in particular from 1 to 20 m/min.

A particularly preferred embodiment comprises from 0.001 to 5% by weight, preferably from 0.05 to 3.0% by weight, in particular from 0.1 to 1.0% by weight, of calcium pimelate or calcium suberate in the microporous layer composed of a propylene homopolymer.

Typically, the microporous label film is a single layer and consists only of the microporous layer. However, it is self-evident that the single-layer film optionally can have a print or a coating or a further cover layer before it is used as a label film in blow molding. The thickness of the porous layer is generally 20 to 150. mu.m, preferably 30 to 100. mu.m. According to the invention, the outer surface of the porous layer is not covered with other layers, i.e. this side of the membrane is neither printed nor coated, laminated or any other type of processing that would result in the pores of the porous layer being covered. Thus, the surface of the porous layer constitutes the surface of the membrane.

If desired, the microporous layer can be corona, flame or plasma treated on the outside in order to improve the adhesion properties and wettability.

The microporous layer typically has a density of 0.3 to 0.85g/cm³Preferably 0.4 to 0.7g/cm³Which corresponds to the density of the film in the single layer embodiment. Surprisingly, it was found that a particularly low density does not lead to an increase in the orange peel effect as in the case of an opaque film containing cavitation bubbles. With regard to vacuole-containing opaque films, the relevant literature teaches that too low a density results in an increase in orange peel effect due to too strong voiding. Surprisingly, this is not the case for the porous membrane. The density can be reduced to extremely low values, while the film can still be applied defect-free in blow molding without the occurrence of a damaging orange peel effect.

In a further embodiment, the microporous layer can have a further cover layer, wherein the microporous layer faces the container in use of the multilayer embodiment according to the invention and is bonded to the molded body in the blow molding. The further cover layer thus forms the outer side of the in-use label according to the invention. The further cover layer may for example be applied by laminating or gluing the porous layer with a further membrane. Preferably, it is a coextruded cover layer. Coatings are also possible, if desired.

The coating may be applied according to conventional methods. The coating is for example made of acrylic, acrylate, PVOH or other polymers suitable as sealable or printable surface layer. These coatings are described in detail, for example, in US 6,013,353 (column 6), the disclosure of which is expressly incorporated herein by reference.

The optionally coextruded cover layer generally comprises at least 70% by weight, preferably from 75 to < 100% by weight, in particular from 90 to 98% by weight, of a polyolefin, preferably a propylene polymer, and optionally further customary additives, such as neutralizing agents, stabilizers, antistatic agents, lubricants, such as fatty acid amides, or siloxanes, or antiblocking agents, each in effective amounts.

The propylene polymer of the cover layer is for example a propylene homopolymer, as already described above for the porous layer, or a copolymer of propylene and ethylene, or a copolymer of propylene and butene, or a copolymer of propylene and another olefin having from 5 to 10 carbon atoms. For the purposes of the present invention, terpolymers of ethylene with propylene and butene or of ethylene with propylene and another olefin having from 5 to 10 carbon atoms are also suitable for the covering layer. Further, mixtures or blends of two or more of the copolymers and terpolymers can be employed.

For the cover layer, preference is given to statistical ethylene-propylene copolymers and ethylene-propylene-butene terpolymers, in particular statistical ethylene-propylene copolymers having an ethylene content of from 2 to 10% by weight, preferably from 5 to 8% by weight, or statistical ethylene-propylene-1-butene terpolymers having an ethylene content of from 1 to 10% by weight, preferably from 2 to 6% by weight, and a 1-butene content of from 3 to 20% by weight, preferably from 8 to 10% by weight, in each case based on the weight of the copolymer or terpolymer.

The statistical copolymers and terpolymers described above generally have melt flow indices of from 1.5 to 30g/10min, preferably from 3 to 15g/10 min. The melting point is 105-140 ℃. The blends of copolymers and terpolymers described above have a melt flow index of 5-9g/10min and a melting point of 120-150 ℃. All melt flow indices given above were determined at 230 ℃ and a force of 2.16kg (DIN 53735).

The thickness of the covering layer is usually 0.1 to 10 μm, preferably 0.5 to 5 μm. If desired, the surface of the cover layer may be corona, flame or plasma treated for improved printability. The density of the membrane is only increased non-significantly relative to the monolayer embodiments by a non-porous cover layer also free of voids, and thus the membrane density of these embodiments is typically 0.35 to 0.85g/cm³Preferably 0.4 to 0.65g/Gm³。

If desired, the cover layer may additionally comprise conventional additives, such as stabilizers, neutralizing agents, antiblocking agents, lubricants, antistatic agents, etc., each in conventional amounts.

The porous film of the present invention is preferably produced according to an extrusion method or a coextrusion method (flat film method) known per se.

Within the scope of the process, this is carried out such that the polypropylene mixed with the β -nucleating agent is melted in an extruder and extruded through a flat die onto a take-off roll, on which the melt solidifies under the formation of β -crystallites. In the case of the two-layer embodiment, the corresponding coextrusion is carried out together with the cover layer. The cooling temperature and cooling time are selected so that a sufficient proportion of beta-crystalline polypropylene is produced in the pre-formed film. The pre-formed film with beta-crystalline polypropylene is then biaxially stretched so that the beta-crystallites are converted to alpha-polypropylene upon stretching. The biaxially stretched film is subsequently heat-set and, if appropriate, corona, plasma or flame-treated on one or both surfaces.

Biaxial stretching (orientation) is usually done sequentially, where it is preferred to first stretch in the machine direction and then stretch in the transverse direction (perpendicular to the machine direction).

To promote the formation of a high proportion of beta-crystalline polypropylene, the take-off roll or rolls are maintained at a temperature of 60 to 130 ℃, preferably 80 to 120 ℃.

The temperature during stretching is less than 140 c, preferably 90-125 c, in the longitudinal direction. The draw ratio is 3: 1 to 5: 1. The stretching in the transverse direction is carried out at a temperature above 140 c, preferably at 145-160 c. The transverse stretching ratio is 3: 1 to 7: 1.

The longitudinal stretching will advantageously be carried out by means of two different fast running rollers, which meet the target stretching ratio, and the transverse stretching by means of a corresponding clamping frame (Kluppenrahmen).

Typically, the heat-setting (heat-treating) of the film is carried out after its biaxial stretching, where the film is held at a temperature of 110-150 ℃ for about 0.5-10 seconds. The film is then typically wound with a winding device.

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

For the alternative corona treatment, the film is conveyed between two conductor elements acting as electrodes, between which a voltage is applied which is so high, in most cases an alternating voltage (approximately 10000V and 10000Hz), that a spray discharge or corona discharge can occur. By this spraying or corona discharge, the air above the membrane surface is ionized and reacts with the molecules of the membrane surface, so that a polar built-in structure (einlagengengengengen) is created in the essentially non-polar polymer matrix. The treatment intensity is in the conventional range, with 38-45mN/m being preferred.

According to this method, a porous film having an opaque appearance is obtained. The porous layer has a network-like structure with pores communicating with each other (see fig. 3a and 3b), which is permeable to gas. In a single layer embodiment, these films have Gurley values of 10000-. In the multilayer embodiment with a gas-impermeable cover layer, the porous layer has a corresponding structure, so that the layer exhibits a comparable Gurley value.

According to the invention, the porous film is used in a blow molding process. The details of the blow molding process have been described above in the prior art. Preferably, the porous film is used for labeling polyethylene containers in blow molding. The membrane is inserted according to the invention so that the porous layer faces the container. Suitable blow molding processes are also described, for example, in ISDN 3-446-15071-4, which is expressly incorporated herein by reference.

For the characterization of the raw materials and the films, the following measurement methods were used:

melt flow index

The melt flow index of the propylene polymer was determined in accordance with DIN53735 at a load of 2.16kg and 230 ℃ and for polyethylene at 190 ℃ and 2.16 kg.

Melting Point

DSC measurement, maximum value of melting curve, heating rate 20K/min.

Density of

The density was determined in accordance with DIN 53479, method A.

Beta-crystal content

To determine the proportion of β -crystals in polypropylene (e.g. in a pre-formed film), the DSC method was used.

Characterization by means of DSC is described in J.o.appl.Polymer Science, Vol.74, 2357. cndot.2368, 1999, Varga and proceeds in the following manner: in DSC, a sample with added β -nucleating agent is initially heated to 220 ℃ at a heating rate of 20 ℃/min and melted (1 st heating). It was then cooled to 100 ℃ at a cooling rate of 10 ℃/min, and then it was melted again at a heating rate of 10 ℃/min (2 nd heating). Melting enthalpy (H) from the beta-crystalline phase at heating 2_β) And the sum of the enthalpies of fusion of the beta-and alpha-crystalline phases (H)_β+H_α) Determining the degree of crystallinity K_β，DSC。

Permeability (Gurley value)

The permeability of the film was determined according to ASTM D726-58 with a Gurley tester 4110. Here, 100cm was measured³Air permeation 1in²(6.452cm²) The time (in seconds) required for the label surface. The pressure difference across the membrane here corresponds to a pressure of 12.4 cm of water. Thus, the time required corresponds to the Gurley value.

Detailed Description

The invention is described below by way of the following examples.

Example 1:

according to the extrusion method, a single layer film was extruded from a wide slit die at an extrusion temperature of 245 ℃. The film had the following composition:

about 50 wt% of a propylene homopolymer (PP) having an n-heptane soluble fraction of 4.5 wt% (relative to 100% PP) and a melting point of 165 ℃; a melt flow index of 3.2g/10min at 230 ℃ and a load of 2.16kg (DIN53735), and

about 49.9% by weight of a propylene-ethylene block copolymer having an ethylene proportion of about 5% by weight relative to the block copolymer and an MFI (230 ℃ and 2.16kg) of 6g/10 min.

0.1 wt% calcium pimelate as a beta-nucleating agent.

The film additionally contains stabilizers and neutralizers in conventional amounts.

After extrusion, the molten polymer mixture is drawn through a first drawing roll and a further set of three rolls and solidified, followed by longitudinal drawing, transverse drawing and setting, wherein the following conditions are specifically chosen:

extruding: extrusion temperature 245 DEG C

Cooling the roller: the temperature was 125 ℃ and the residence time on the draw-off rolls was 17 seconds

Longitudinal stretching: stretching roller T95 deg.C

Longitudinally stretched by 4 times

And (3) transverse stretching: heating zone T145 deg.C

Tensile zone T140 deg.c

Transversely stretched by 5.5 times

The porous film thus produced was about 95 μm thick and had a thickness of 0.50g/cm³And a uniform white opaque appearance. Gurley value of95000sec。

Comparative example 1

Membranes were prepared as described in example 1. Unlike example 1, the residence time on the take-off roll was increased to 55 sec. Thus, the production speed of example 1 was higher than 3 times the speed in this comparative example 1. The film of comparative example 1 had a Gurley value of about 1040sec and a density of 0.35g/cm at a film thickness of about 80 μm³。

Comparative example 2

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

Bottom layer B (vacuole-containing layer)

93 wt.% propylene homopolymer with a melting point of 165 ℃

7.0% by weight of Millicarb type CaCO having a mean diameter of 3 μm₃

Coating A

99.67 wt.% of a statistical ethylene-propylene copolymer, C₂The content is 3.5 wt%

0.33 wt.% SiO with an average diameter of 2 μm₂As antiblocking agents

Coating B is the same as coating A

The preparation conditions in each process step are as follows:

extrusion temperature: 280 deg.C

Temperature of the drawing roller: 30 deg.C

Longitudinal stretching temperature: 122 deg.C

Longitudinal stretching ratio: 6.0

Transverse stretching temperature: 155 deg.C

Transverse stretching ratio: 8.0

Shaping: temperature: 140 deg.C

Convergence degree: 15 percent of

In this way, a density of 0.6g/cm was obtained³The vacuole-containing opaque film of (a). The membrane was non-porous and therefore the Gurley value of the membrane could not be determined.

Use according to the invention

The films according to examples and comparative examples were cut into label shapes, as commonly provided on blow molding machines, and inserted into a mold prior to the blow molding process, where the films according to example 1 and comparative example 1 were placed in such a way that the microporous layer faced the container. The blow moulding machine is fitted with a mould for a belly (bauchig) bottle. The blow moulding machine was loaded with HD-PE blow mouldings having an MFI of 0.4g/10 min. The HDPE is extruded as a hose through an annular die at a mass temperature of about 200 ℃. The mold is closed and the lower end of the melt tube is sealed off. An inflation lance (Lanze) was inserted into the upper end of the hose and the hose was inflated in a mould at a pressure of 10 bar. Subsequently, the mold is pulled open and the container is removed.

The porous label films of example 1 and comparative example 1 were firmly bonded to the container and all showed a smooth appearance without blemishes, without any evidence of orange peel. The opaque, void-containing film according to the comparative example was also bonded to the container and exhibited a characteristic orange peel appearance. Despite the significantly increased Gurley number, lower air permeability, the inventive film filaments according to example 1 showed no impairment of adhesion or appearance. Thus, the film can be advantageously produced at a production speed of more than 2 times with respect to comparative example 1, without causing problems in the application according to the present invention.

Claims (13)

1. Biaxially oriented film having a microporous layer, said microporous layer comprising a propylene polymer and at least one beta-nucleating agent and having a microporosity resulting from the transformation of beta-crystalline polypropylene upon stretching of said film, characterized in that said microporous layer has a Gurley value of 10000-300000sec and forms an outer layer of said film.

2. The membrane of claim 1, characterized in that said microporous layer has a Gurley value of 30000-150000 Gurley.

3. The film of claim 2, characterized in that the film has a density of 0.3 to 0.85g/cm 3.

4. The membrane according to claim 2 or 3, characterized in that the microporous layer comprises a propylene homopolymer and/or a propylene block copolymer.

5. The membrane of claim 1, characterized in that the microporous layer comprises a mixture of propylene homopolymer and propylene block copolymer and the ratio is from 90: 10 to 10: 90.

6. The membrane according to any one of claims 1 to 3, characterized in that the microporous layer comprises 0.001 wt% to 5 wt% of a beta-nucleating agent, based on the weight of the layer.

7. A film according to any one of claims 1-3, characterized in that said nucleating agent is the calcium salt of pimelic acid or the calcium salt of suberic acid or a carboxamide.

8. The film according to any of claims 1 to 3, characterized in that the film is made according to a tenter frame process and the take-off roll temperature is 60 to 130 ℃.

9. The membrane according to any one of claims 1 to 3, characterized in that the membrane is mono-layered and has only the microporous layer.

10. The membrane according to any one of claims 1 to 3, characterized in that the membrane is multi-layered and an additional cover layer is applied on one surface of the microporous layer.

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

12. Use of a film according to any of claims 1 to 11 for labelling containers in a blow moulding process, characterised in that the porous layer faces the containers.

13. Process for the preparation of labeled containers by means of the blow molding process, wherein a thermoplastic polymer is extruded through an annular die as a melt hose into a two-part mold, into which a film or at least one film segment is inserted, the melt hose is pressed at one end by closing of the two-part mold, air is introduced at the opposite end to inflate the melt hose and match it to the mold, so that a hollow body is formed and simultaneously an inserted label is applied thereto, characterized in that the label consists of a film according to one of claims 1 to 11 and the microporous layer of the film is joined to the container.