Multilayer film

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

[0001] MULTILAYER FILM

[0002] The present invention relates to a multilayer film characterized by controlled shrinkages, and a method for obtaining such film.

[0003] To carry out the packaging of specific products, currently the use is known of multilayer films which comprise, for example, layers of polyethylene terephthalate (PET), aluminum, and other polymers.

[0004] Recent regulations on recycling and disposal of product packaging are phasing out this type of multilayer sheets owing to the fact that it is not possible to recycle them (without sustaining impractical costs). Multilayer films are currently used for making many types of packages, in particular doypacks (standup pouches) which have great applicative interest on the market.

[0005] Furthermore, the use is also known of sheets made of PET for closing single-use trays (in particular for packaging hot products): in this case too, the use of PET complicates recycling operations. In fact, if any contrivances are adopted to keep at least one flap of the closure film integral with the tray, then it must be treated as multi-material refuse, with consequent burdens associated with operations of categorization and separation; on the other hand, if the closure film made of PET is completely removable, then often it will not enter the recycling process owing to inefficient disposal.

[0006] Alternatively, nowadays films of bi-oriented polypropylene (“BOPP”) are used, which can be made using different production technologies: one of these is a process of sequential production on a flat head; other processes are “bubble-blown”, or “double-bubble blown”, “triple-bubble blown”, and “bubble-blown with subsequent stretching” in the machine direction (MD).

[0007] However, with the technologies described above, it is possible to produce films that are heat- shrinkable or thermally stable exclusively in very narrow operating ranges, and which in particular are extremely sensitive to high temperatures, at which they present dimensional variations that are unacceptable for the majority of the applications described above. This is found to be particularly problematic, considering the temperatures required by sterilization processes (for example in an autoclave or in an atmosphere of thermally-controlled vapors), or by heat-sealing welding operations to seal the packages.

[0008] In light of the limitations described above, the aim of the present invention is to provide a single-material multilayer film characterized by dimensional contractions that are controlled and limited within thresholds imposed by the industrial applications for which the film is intended.

[0009] Within this aim, an object of the invention is to provide a singlematerial film that, when subjected to high temperatures, exhibits dimensional variations in the two axial directions (MD / TD) that are similar to those of the multi-material films used nowadays.

[0010] Another object of the invention is to provide a single-material film that can be recycled more efficaciously.

[0011] Another object of the present invention is to provide a thermally stable single-material film made of bi-oriented polypropylene (BOPP).

[0012] In more detail, the present invention sets out to obtain a thermally stable single-material film of bi-oriented polypropylene (BOPP) that does not undergo significant dimensional deformations, in any case of the same order of magnitude as the deformations exhibited by multi-layer films currently in use and which comprise PET, at the usual sterilization temperatures (of the order of approximately 135°C), even for extended periods (longer than 2 minutes).

[0013] Moreover, the present invention sets out to obtain a thermally stable single-material film of bi-oriented polypropylene (BOPP) that does not undergo significant dimensional deformations, in any case of the same order of magnitude as the deformations exhibited by multi-layer films currently in use and which comprise PET, at the usual heat- sealing temperatures employed in industry for closing packages (of the order of at least 170°C), even for periods longer than 1 second. Another aim of the present invention is to identify a method for manufacturing a film according to the invention that is simple to carry out and which makes it possible to obtain a bi-oriented polypropylene film that exhibits high dimensional stability even at high temperatures.

[0014] Another object of the present invention is to provide a film that is highly reliable, easy to implement, and at low cost.

[0015] This aim and these and other objects which will become better apparent hereinafter are achieved by a multilayer film comprising a first layer (A), a third layer (C), and a second layer (B) interposed between the first layer (A) and the third layer (C), characterized in that each of said layers (A, B, C) comprises at least one propylene polymer chosen from the group consisting of propylene homopolymers, heterophasic polymers, propylene copolymers with at least one comonomer chosen from ethylene and a linear or branched C4-C12 alpha olefin, and mixtures thereof, and in said second layer (B) the propylene polymer has a melt mass-flow rate (MFR) not exceeding 5 g / 10 min, where the melt mass-flow rate is determined according to the ISO 1133 standard.

[0016] The aims and the objects of the present invention are also achieved by a method for obtaining a multilayer film according to the invention, characterized in that it comprises the steps of:

[0017] (i) co-extruding, at a temperature comprised between 200°C and 270 °C, the layers (A, B, C) according to any one of the preceding claims, resulting in a multilayer sheet;

[0018] (ii) cooling the multilayer sheet obtained in step (i) to a temperature comprised between 30-60°C;

[0019] (iii) heating the multilayer sheet, previously cooled in step (ii), in an environment with a temperature comprised between 30°C and 140°C;

[0020] (iv) stretching and orienting the sheet heated in step (iii) inside an oven at a temperature comprised between 150°C and 190°C by gripping the edges of said sheet, with a series of clamps which can move according to respective divergent rules of motion, resulting in a longitudinal stretch ratio (MD) comprised between 5.0 and 8 and a transverse stretch ratio (TD) comprised between 5.5 and 8;

[0021] (v) thermally stabilizing, by performing a final heat setting, the stretch ratio, both longitudinal (MD) and transverse (TD) by means of a convergence of the clamps fastened on mutually opposite edges of said multilayer sheet while keeping the stretched multilayer sheet at temperatures of about 130°C-185°C, resulting in said multilayer film.

[0022] Further characteristics and advantages of the invention will become better apparent from the detailed description that follows of a preferred, but not exclusive, embodiment of the multilayer film and of the method for obtaining a multilayer film according to the invention, which are illustrated by way of non-limiting example in the accompanying drawings wherein:

[0023] Figure 1 is a block diagram of the steps of the method according to the present invention;

[0024] Figure 2 is a schematic front elevation view of a packaging pouch made with a film of the conventional type after its sterilization;

[0025] Figure 3 is a schematic front elevation view of the heat-sealed closure of a packaging pouch made with a film of the conventional type after its sterilization;

[0026] Figure 4 is a schematic front elevation view of a packaging pouch made with the multilayer film according to the invention after its sterilization;

[0027] Figure 5 is a schematic front elevation view of the heat-sealed closure of a packaging pouch made with the multilayer film according to the invention.

[0028] With reference to the figures, the multilayer film according to the present invention is generally shown in Figure 1.

[0029] In a first aspect, the present invention relates to a multilayer film comprising a first layer A, a third layer C, and a second layer B interposed between the first layer A and the third layer C, characterized in that:

[0030] - each layer A, B, C comprises at least one propylene polymer chosen from the group consisting of propylene homopolymers, heterophasic polymers, propylene copolymers with at least one comonomer chosen from ethylene and a linear or branched C4-C12 alpha olefin, and mixtures thereof;

[0031] - in the second layer B the propylene polymer has a melt mass-flow rate (MFR) not exceeding 5 g / 10 min, where the melt mass-flow rate is determined according to the ISO 1133 standard.

[0032] In an embodiment, the at least one propylene polymer in layers A, B and C has a melting temperature (Tm) higher than or equal to 115°C, preferably higher than or equal to 118°C, more preferably comprised between 120°C and 167°C.

[0033] In an embodiment, the at least one propylene polymer is a homopolymer with isotacticity greater than 80%, preferably greater than 90%, more preferably greater than 97%, measured according to the ISO 24076 standard.

[0034] In another embodiment, the at least one propylene polymer is a copolymer with at least one comonomer chosen from the group consisting of ethylene, butene, hexene, octene, decene, and mixtures thereof, preferably from the group consisting of ethylene, butene, and mixtures thereof.

[0035] Preferably, the above-mentioned propylene copolymer is characterized by a copolymerized comonomer content of 0.5-20%, preferably 5-20%, more preferably about 8-15% by weight on the total weight of the copolymer.

[0036] In an embodiment, the at least one propylene polymer is a propylene / ethy lene / butene terpolymer .

[0037] In an embodiment of the multilayer film according to the invention, at least one of the layers A and C comprises a propylene polymer having a melt mass-flow rate comprised between 1.5 and 10 g / 10 min, preferably between 2 and 8 g / 10 min, wherein the melt mass-flow rate is determined according to the ISO 1133 standard.

[0038] In a preferred embodiment, the multilayer film of the invention is characterized in that at least one of the layers A and C comprises a propylene polymer having:

[0039] (i) a melt mass-flow rate comprised between 1.5 and 10 g / 10 min, preferably between 2 and 8 g / 10 min, wherein the melt mass-flow rate is determined according to the ISO 1133 standard, and

[0040] (ii) a melting temperature (Tm) higher than or equal to 115°C, preferably higher than or equal to 118°C, more preferably comprised between 120°C and 167°C.

[0041] In an embodiment, the multilayer film of the invention is characterized in that at least one of the layers A and C further comprises at least one compound chosen from a slip agent and an anti-blocking agent in an amount greater than or equal to 20% by weight, preferably in an amount comprised between 1 and 20% by weight, more preferably in an amount comprised between 3 and 15% by weight on the total weight of the layer, wherein said compound is chosen from the group consisting of:

[0042] - organic agents,

[0043] - organic-inorganic elements,

[0044] - inorganic agents, and mixtures thereof.

[0045] In an embodiment, the above-mentioned compound is polymethylmethacrylate (PMMA), preferably in the form of spherical particles with an average diameter comprised between 2 and 8 pm, preferably between 3 and 5 pm.

[0046] In another embodiment, the above-mentioned compound is chosen from the group consisting of polyorganoxyloxanes, polyorganoxylanes and mixtures thereof. In another embodiment, the above-mentioned compound is silica, preferably in the form of spherical or irregular particles with an average dimension comprised between 3 and 5 pm, measured with a SediGraph using X-rays (marketed by Micrometrics®).

[0047] In an embodiment, the multilayer film according to the invention is characterized in that said layer B comprises a propylene homopolymer having a melting temperature (Tm) higher than or equal to 150°C, preferably greater than or equal to 160°C, more preferably comprised between 162° and 167°C.

[0048] In an embodiment, the above-mentioned layer B further comprises a hydrocarbon resin chosen from the group consisting of resins derived from the polymerization of styrene, methylstyrene, vinyltoluene, indene, pentadiene, cyclopentadiene, and mixtures thereof, preferably amorphous, in an amount lower than or equal to 20% by weight, preferably 1-20% by weight, more preferably 5-10% by weight, on the total weight of the layer. Preferably, the above-mentioned hydrocarbon resin has a softening point comprised between 130° and 180°C, more preferably between 130° and 160°C, determined according to the ASTM E28 method. Preferably, the hydrocarbon resin is a hydrogenated resin, more preferably a cyclopentadienic hydrogenated resin.

[0049] In an embodiment, the multilayer film of the invention is characterized in that the layer B comprises a propylene copolymer chosen from propylene copolymers with at least one comonomer chosen from the group consisting of ethylene, linear or branched C4-C12 alpha olefins, and mixtures thereof, preferably chosen from the group consisting of ethylene, butene, hexene, octene, decene, and mixtures thereof, more preferably chosen from the group consisting of ethylene, butene- 1, and mixtures thereof, wherein said copolymer has a copolymerized comonomer content comprised between 0.5 and 20% by weight, preferably between 5 and 20% by weight, more preferably between 8 and 15% by weight on the total weight of the copolymer. Preferably, the layer B comprises at least one regranulate propylene polymer in an amount lower than 15% by weight, preferably comprised between 10 and 15% by weight, of the total weight of the layer.

[0050] In a second aspect, the present invention relates to a method for obtaining a multilayer film according to any one of the preceding claims, characterized in that it comprises the steps of:

[0051] (i) co-extruding, at a temperature comprised between 200°C and 270°C, the layers A, B, C resulting in a multilayer sheet;

[0052] (ii) cooling the multilayer sheet obtained in step (i) to a temperature comprised between 30-60°C;

[0053] (iii) heating the multilayer sheet, previously cooled in step (ii), to a temperature comprised between 30°C and 140°C;

[0054] (iv) stretching and orienting the sheet heated in step (iii) inside an oven at a temperature comprised between 150°C and 190°C by gripping the edges of said sheet, with a series of clamps which can move according to respective divergent rules of motion, resulting in a longitudinal stretch ratio (MD) comprised between 5.0 and 8 and a transverse stretch ratio (TD) comprised between 5.5 and 8;

[0055] (v) thermally stabilizing, by performing a final heat setting, the stretch ratio, both longitudinal (MD) and transverse (TD) by means of a convergence of the clamps fastened on mutually opposite edges of said multilayer sheet while keeping the stretched multilayer sheet at temperatures of about 130°C-180°C, resulting in said multilayer film. Preferably, the multilayer sheet obtained in step (i) has a thickness comprised between 0.5 mm and 4 mm.

[0056] In an embodiment, the method of the invention is characterized in that step (iv) comprises:

[0057] - a first deformation operation (iv.a), during which a longitudinal stretch ratio (MD) comprised between 200% and 700%, preferably between 280% and 620%, more preferably between 300% and 580%, and a transverse stretch ratio comprised between 550% and 900%, preferably between 590% and 800%, more preferably between 620% and 760%, are applied,

[0058] - a second deformation operation (iv.b), during which a longitudinal stretch ratio (MD) comprised between -100% and 500%, preferably between -50% and 400%, more preferably between -30% and 380%, and a transverse stretch ratio comprised between -50% and 200%, preferably between -30% and 150%, more preferably between -20% and 110%, are applied,

[0059] - a third deformation operation (iv.c), during which a longitudinal stretch ratio (MD) comprised between -250% and 10%, preferably between -140% and 0%, even more preferably between -120% and -20%, and a transverse stretch ratio comprised between -150% and -100%, preferably between -100% and -400%, more preferably between -50% and -20, are applied. Preferably, the first deformation operation (iv.a) on the multilayer sheet is performed in a first portion of its travel in step (iv) with a length comprised between 0 m and 10 m, preferably between 0 m and 3 m, the second deformation operation (iv.b) on the multilayer sheet is performed in a second portion of its travel in step (iv) with a length comprised between 2 m and 30 m, preferably between 3 m and 20 m, the third deformation operation (iv.c) on the multilayer sheet is performed in a third portion of its travel in step (iv) with a length comprised between 15 m and 50 m in length, preferably between 20 m and 38 m.

[0060] The multilayer film 1 according to the invention makes it possible to produce packages (for example doypacks, also known as standup pouches, which have great applicative interest on the market) which can be sterilized in an autoclave or in a steam atmosphere (at temperatures of the order of 120°-150°, preferably in the neighborhood of 135°C) without undergoing any type of deformation. The accompanying Figure 2 shows a package X of the type of a doypack, made with a BOPP film of the conventional type: the figure graphically represents the effects of the uneven thermal contraction that occurs on such materials, which are such as to give the package X a distressed and damaged appearance, to the point of rendering it unacceptable for the majority of applications.

[0061] Figure 4, on the other hand, shows a package Y made with the multilayer film 1 according to the invention at the end of the sterilization process (under temperature conditions like those described in the previous paragraph): the package Y will be perfectly smooth and regular in that the multilayer film 1 according to the invention is thermally stable, i.e. it does not undergo thermal deformations at the usual temperatures employed for the sterilization.

[0062] Similarly, Figure 3 shows the closure strip W of a package X of the type of a doypack, made with a BOPP film of the conventional type: such closure W is obtained by heat-sealing two superimposed flaps of conventional film, executed by clamping them between a pair of heatsealing bars (generally maintained at a temperature comprised between 130°C and 160°C) forced against each other. It is clear that the heat-sealing process determines the formation of wrinkles and creases on the material which compromise the appearance and the quality of the package X, rendering it unacceptable for a plurality of different applications.

[0063] By contrast, Figure 5 shows the closure strip K heat-sealed with bars (of similar type to those described in the previous paragraph, operating under the same operating conditions) of a package Y of the type of a doypack, made with a multilayer film 1 according to the invention: it can be seen that the heat-sealing process in this case does not result in the formation of substantially any wrinkles or creases, because the film 1 is thermally stable, i.e. it does not undergo thermal deformations at the usual heat-sealing temperatures if applied for the standard times envisaged for such operations.

[0064] The invention will now be described with reference to the following non-limiting examples:

[0065] EXAMPLE 1 : PREPARATION OF A MULTILAYER FILM

[0066] A multilayer film was produced by means of the following steps:

[0067] (i) the following three films A, B and C were co-extruded at a temperature of 255°C:

[0068] A: 96% by weight of a polypropylene homopolymer (HP522H marketed by LyondellBasell Industries Holdings B.V.), 1% by weight of an anti-blocking agent (AB06001PP marketed by CONSTAB Polyolefin Additives GmbH), and 3% of a slip agent (ABVT34SC marketed by LyondellBasell Industries Holdings B.V.),

[0069] B: 100% by weight of a polypropylene homopolymer (HP522H marketed by LyondellBasell Industries Holdings B.V.), and

[0070] C: 98% by weight of a polypropylene copolymer (ADSYL3C30FHP marketed by LyondellBasell Industries Holdings B.V.), 2% by weight of an anti-blocking agent (AB06001PP marketed by CONSTAB Polyolefin Additives GmbH), so obtaining a multilayer sheet with a thickness of 1.065 mm;

[0071] (ii) the multilayer sheet was cooled to a temperature of 45°C;

[0072] (iii) subsequently the sheet was reheated to 125°C for a period of 60 seconds.

[0073] (iv) The sheet was then heated to and kept at a temperature of 165°C during the following steps of stretching and orientation:

[0074] (iv.a) a first deformation operation, for a distance, from the start of the stretch, of 3 meters, over the course of which a longitudinal stretch ratio equal to ME) = 300%, and a transverse stretch ratio equal to TD = 750%, are executed,

[0075] (iv.b) a second deformation operation, for a distance, from the start of the stretch, of 10 meters, over the course of which a longitudinal stretch ratio equal to ME) = 360%, and a transverse stretch ratio equal to TD = -20%, are executed, (iv.c) a third deformation operation, for a distance, from the start of the stretch, of 25 meters, over the course of which a longitudinal stretch ratio equal to MD = -80%, and a transverse stretch ratio equal to TD = -20%, are executed;

[0076] (v) the stretch ratio, both longitudinal (MD) and transverse (TD), was thermally stabilized, by performing a "final heat setting", by means of a convergence of clamps fastened on mutually opposite edges of the multilayer sheet while keeping the stretched multilayer sheet at 170°C, resulting in a multilayer film where the layer A is 1 pm thick, the layer B is 23 pm thick, and the layer C is 1 pm thick. The multilayer film thus obtained is characterized by a longitudinal stretch ratio (MD) of 6.0, and a transverse stretch ratio (TD) of 7.1.

[0077] EXAMPLE 2: EVALUATION OF THE DIMENSIONAL STABILITY OF A FILM ACCORDING TO EXAMPLE 1 OF THE INVENTION

[0078] In a ventilated stove at 150°C for 5 minutes, the dimensional stability of the film preparation in Example 1, of a standard BOPP film 25 pm thick, and of a PET film 12 pm thick were evaluated. The results are given in Table 1 below:

[0079] Table 1

[0080] It is therefore possible to conclude that the multilayer film of Example 1 (with a thickness of 25 pm) has a minimum dimensional contraction, owing to heat- shrinking, in the longitudinal direction, equal to -3.1%. This value is in the same order of magnitude as that of a film just 12 pm thick made of polyethylene terephthalate (a contraction of 1.1% in the longitudinal direction), and in a lower order of magnitude than that of a sheet of standard-type BOPP (a contraction of 11.6% in the longitudinal direction), i.e. a film without the layering illustrated of the films A, B, C and not subjected to a procedure of stretching of the type shown above and applying the processing parameters listed.

[0081] In the transverse direction the film of Example 1 has a minimum dimensional contraction, owing to heat- shrinking, in the transverse direction, equal to -1.8%. This is a transverse direction contraction of a lower order of magnitude than that obtainable with a standard BOPP film of equal thickness (25 pm), which is -13%; even compared with the transverse direction contraction of a sheet just 12 pm thick made of polyethylene terephthalate (-0.5%), it is still a close and consistent value.

[0082] It has therefore been shown that the film according to Example 1 exhibits a surprising thermal stability if placed in a ventilated stove at 150°, and therefore it is perfectly suitable for being subjected to sterilization operations.

[0083] To verify the suitability of the film according to Example 1 for heatsealing using heat-sealing bars heated to a temperature of 180°C, the respective dimensional variations were verified upon contact with a bar heated to that temperature for a period of 2 seconds, while a pressure of 32 psi is applied. The data for such a test are given in Table 2 below:

[0084] Table 2

[0085] In this case, it can be seen that the layered film according to Example 1 presents contraction percentages of intermediate value with respect to those obtainable with a standard BOPP film of identical thickness and those obtainable with a polyethylene terephthalate film 12 pm thick, so ensuring performance levels that are entirely satisfactory for use, for example, in the packaging industry (as well as in many other industrial sectors).

[0086] EXAMPLE 3: PREPARATION OF A MULTILAYER FILM

[0087] A multilayer film was produced by means of the following steps:

[0088] (i) the following three films A, B and C were co-extruded at a temperature of 255°C:

[0089] A: 98% by weight of a polypropylene homopolymer (HP522H marketed by LyondellBasell Industries Holdings B.V.), 2% by weight of an anti-blocking agent (AB06001PP marketed by CONSTAB Polyolefin Additives GmbH),

[0090] B: 100% by weight of a polypropylene homopolymer (HP522H marketed by LyondellBasell Industries Holdings B.V.), and

[0091] C: 98% by weight of a polypropylene copolymer (ADSYL3C30FHP marketed by LyondellBasell Industries Holdings B.V.), 2% by weight of an anti-blocking agent (AB06001PP marketed by CONSTAB Polyolefin Additives GmbH), so obtaining a multilayer sheet with a thickness of 0.8875 mm;

[0092] (ii) the multilayer sheet was cooled to a temperature of 45°C;

[0093] (iii) subsequently the sheet was reheated to 130°C for a period of 120 seconds.

[0094] (iv) The sheet was then heated to and kept at a temperature of 165°C during the following steps of stretching and orientation:

[0095] (iv.a) a first deformation operation, for a distance, from the start of the stretch, of 3 meters, over the course of which a longitudinal stretch ratio equal to ME) = 560%, and a transverse stretch ratio equal to TD = 750%, are executed,

[0096] (iv.b) a second deformation operation, for a distance, from the start of the stretch, of 10 meters, over the course of which a longitudinal stretch ratio equal to MD = -20%, and a transverse stretch ratio equal to TD = -20%, are executed,

[0097] (iv.c) a third deformation operation, for a distance, from the start of the stretch, of 15 meters, over the course of which a longitudinal stretch ratio equal to MD = -40%, and a transverse stretch ratio equal to TD = -20%, are executed;

[0098] (v) the stretch ratio, both longitudinal (MD) and transverse (TD), was thermally stabilized, by performing a "final heat setting", by means of a convergence of clamps fastened on mutually opposite edges of the multilayer sheet while keeping the stretched multilayer sheet at 178°C, resulting in a multilayer film where the layer A is 1 pm thick, the layer B is 23 pm thick, and the layer C is 1 pm thick. The multilayer film thus obtained is characterized by a longitudinal stretch ratio (MD) of 5.0, and a transverse stretch ratio (TD) of 7.1.

[0099] EXAMPLE 4: EVALUATION OF THE DIMENSIONAL STABILITY OF A FILM ACCORDING TO EXAMPLE 3 OF THE INVENTION

[0100] In a ventilated stove at 150°C for 5 minutes, the dimensional stability was evaluated of the film preparation in Example 1, of a standard BOPP film 25 pm thick, and of a PET film 12 pm thick. The results are given in Table 3 below:

[0101] Table 3

[0102] It can therefore be concluded that the multilayer film of Example 1 (with a thickness of 25 pm) presents the same minimum dimensional contraction, owing to heat- shrinking, in the longitudinal direction of a film just 12 pm thick made of polyethylene terephthalate, and a lower order of magnitude than that of a film of standard-type BOPP (a contraction of 11.6% in the longitudinal direction), i.e. a film without the layering illustrated of the films A, B, C and not subjected to a procedure of stretching of the type shown above and applying the processing parameters listed.

[0103] In the transverse direction, the film of Example 1 has a minimum dimensional contraction of a lower order of magnitude than that obtainable with a standard BOPP film of equal thickness (25 pm), which is -13%; even compared with the transverse direction contraction of a sheet just 12 pm thick made of polyethylene terephthalate (-0.5%), it is still a very similar value.

[0104] It has therefore been shown that the film according to Example 2 exhibits a surprising thermal stability if placed in a ventilated stove at 150°, and therefore it is perfectly suitable for being subjected to sterilization operations.

[0105] To verify the suitability of the film according to Example 1 for heatsealing using heat-sealing bars heated to a temperature of 180°C, the respective dimensional variations were verified upon contact with a bar heated to that temperature for a period of 2 seconds, while a pressure of 32 psi is applied. The data for such a test are given in Table 4 below:

[0106] Table 4

[0107] In this case too, it can be seen that the layered film according to Example 2 (according to the invention) presents contraction percentages of intermediate value with respect to those obtainable with a standard BOPP film of identical thickness and those obtainable with a polyethylene terephthalate film 12 pm thick, so ensuring performance levels that are entirely satisfactory for use, for example, in the packaging industry (as well as in many other industrial sectors).

[0108] In practice it has been found that the method and the film according to the invention fully achieve the set aim and objects, in that the film of the invention makes it possible to minimize the dimensional variations in the packaging during the heat-sealing operations and subsequent operations to sterilize the products. In particular, the multilayer film according to the invention makes it possible to minimize the formation of wrinkles or creases on the final package with respect to BOPP film produced using different technologies and processes from those according to the present invention.

[0109] Precise modulation of the dimensional stability of the film therefore has a surprising advantage during packaging of the finished products, solving application problems that compromise the quality of appearance and of function of the packaging. In particular, such improvements are evident from the use of the film according to the invention in the production of stand up pouches and lidding film for heat-formed trays which can in some cases be subjected to the “retort” process. In particular, a dimensional stability value in MD < -3.5% and in TD < -2%, measured by way of the OPMA CT 4 (a) standard (described below) in a ventilated stove at a temperature of 150°C for a period of 5 min, solves the distortion problems of packaging subjected to the "high retort" process. “High retort” means sterilization in a steam autoclave at 135°C for 60min.

[0110] Furthermore it has been observed that a value of dimensional stability in MD < -6% and in TD ~ -6%, measured with the “IRPLAST 36” method (described below) in contact with a heat-sealing bar heated to 180°C at a pressure of 32 psi for 2 secs, solves the problem of creases generated in the areas of packaging subjected to heat- sealing.

[0111] IRPLAST 36 method: MEASUREMENT OF CONTACT HEAT-SEALING

[0112] 1.

[0113] The purpose of the method is to establish the change in the dimensions of a portion of film after it has been subjected to contact heatsealing.

[0114] - SENTINEL SENCORP heat-sealer, equipped with a smooth single upper heat-sealing bar and a lower mat in rubber

[0115] - Hirlinger precision ruler

[0116] - Calibrated 40x40 mm template

[0117] 3. Execution of the test

[0118] - Cut a sheet measuring 15x10 (length x height) from the sample under examination;

[0119] - With the calibrated 40x40 mm template, locate the center of the sheet and with a black felt-tip marker draw the outline;

[0120] - Measure the distance between the edges with the Hirlinger ruler, both in MD and in TD, and record the initial measurement;

[0121] - Introduce the sample into the heat-sealer and at the required temperature for the product type, setting the following conditions on the heat- sealer:

[0122] Pressure = 32 psi (2.2 bar)

[0123] Contact time = 2 sec

[0124] - Position the piece so that the smooth bar of the heat-sealer affects the entire portion of film under examination;

[0125] - At the end of the test, measure with the Hirlinger ruler and record the distance between the edges, both in MD and in TD.

[0126] 4. Calculation

[0127] The shrinkage, expressed as a percentage, is calculated with the formula: PERCENTAGE SHRINKAGE = ((A-B)*100) / A

[0128] A = initial measurement of the test piece

[0129] B = final measurement of the test piece

[0130] This calculation is repeated for the two directions, Longitudinal (MD) and Transverse (TD).

[0131] Precision

[0132] The precision of the measurement method was estimated using the standard deviations of repeatability and reproducibility, in accordance with the calculation methodology indicated in the UNI-ISO 5725-2 standard. The starting data for performing the calculations are given in the following three tables:

[0133] Table 1

[0134] Table 2

[0135] Table 3

[0136] For each level three variances were calculated. They are the repeatability variance, the inter-laboratory variance and the reproducibility variance:

[0137] The precision of the method is indicated as:

[0138] Standard deviation of repeatability Sr: 0.175

[0139] Standard deviation of reproducibility Sr: 0.155 QPMA TC4 (a) METHOD

[0140] METHOD FOR DETERMINING THE SHRINKAGE FILM

[0141] 1. Introduction

[0142] The shrinkage of the films is determined by measuring the change in the dimensions of strips of film after exposure in a stove to circulating air under preset conditions of time and temperature.

[0143] 2, Safety

[0144] This test requires the use of blades and of a stove at temperatures higher than 100°C.

[0145] The use of protective gloves is recommended. 3 , Method

[0146] - Take a section of film from the spool and lay it on a bench. Adhesive tape can be used to keep the film flat with a slight tension.

[0147] - Mark two parallel lines, 250 mm apart, in the longitudinal (MD) and transverse (TD) directions. - Measure the length of each strip, slightly tensed, with a tolerance of 0.1 mm.

[0148] - Place the film in the stove for 300 seconds at the temperature necessary for the test. Remove and allow to cool.

[0149] - Measure the length of each strip, slightly tensed, with a tolerance of 0.1 mm.

[0150] 4. Calculation

[0151] The shrinkage is calculated with the formula:

[0152] PERCENTAGE SHRINKAGE = ((A-B)*100) / A

[0153] A = initial measurement of the test piece

[0154] B = final measurement of the test piece

[0155] (Indicate the two values for the directions MD and TD)

[0156] Area shrinkage factor: MD + TD

[0157] 5. Results

[0158] The results are expressed as a percentage shrinkage in the two directions, specifying the conditions of the test. The area shrinkage factor is defined as MD + TD.

[0159] Precision

[0160] The precision of the method is indicated as:

[0161] Standard deviation of repeatability Sr: 0.167

[0162] Standard deviation of reproducibility Sr: 0.135

[0163] The method and the multilayer film, thus conceived, are susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; moreover, all the details may be substituted by other, technically equivalent elements.

[0164] In practice, the materials employed, as well as the dimensions, may be any according to requirements and to the state of the art.

[0165] The disclosures in Italian Patent Application No. 102023000025677 from which this application claims priority are incorporated herein by reference.

[0166] Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

CLAIMS1. A multilayer film comprising a first layer (A), a third layer (C), and a second layer (B) interposed between the first layer (A) and the third layer (C), characterized in that- each of said layers (A, B, C) comprises at least one propylene polymer chosen from the group consisting of propylene homopolymers, heterophasic polymers, propylene copolymers with at least one comonomer chosen from ethylene and a linear or branched C4-C12 alpha olefin, and mixtures thereof;- and in said second layer (B) the propylene polymer has a melt massflow rate (MFR) not exceeding 5 g / 10 min, where the melt mass-flow rate is determined according to the ISO 1133 standard.

2. The multilayer film according to claim 1, characterized in that said at least one propylene polymer in layers A, B and C has a melting temperature (Tm) higher than or equal to 115°C, preferably higher than or equal to 118°C, more preferably comprised between 120°C and 167°C.

3. The multilayer film according to claim 1 or 2, characterized in that said at least one propylene polymer is a homopolymer with isotacticity greater than 80%, preferably greater than 90%, more preferably greater than 97%, measured according to the ISO 24076 standard.

4. The multilayer film according to claim 1 or 2, characterized in that said at least one propylene polymer is a copolymer with at least one comonomer chosen from the group consisting of ethylene, butene, hexene, octene, decene, and mixtures thereof, preferably from the group consisting of ethylene, butene, and mixtures thereof.

5. The multilayer film according to claim 4, characterized in that said propylene copolymer is characterized by a copolymerized comonomer content of 0.5-20%, preferably 5-20%, more preferably about 8-15% by weight on the total weight of the copolymer.

6. The multilayer film according to any one of the preceding claims,characterized in that at least one of the layers (A) and (C) comprises a propylene polymer having a melt mass-flow rate comprised between 1.5 and 10 g / 10 min, preferably between 2 and 8 g / 10 min, wherein the melt massflow rate is determined according to the ISO 1133 standard.

7. The multilayer film according to any one of the preceding claims, characterized in that at least one of the layers (A) and (C) comprises a propylene polymer having:(i) a melt mass-flow rate comprised between 1.5 and 10 g / 10 min, preferably between 2 and 8 g / 10 min, wherein the melt mass-flow rate is determined according to the ISO 1133 standard, and(ii) a melting temperature (Tm) higher than or equal to 115°C, preferably higher than or equal to 118°C, more preferably comprised between 120°C and 167°C.

8. The multilayer film according to any one of the preceding claims, characterized in that at least one of the layers (A) and (C) further comprises at least one compound chosen from a slip agent and an anti-blocking agent in an amount greater than or equal to 20% by weight, preferably in an amount comprised between 1 and 20% by weight, more preferably in an amount comprised between 3 and 15% by weight on the total weight of the layer, wherein said compound is chosen from the group consisting of:- organic agents,- organic-inorganic elements,- inorganic agents, and mixtures thereof.

9. The multilayer film according to any one of the preceding claims, characterized in that said layer (B) comprises a propylene homopolymer having a melting temperature (Tm) higher than or equal to 150°C, preferably greater than or equal to 160°C, more preferably comprised between 162° and 167°C.

10. The multilayer film according to any one of the preceding claims,characterized in that said layer (B) further comprises a hydrocarbon resin chosen from the group consisting of resins derived from the polymerization of styrene, methylstyrene, vinyltoluene, indene, pentadiene, cyclopentadiene, and mixtures thereof, preferably amorphous, in an amount lower than or equal to 20% by weight, preferably 1-20% by weight, more preferably 5-10% by weight, on the total weight of the layer.

11. The multilayer film according to any one of the preceding claims, characterized in that said layer (B) comprises a propylene copolymer chosen from propylene copolymers with at least one comonomer chosen from the group consisting of ethylene, linear or branched C4-C12 alpha olefins, and mixtures thereof, preferably chosen from the group consisting of ethylene, butene, hexene, octene, decene, and mixtures thereof, more preferably chosen from the group consisting of ethylene, butene- 1, and mixtures thereof, wherein said copolymer has a copolymerized comonomer content comprised between 0.5 and 20% by weight, preferably between 5 and 20% by weight, more preferably between 8 and 15% by weight on the total weight of the copolymer.

12. A method for obtaining a multilayer film according to any one of the preceding claims, characterized in that it comprises the steps of:(i) co-extruding, at a temperature comprised between 200°C and 270 °C, the layers (A, B, C) according to any one of the preceding claims, resulting in a multilayer sheet;(ii) cooling the multilayer sheet obtained in step (i) to a temperature comprised between 30-60°C;(iii) heating the multilayer sheet, previously cooled in step (ii), to a temperature comprised between 30°C and 140°C;(iv) stretching and orienting the sheet heated in step (iii) inside an oven at a temperature comprised between 150°C and 190°C by gripping the edges of said sheet, with a series of clamps which can move according to respective divergent rules of motion, resulting in a longitudinal stretch ratio(MD) comprised between 5.0 and 8 and a transverse stretch ratio (TD) comprised between 5.5 and 8;(v) thermally stabilizing, by performing a final heat setting, the stretch ratio, both longitudinal (MD) and transverse (TD) by means of a convergence of the clamps fastened on mutually opposite edges of said multilayer sheet while keeping the stretched multilayer sheet at temperatures of 130°C-180°C, resulting in said multilayer film.

13. The method according to claim 12, characterized in that said multilayer sheet obtained in step (i) has a thickness comprised between 0.5 mm and 4 mm.

14. The method according to any one of claims 12 and 13, characterized in that said step (iv) comprises:- a first deformation operation (iv.a), during which a longitudinal stretch ratio (MD) comprised between 200% and 700%, preferably between 280% and 620%, more preferably between 300% and 580%, and a transverse stretch ratio comprised between 550% and 900%, preferably between 590% and 800%, more preferably between 620% and 760%, are applied,- a second deformation operation (iv.b), during which a longitudinal stretch ratio (MD) comprised between -100% and 500%, preferably between -50% and 400%, more preferably between -30% and 380%, and a transverse stretch ratio comprised between -50% and 200%, preferably between -30% and 150%, more preferably between -20% and 110%, are applied,- a third deformation operation (iv.c), during which a longitudinal stretch ratio (MD) comprised between -250% and 10%, preferably between -140% and 0%, more preferably between -120% and -20%, and a transverse stretch ratio comprised between -150% and -100%, preferably between -100% and -400%, more preferably between -50% and -20, are applied.

15. The method according to claim 14, characterized in that said first deformation operation (iv.a) on the multilayer sheet is performed in a firstportion of its travel in step (iv) with a length comprised between 0 m and 10 m, preferably between 0 m and 3 m, said second deformation operation (iv.b) on the multilayer sheet is performed in a second portion of its travel in step (iv) with a length comprised between 2 m and 30 m, preferably between 3 m and 20 m, said third deformation operation (iv.c) on the multilayer sheet is performed in a third portion of its travel in step (iv) with a length comprised between 15 m and 50 m in length, preferably between 20 m and 38 m.