Patent Description:
The present invention also concerns a method and a plant to make the film, by means of the technique of flat head co-extrusion and sequential stretching.

In particular, the film according to the present invention is usable to make packing, typically of the flexible type, in the food sector, labels both of the type wrapping around a container (wrap around), and of the type incorporated into the container (in-mould label), fastenings, laminated products and similar products, and has effective properties as a barrier against oxygen.

It is known to make a film, used to package food, made from at least one filling layer or core, with a polypropylene homopolymer base and two lateral layers or external layers, with a polypropylene copolymer, terpolymer and/or homopolymer base. Two further internal layers (interlayers) can be provided, always based on a mix of the above materials, according to requirements.

To make the films, the standard and most economical technique, and thus preferred in the field, is flat head co-extrusion with sequential stretching.

After the co-extruded product has been cast onto the cooling roll, a stretching in the longitudinal direction of the machine is provided (MD stretching) and a subsequent stretching in the transverse direction (TD stretching).

This determines the known axially bi-orientated structure of polypropylene (BOPP).

A film as described above has good barrier properties against humidity and oxygen, but the barrier against oxygen is not sufficient for more critical applications.

The barrier against oxygen is an important property for a film used for packing foods.

To obtain the barrier effect against oxygen a metalized film can be made by depositing a thin metal layer on the film (lacquering).

If we want to avoid lacquering by means of metallization and still obtain a film with a barrier against oxygen, the use of ethylene vinyl alcohol (EVOH) is in general known, because of its good properties as a barrier against oxygen.

However, ethylene vinyl alcohol is not normally compatible with the polypropylene as indicated above and with the processing parameters provided in the standard technique of making the films in question. It is very rigid and can only be stretched with great difficulty by means of said technique with sequential stretching.

Moreover, its crystallization temperature is near to the stretching temperature traditionally used, so that, during stretching, the crystals can break and there is a drastic reduction in the effect of the oxygen barrier.

<CIT> discloses a laminate film comprising a biaxially oriented polymer film substrate, a metal-containing layer; and an EVOH layer coextruded or laminated directly on the metal-containing layer.

For these reasons, so far it has not been possible to make a film with a polypropylene base inside which there is a barrier layer of ethylene vinyl alcohol, using the technique of flat head co-extrusion with sequential stretching.

There are films commonly available with a polypropylene cast base or films with a bi-orientated polypropylene base produced with bubble film technology. These techniques are not very economical, however.

Purpose of the present invention is to make a film, and to perfect a method and a plant for making said film, made of layers with a base of axially bi-orientated polypropylene, which has good properties as a barrier against oxygen, which can be made cheaply using the standard and economical technique of flat head co-extrusion with sequential stretching.

According to one feature of the present invention, a method to make a film based on axially bi-orientated polypropylene (BOPP), suitable for making packaging for food having a barrier effect against oxygen and formed by a plurality of layers one on top of the other having a polypropylene base, wherein the layers comprise at least a core layer with a polypropylene homopolymer base, and a first and a second external layers based on polypropylene copolymer, terpolymer and/or homopolymer and additives, the film further comprising a barrier layer, positioned intermediate said core layer and one of said external layers and having an ethylene vinyl alcohol EVOH base, at least an adhesive layer to couple, at least on one side, the barrier layer with a respective adjoining layer, and a layer with a polypropylene copolymer, terpolymer and/or homopolymer base, provided between the filling layer and the first external layer, comprises the following steps:.

According to one characteristic of the present invention, the trimming step of the edges does not affect the barrier layer, as there is a safety distance between the lateral ends of said barrier layer and the lateral ends of said intermediate product. Advantageously, the co-extrusion is made so that the value of the distance of one of the lateral ends of the barrier layer from the relative lateral end of the intermediate product itself, before the trimming of the selvages, is greater than or equal to about <NUM>.

Advantageously, the value of the distance is comprised between about <NUM> and <NUM>, preferably between about <NUM> and <NUM>, even more preferably between about <NUM> and <NUM>.

This distance allows to obtain a co-extruded film with ethylene vinyl alcohol incorporated which is easily stretched by means of sequential stretching, in the desired stretching ratios, to obtain overall thicknesses of the film and in particular of the barrier layer, suitable to create an oxygen barrier effect.

In fact, thanks to said value of the distance adopted in the first step, once the stretching ratio has been defined as the ratio between the transverse dimension of the intermediate product before and after the stretching operation, in order to have the desired final properties of the film, the stretching ratio of the second stretching operation is advantageously comprised between about <NUM>:<NUM> and about <NUM>:<NUM>.

The stretching time of said second stretching operation can be comprised between <NUM> and <NUM> seconds.

Advantageously, the method also comprises a recirculation step able to recirculate the material of the selvages cut by the trimming unit to the extrusion unit, with obvious advantages in terms of cost of the raw material.

The distance of the barrier layer from the end of the intermediate co-extrusion product represents a safety measure by means of which the material based on ethylene vinyl alcohol is guaranteed not to be cut/trimmed and therefore re-circulated.

The material based on ethylene vinyl alcohol for the barrier layer may be obtained by means of copolymerization of ethylene and vinyl alcohol, where the ethylene is present in a percentage in weight comprised between <NUM>% and <NUM>%.

This innovative starting composition for the copolymerization of ethylene vinyl alcohol achieves a good compromise between the required performance of processability and as oxygen barrier.

In particular this composition allows to have a stretching temperature, in the second stretching operation, which is advantageously comprised between about <NUM> and <NUM>, that is, a temperature of the conventional type, but substantially without the disadvantages due to the crystallization of ethylene vinyl alcohol.

The stretching temperature which is used in said first stretching operation can be comprised between about <NUM> and <NUM>.

Preferably, the thickness of the barrier layer is comprised between about <NUM> and <NUM> micron (µm), more preferably between about <NUM> and <NUM> micron (µm), even more preferably between <NUM> and <NUM> micron (µm), so as to obtain the desired oxygen barrier effect.

The film which is obtained from the present invention is made by means of the standard and economical technique of flat head co-extrusion with sequential stretching, as described above.

The film can advantageously be transparent.

Alternatively, the film can also be metalized, obtaining better barrier performances with respect to a traditional metalized BOPP film.

Moreover, the film can have an aesthetic white appearance, thanks to the insertion of TiO<NUM> in the filling or core layer.

The external aesthetic appearance can also be made pearly, thanks to the insertion of CaCO<NUM> in the filling or core layer.

The combination which is obtained by inserting both TiO<NUM> and CaCOs in the filling or core layer, allows to obtain a white, expanded film.

The presence of a single barrier layer of ethylene vinyl alcohol contributes to rendering the film made according to the present invention economically convenient, at the same time also obtaining the desired oxygen barrier effect.

According to a further feature of the present invention, a film with an axially bi-orientated polypropylene (BOPP) base obtainable by the method of the invention, and suitable to make packing, flexible packages, laminated products and labels, comprises at least a partial oxygen barrier effect, the film being formed by a plurality of layers having a polypropylene base, wherein said layers comprise at least a core layer with a polypropylene homopolymer base, a first and a second external layers based on polypropylene copolymer, terpolymer and/or homopolymer, the film further comprising a barrier layer, positioned intermediate said core layer and one of said external layers and having an ethylene vinyl alcohol (EVOH) base, at least an adhesive layer to couple, at least on one side, the barrier layer with a respective adjoining layer, and a layer with a polypropylene copolymer, terpolymer and/or homopolymer base, provided between the core layer and the first external layer.

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:.

With reference to the attached drawings, a plant <NUM> is used to make a final film <NUM> formed by seven layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> positioned one above the other.

The final film <NUM> is used preferentially, even if not exclusively, to make welded/hermetically closed containers by means of thermo-welding, or labels, as indicated above.

In particular there is the following sequence and disposition, so-called asymmetrical, for the layers of the final film <NUM>, as shown in <FIG>:.

The plant <NUM> is of the flat head co-extrusion type with sequential stretching and comprises:.

In particular, the co-extrusion of the ethylene vinyl alcohol for the barrier layer <NUM> is achieved by feeding the raw material at a temperature comprised between <NUM> and <NUM>. There is then an extrusion zone (fusion, filter and adapter) between <NUM> and <NUM>.

It must be noted that, due to the different chemical-physical characteristics, among which the melting temperature in particular, the feed of ethylene vinyl alcohol is distinct from that of the other materials, in particular from the polypropylene homopolymer extruded by the main extruder <NUM>.

As has been said, the extrusion unit <NUM> has an asymmetric configuration, directly correlated with the asymmetric geometry of the final film <NUM> which is to be obtained.

This configuration allows a greater versatility and flexibility in production, allowing to pass from making a film of the traditional type, of the type with <NUM> or <NUM> layers as in the state of the art, to the film according to the present invention, without modifications to the plant itself.

The co-extruded intermediate film <NUM> comes out of the extrusion unit <NUM> which, once cast onto the cooling roll <NUM>, is subjected to sequential stretching, is trimmed, treated and refined by means of slitting so as to make the final film <NUM> final, which is then packed.

The width of the co-extruded intermediate film <NUM> exiting from the extrusion head of the extrusion unit <NUM>, in particular the width of the extrusion head used, determines the so-called width of the productive draw-plate <NUM> of the plant <NUM>, as one would see looking in the direction indicated by the arrow G in <FIG> and as schematically shown also in section in <FIG>.

<FIG> illustrates, on the other hand, a schematic section of a productive draw-plate F for a film P known in the state of the art, consisting of a filling or core layer, S <NUM> of polypropylene homopolymer and of two external layers S2 and S3 of polypropylene copolymer, terpolymer and/or homopolymer.

In this case the width of the state-of-the-art draw-plate F can be identified as the distance indicated by L1, between the ends of the co-extruded film P exiting from the extruder, while the distance L2 indicates the width of the layers S2 and S3 and, finally, the segments L3 indicate the distance of said layers S2 and S3 from the ends of the film P.

With regard to the present invention, the width of the draw-plate <NUM>, univocally determined by the width of the extrusion head, is represented by the distance D1 between the ends of the co-extruded intermediate film <NUM> exiting from the extrusion unit <NUM>. The width of the extrusion head substantially coincides with the value of the distance D <NUM>.

The distance D2 represents the width of each of the layers <NUM>, <NUM> and <NUM>, while the segment D3 represents the distance of each of said layers <NUM>, <NUM>, and <NUM> from the ends of the co-extruded intermediate film <NUM>.

Finally, the distance D4 represents the width of each of the layers <NUM>, <NUM> and <NUM>, while the segment D5 represents the distance of the barrier layer <NUM> from the corresponding end, left or right, of the intermediate film <NUM>, before the selvages are cut (<FIG>).

The segment D6 consists of material that is generated in excess at the sides of the co-extruded intermediate film <NUM> to define a gripping zone, used by the grippers <NUM> for the transverse stretching. The value D6 is normally constant for any thickness of the film and for any width of the draw-plate, usually equal to <NUM> for the present invention.

As we said, to obtain the final film <NUM> it is necessary to have the trimming operation to eliminate the selvages at the sides (usually a segment with a width of about <NUM> - <NUM>), which entails the production of scales of cut off lateral material which are normally re-circulated to the main extruder <NUM>.

In the recirculation material only polypropylene homopolymer must be present, and not ethylene vinyl alcohol too, since the presence of ethylene vinyl alcohol, which is a polymer incompatible with polypropylene, would entail the presence of serious aesthetic defects.

Therefore, to ensure that the trimming operation does not also cut strips of material containing ethylene vinyl alcohol, according to the present invention the barrier layer <NUM> must have a distance D5 from the ends of the co-extruded intermediate film <NUM>, before the selvages are cut, such as to prevent even the partial cutting of the barrier layer <NUM> itself. The distance D5 therefore constitutes a safety measure to prevent the recycling of ethylene vinyl alcohol. Taking into account the space necessary for gripping, segment D6, it is necessary to provide a segment where the cut may be made safely. Keeping this in mind, according to the present invention, the length of the segment D5 is greater than or equal to <NUM>, advantageously comprised between about <NUM> and <NUM>, preferably between <NUM> and <NUM>, more preferably between <NUM> and <NUM>.

The following Table <NUM> shows examples of values in millimeters for the values of L1, L2 and L3 in the state of the art, and also for values of D1, D2, D3, D4, D5 and D6 of the present invention.

As we can see, the distance of the barrier layer <NUM> from the ends of the co-extruded intermediate film <NUM> has been increased, with respect to the same distance L3 of the innermost layer S3 in the state of the art.

Furthermore, again in comparison with the state of the art, the width of the draw-plate D1 has been increased with respect to the analogous value of L1 and the value of the length L2, which in the state of the art related to the layers S2 and S3, corresponds, in the present invention, to the value D4 of the width of the barrier layer <NUM>, whereas the value D2 of the width of the layers <NUM>, <NUM> and <NUM> has been increased.

Another fundamental parameter is the ratio of transverse stretching, that is, the ratio between the transverse size before and after stretching TD.

Generally speaking, the ratios of stretching in the present invention are reduced with respect to what is known in the state of the art.

As a correlation, the stretching temperature and the stretching times have consequently been optimized.

Table <NUM> shows the ranges of values advantageously adopted for transverse stretching TD (temperature in °C, time in seconds).

For example, for a draw-plate with a width of <NUM>, the co-extruded intermediate film <NUM> will have a width of <NUM>, and once it has been cast on the cooling roll <NUM>, it cools and shrinks to a width of <NUM>. Afterward, we have the longitudinal stretching and the film shrinks to about <NUM>. Then, the co-extruded intermediate film <NUM> is gripped at the sides and the transverse stretching is carried out, with a stretching ratio of <NUM>:<NUM>.

The optimum values of stretching ratio, a compromise between the rigidity of ethylene vinyl alcohol and the necessary stretching, are advantageously obtained also by using a particular starting mixture in the copolymerization of ethylene and vinyl alcohol, to obtain ethylene vinyl alcohol.

Normally, the higher the ethylene content, the better the processing properties are and the lower the oxygen barrier properties.

The starting mixture is formed by a composition (percentage in moles) of ethylene comprised between about <NUM>% and <NUM>%, the rest being substantially vinyl alcohol. The characteristic composition according to the present invention provides the desired and optimum balance between the oxygen barrier properties and the processability and working of the copolymer.

The stretching step is extremely important, since it determines the BOPP structure of the final film; the ratios of sequential stretching applied to the co-extruded intermediate film <NUM> also influence the thickness of the various layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>.

In turn, the thicknesses of the various layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are directly correlated to the effectiveness of the respective properties that they must confer on the final film <NUM> (mechanical properties, oxygen barrier, adhesion, thermo-weldability, etc).

The following Table <NUM> shows the ranges of thicknesses obtainable with the present invention for said various layers, as the overall thickness of the film varies (thicknesses in micron µm). The thicknesses of the adhesive layers <NUM> and <NUM> are the same.

It should be noted that for each overall thickness of the final film <NUM>, the values of thicknesses of the various layers that make it up are provided between a minimum value and a maximum value, but it is clear that the choice of individual thicknesses is determined by production needs, so as to have in any case the desired final properties of the film, with the desired final thickness.

The following Table <NUM> shows the oxygen barrier values, expressed in cm<NUM>/m<NUM> d at <NUM> and at <NUM>% relative humidity, method ASTM D <NUM>-<NUM>, as the thickness (in micron µm) of the barrier layer <NUM> varies. The lower the permeability to oxygen, the greater the barrier effect is.

It is clear that these values are examples, which indicate the effectiveness of the present invention as an oxygen barrier, and that these values depend not only on the thickness but also on the composition of the starting mixture for the copolymerization of the ethylene vinyl alcohol.

By increasing the thickness of the barrier layer <NUM>, the oxygen barrier properties increase, but also the overall cost of the final film.

According to the present invention, the barrier layer <NUM> preferably has a thickness of at least <NUM> micron (µm), so as to have a good compromise between optimum oxygen barrier properties and cost of the film.

To have sufficient adhesion properties, using the adhesive with the polyolefin base modified with maleic anhydride, the thickness of each of the adhesive layers <NUM> and <NUM> should preferably not go below <NUM> micron (µm), whereas optimum conditions are obtained with a thickness of around <NUM>-<NUM> microns (µm).

The thicknesses depend on the type of adhesive used, and it is not excluded that lower optimum thicknesses may be achieved with other adhesives.

For thermo-weldability of the final film, the thickness of each of the external layers <NUM> and <NUM> should preferably not go below <NUM> micron (µm).

A preferred final film <NUM> has an overall thickness of <NUM> micron (µm) and provides the core layer <NUM> of <NUM> micron (µm), the barrier layer <NUM> of <NUM> micron (µm), layer <NUM> and layer <NUM> of <NUM> micron (µm) each, layer <NUM> of <NUM> (µm), and the adhesive layers <NUM> and <NUM> of <NUM> micron (µm) each.

As we said, the trimming operation entails the recycling of the cut material, the percentage in weight of which with respect to the core is much higher than in the state of the art. For a film with <NUM> or <NUM> layers, therefore, we have a high percentage weight of re-circled scales, normally even higher than the percentage in weight or quantity that is dosed by the dosing unit <NUM>.

In the state of the art, the feed mouth for standard production (lines of <NUM>, <NUM> and <NUM>) has a diameter that can vary from <NUM> to <NUM>, therefore with a section from about <NUM><NUM> to about <NUM><NUM>.

For example, for a draw-plate <NUM> wide (L1) and with a width of <NUM> (L2) of the layer S2, we have a feed mouth with a circular section with a diameter of <NUM>, and therefore with a surface of about <NUM><NUM>.

Normally, the flow rate of the extrusion line is constant, so that the plant <NUM> can be used in a versatile manner, both to achieve films of a known type, and also for the film according to the present invention. Therefore, when the new film is produced, the percentage weight in the core of the scales deriving from the trimming increases by as much as <NUM> - <NUM>% by virtue of the reduction in thickness of the filling layer or core <NUM>. In the state of the art, instead, in order to achieve the traditional film shown in <FIG>, the scales make up at most <NUM>% of the material that is extruded in the main extruder. The possibility of versatility and flexibility of the plant <NUM>, which allows to pass from traditional production to that according to the present invention, is facilitated by the asymmetric configuration of the final film <NUM> which is produced, as described above.

For one reason and the other, the usable surface A (<FIG>) of the standard type feed mouth has proved to be insufficient for the passage of the main feed and the large quantity of re-circled material as described above.

It has therefore been necessary to modify and oversize the feed mouth <NUM> of the main extruder <NUM>, varying and increasing the section or usable surface A for the passage of polypropylene, with respect to the state of the art.

Therefore, the value of surface A will be directly correlated to the quantity of re-circled material.

To this purpose, the sizes of the feed mouth must be such as to allow a greater feed by a multiplication factor from <NUM> to <NUM> with respect to that of standard productions.

Consequently, the usable surface A is also made supersized by said multiplication factor of <NUM> to <NUM>.

Preferably, the multiplication factor with respect to standard production is equal to <NUM>.

As can be seen in <FIG>, the section or usable surface A of the feed mouth <NUM> can be substantially rectangular, with beveled angles, with a length (M1 and M2) of the two sides of the rectangle respectively of <NUM> and <NUM>. Obviously, the extrusion screw must necessarily have a larger section, compatible with the increase in the re-circled material. In this way, thanks to this surface A of the feed mouth <NUM>, it is possible to prevent blockages of the feed due to excess feed.

Apart from the asymmetric configuration of the final film <NUM>, as described above, alternatively there can also be a symmetrical disposition, so as to achieve a final film <NUM> as in <FIG>. In this case, an intermediate film <NUM> is produced, and the disposition and material of the layers <NUM>, <NUM> is the same, as in the intermediate film <NUM> and the final film <NUM>. Instead, the barrier layer <NUM>, made of the same material as in the final film <NUM>, is substantially at the center of the film <NUM>, in a symmetrical position, with the adhesive layers <NUM> and <NUM> at the sides, all of this surrounded by the material of the core layer <NUM> which therefore makes up two layers, 112a upper and 112b lower. In this case therefore, the adhesive layers <NUM> and <NUM> stick the barrier layer <NUM> of ethylene vinyl alcohol directly to the polypropylene homopolymer of the layers 112a, 112b.

Claim 1:
Method to make a film with an axially bi-orientated polypropylene (BOPP) base and suitable to make packages and labels having at least a partial barrier effect against oxygen and formed by a plurality of layers having a polypropylene base (<NUM>, 112a, 112b; <NUM>; <NUM>; <NUM>, <NUM>, <NUM>), wherein said layers comprise at least a core layer (<NUM>) with a polypropylene homopolymer base, a first (<NUM>) and a second (<NUM>) external layers based on polypropylene copolymer, terpolymer and/or homopolymer, the film further comprising a barrier layer (<NUM>), positioned intermediate said core layer (<NUM>) and one of said external layers (<NUM>, <NUM>) and having an ethylene vinyl alcohol (EVOH) base, at least an adhesive layer (<NUM>, <NUM>) to couple, at least on one side, the barrier layer (<NUM>) with a respective adjoining layer, and a layer (<NUM>) with a polypropylene copolymer, terpolymer and/or homopolymer base, provided between the core layer (<NUM>) and the first external layer (<NUM>), the method comprising the following steps,
- a first co-extrusion step, by means of a flat head extrusion unit (<NUM>), to make a co-extruded intermediate product (<NUM>, <NUM>), in the form of a film, formed by said layers (<NUM>, 112a, 112b; <NUM>, <NUM>; <NUM>; <NUM>, <NUM>, <NUM>) and by said barrier layer (<NUM>),
- a second step of sequential stretching, respectively in the machine direction (MD) and in a direction transverse to the machine (TD), of the intermediate product (<NUM>, <NUM>), so as to determine the desired axially bi-orientated structure (BOPP) and define the final thickness of each of the layers (<NUM>, 112a, 112b; <NUM>; <NUM>; <NUM>);
- a third trimming step, along the longitudinal direction, by means of which the edges of the intermediate product (<NUM>, <NUM>) in excess of the desired final width are cut, so as to define the final film;
characterized in that the trimming step of the edges does not affect the barrier layer (<NUM>), as there is a safety distance (D5) between the lateral ends of said barrier layer (<NUM>) and the lateral ends of said intermediate product (<NUM>, <NUM>).