Multilayer film

Abstract

A multilayer film includes a first layer (A), a third layer (C), and a second layer (B) sandwiched between the first layer (A) and the third layer. Each layer (A, B, C) includes at least one propylene polymer selected from propylene homopolymers, multiphase polymers, propylene copolymers having at least one comonomer selected from ethylene and linear or branched C4-C12 α-olefins, and mixtures thereof. In the second layer (B), the propylene polymer has a melt flow rate (MFR) of not more than 5 g / 10 min, wherein the melt flow rate is determined according to ISO 1133.

Description

Technical Field

[0001] The present invention relates to a multilayer film characterized by controlled shrinkage, and a method for obtaining such a film. Background Technology

[0002] To perform packaging for a specific product, it is known to use multilayer films, including layers of materials such as polyethylene terephthalate (PET), aluminum, and other polymers.

[0003] Recent regulations on product packaging recycling and disposal are gradually phasing out these multi-layered sheets because they cannot be recycled (without incurring unrealistic costs). Currently, multi-layered films are widely used in the manufacture of various types of packaging, especially stand-up pouches (flexible stand-up pouches), which have significant market value.

[0004] Furthermore, it is known that PET sheets are used to close disposable pallets (especially for packaging hot products): in this case, the use of PET complicates recycling operations. In fact, if any design keeps at least one flap of the sealing film integrated with the pallet, it must be treated as multi-material waste, thus creating an additional burden on sorting and grading operations; on the other hand, if the PET sealing film is completely removable, it often fails to enter the recycling process due to inefficient disposal.

[0005] Alternatively, biaxially oriented polypropylene (BOPP) films are currently used, which can be manufactured using different production technologies: one of which is a continuous production process on a flat die; other processes are “bubble-blown”, or “double-bubble”, “triple-bubble”, and “blow-and-stretch”, which are in the machine orientation (MD).

[0006] However, using the aforementioned technologies, it is possible to specifically produce heat-shrinkable or heat-stable films within a very narrow operating range, and they are particularly sensitive to high temperatures, at which they exhibit dimensional changes unacceptable for most of the aforementioned applications. This is found to be especially problematic considering the temperatures required for sterilization processes (e.g., in autoclaves or in temperature-controlled steam environments) or for sealing packages via heat-sealing welding operations. Summary of the Invention

[0007] In view of the above limitations, the object of the present invention is to provide a single-material multilayer film characterized by controlled dimensional shrinkage, which is limited to a threshold specified by the intended industrial application of the film.

[0008] Within this scope, the object of the present invention is to provide a single-material thin film that, when subjected to high temperatures, exhibits dimensional changes in two axial directions (MD / TD) similar to the dimensional changes of currently used multi-material thin films.

[0009] Another object of the present invention is to provide a single-material thin film that can be recycled more effectively.

[0010] Another object of the present invention is to provide a heat-stable single-material film made of biaxially oriented polypropylene (BOPP).

[0011] More specifically, the present invention aims to obtain a heat-stable biaxially oriented polypropylene (BOPP) monolithic film that does not exhibit significant dimensional deformation at conventional sterilization temperatures (on the order of approximately 135°C), even after prolonged periods (more than 2 minutes), and whose deformation magnitude is in all cases the same as that exhibited by currently used PET-containing multilayer films.

[0012] Furthermore, the present invention aims to obtain a thermally stable biaxially oriented polypropylene (BOPP) monolithic film that does not exhibit significant dimensional deformation at conventional heat-sealing temperatures (on the order of at least 170°C) used in industrial sealed packaging, even for periods exceeding 1 second, and in any case, its deformation magnitude is the same as that exhibited by currently used PET-containing multilayer films.

[0013] Another object of the present invention is to identify a method for manufacturing a film according to the invention, which is easy to implement and enables the production of a biaxially oriented polypropylene film that exhibits high dimensional stability even at high temperatures.

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

[0015] This objective, and these and other objectives as will become more apparent below, are achieved by a multilayer film comprising a first layer (A), a third layer (C), and a second layer (B) sandwiched between the first layer (A) and the third layer (C), characterized in that each of the layers (A, B, C) comprises at least one propylene polymer selected from the group consisting of propylene homopolymers, multiphase polymers, propylene copolymers having at least one comonomer selected from ethylene and linear or branched C4-C12 α-olefins, and mixtures thereof, and in the second layer (B), the propylene polymer has a melt mass flow rate (MFR) of not more than 5 g / 10 min, wherein the melt mass flow rate is determined according to ISO 1133.

[0016] The object and purpose of this invention are also achieved by a method for obtaining a multilayer thin film according to the invention, characterized by comprising the following steps:

[0017] (i) At a temperature between 200°C and 270°C, the layers (A, B, C) according to any one of the preceding claims are co-extruded to obtain a multilayer sheet;

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

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

[0020] (iv) The sheet heated in step (iii) is stretched and oriented in an oven at a temperature between 150°C and 190°C by clamping the edges of the sheet with a series of clamps that can move according to the corresponding divergent motion rules, resulting in a longitudinal stretch ratio (MD) between 5.0 and 8 and a transverse stretch ratio (TD) between 5.5 and 8.

[0021] (v) The multilayer film is obtained by performing a final heat setting, using clamps fastened to the opposite edges of the multilayer sheet, while holding the stretched multilayer sheet at a temperature of about 130°C-185°C, to thermally stabilize the stretch ratio of both the longitudinal (MD) and transverse (TD) directions. Attached Figure Description

[0022] Further features and advantages of the invention will become more apparent from the following detailed description of preferred, but not exclusive, embodiments of the multilayer thin films and methods for obtaining multilayer thin films according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

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

[0024] Figure 2 This is a schematic front view of a package made of conventional film after sterilization;

[0025] Figure 3 This is a schematic front view of the heat-sealed opening of a packaging bag made of conventional film after sterilization;

[0026] Figure 4 This is a schematic front view of a packaging bag made of multilayer film according to the present invention after sterilization;

[0027] Figure 5 This is a schematic front view of the heat-sealed opening of a packaging bag made of a multilayer film according to the present invention. Detailed Implementation

[0028] Referring to the accompanying drawings, the multilayer thin film according to the present invention is generally in Figure 1 As shown in the image.

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

[0030] - Each layer A, B, C includes at least one propylene polymer selected from propylene homopolymers, multiphase polymers, propylene copolymers having at least one comonomer selected from ethylene and linear or branched C4-C12 α-olefins, and mixtures thereof;

[0031] - In the second layer B, the propylene polymer has a melt mass flow rate (MFR) of no more than 5 g / 10 min, wherein the melt mass flow rate is determined according to ISO 1133 standard.

[0032] In one embodiment, at least one of the propylene polymers in layers A, B, and C has a melt temperature (Tm) of 115°C or higher, preferably 118°C or higher, and more preferably between 120°C and 167°C.

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

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

[0035] Preferably, the propylene copolymer is characterized in that the content of the comonomer in the copolymer is 0.5-20% of the total weight of the copolymer, preferably 5-20%, and more preferably about 8-15% by weight.

[0036] In one embodiment, at least one propylene polymer is a propylene / ethylene / butene terpolymer.

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

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

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

[0040] (ii) A melting temperature (Tm) of 115°C or higher, preferably 118°C or higher, and more preferably between 120°C and 167°C.

[0041] In one embodiment, the multilayer film of the present invention is characterized in that at least one of layers A and C further comprises at least one compound selected from slip agents and anti-blocking agents, wherein the content is greater than or equal to 20% by weight, preferably 1% to 20% by weight, more preferably 3% to 15% by weight, based on the total weight of the layers, wherein the compound is selected from the group consisting of:

[0042] -Organic reagents,

[0043] -Organic-inorganic elements,

[0044] -Inorganic reagents,

[0045] and its mixtures.

[0046] In one embodiment, the compound is polymethyl methacrylate (PMMA), preferably in the form of spherical particles with an average diameter between 2-8 μm, more preferably between 3-5 μm.

[0047] In another embodiment, the above-mentioned compound is selected from the group consisting of polyorganoxyloxane, polyorganoxylane, and mixtures thereof.

[0048] In another embodiment, the compound is silicon dioxide, preferably in the form of spherical or irregular particles with an average size between 3 μm and 5 μm, which is measured using an X-ray settling particle size analyzer (sold by Micrometrics®).

[0049] In one embodiment, the multilayer film according to the invention is characterized in that layer B comprises a propylene homopolymer with a melt temperature (Tm) greater than or equal to 150°C, preferably greater than or equal to 160°C, and more preferably between 162°C and 167°C.

[0050] In one embodiment, layer B further comprises a hydrocarbon resin selected from the group consisting of resins obtained by polymerizing styrene, methylstyrene, vinyltoluene, indene, pentadiene, cyclopentadiene, and mixtures thereof, preferably amorphous, and comprising less than or equal to 20% by weight, preferably 1-20% by weight, and more preferably 5-10% by weight, based on the total amount of the layer. Preferably, the hydrocarbon resin has a softening point determined according to ASTM E28 method, between 130°C and 180°C, more preferably between 130°C and 160°C. Preferably, the hydrocarbon resin is a hydrogenated resin, more preferably a hydrogenated cyclopentadiene resin.

[0051] In one embodiment, the multilayer film of the present invention is characterized in that layer B comprises a propylene copolymer selected from propylene copolymers having at least one comonomer, said at least one comonomer being selected from the group consisting of ethylene, linear or branched C4-C12 α-olefins and mixtures thereof, preferably selected from the group consisting of ethylene, butene, hexene, octene, decene and mixtures thereof, more preferably selected from the group consisting of ethylene, butene-1 and mixtures thereof, wherein, based on the total weight of the copolymer, the copolymer has a comonomer content comprising between 0.5-20% by weight, preferably between 5-20% by weight, and more preferably between 8-15% by weight. Preferably, layer B comprises at least one regranulated propylene polymer, the content of which is less than 15% by weight, preferably between 10% and 15% by weight, based on the total weight of the layer.

[0052] In a second aspect, the present invention relates to a method for obtaining a multilayer thin film according to any one of the preceding claims, characterized by comprising the following steps:

[0053] (i) Co-extrude layers A, B, and C at a temperature between 200°C and 270°C to obtain a multilayer sheet;

[0054] (ii) Cool the multilayer sheet obtained in step (i) to a temperature between 30-60°C;

[0055] (iii) Heating the multilayer sheet that was previously cooled in step (ii) to a temperature between 30°C and 140°C;

[0056] (iv) The sheet heated in step (iii) is stretched and oriented in an oven at a temperature between 150°C and 190°C by clamping the edges of the sheet with a series of clamps that can move according to the corresponding divergent motion rules, resulting in a longitudinal stretch ratio (MD) between 5.0 and 8 and a transverse stretch ratio (TD) between 5.5 and 8.

[0057] (v) By means of clamps fastened to the opposing edges of the multilayer sheet, while holding the stretched multilayer sheet at a temperature of approximately 130°C–185°C, the stretch ratio of both the longitudinal (MD) and transverse (TD) directions is thermally stabilized through final heat setting to obtain the multilayer film. Preferably, the multilayer sheet obtained in step (i) has a thickness between 0.5 mm and 4 mm.

[0058] In one embodiment, the method of the present invention is characterized in that step (iv) includes:

[0059] - First deformation operation (iv.a), during which a longitudinal stretch ratio (MD) is applied, which includes a stretch ratio between 200% and 700%, preferably between 280% and 620%, more preferably between 300% and 580%, and a transverse stretch ratio is applied, which includes a stretch ratio between 550% and 900%, preferably between 590% and 800%, more preferably between 620% and 760%.

[0060] --Second deformation operation (iv.b), during which a longitudinal stretch ratio (MD) is applied, which includes a range between -100% and 500%, preferably between -50% and 400%, more preferably between -30% and 380%, and a transverse stretch ratio is applied, which includes a range between -50% and 200%, preferably between -30% and 150%, more preferably between -20% and 110%.

[0061] - Third deformation operation (iv.c), during which a longitudinal stretch ratio (MD) is applied, which includes between -250% and 10%, preferably between -140% and 0%, even more preferably between -120% and -20%, and a transverse stretch ratio is applied, which includes between -150% and -100%, preferably between -100% and -400%, even more preferably between -50% and -20%. Preferably, the first deformation operation (iv.a) on the multilayer sheet is performed in the first part of the travel in step (iv), the length of which includes between 0m and 10m, preferably between 0m and 3m; the second deformation operation (iv.b) on the multilayer sheet is performed in the second part of the travel in step (iv), the length of which includes between 2m and 30m, preferably between 3m and 20m; and the third deformation operation (iv.c) on the multilayer sheet is performed in the third part of the travel in step (iv), the length of which is between 15m and 50m, preferably between 20m and 38m.

[0062] The multilayer film 1 according to the invention enables the production of packaging (e.g., stand-up pouches, also known as flexible stand-up pouches, which have significant market applications) that can be sterilized in an autoclave or in a steam environment (at temperatures on the order of 120°-150°, preferably around 135°) without undergoing any type of deformation. Appendix Figure 2 The diagram shows a stand-up pouch-type package X made of a conventional type of BOPP film: the figure illustrates the effect of uneven heat shrinkage occurring on such a material, which causes the package X to have a damaged appearance due to pressure, which is unacceptable for most applications.

[0063] on the other hand, Figure 4 The following is shown: at the end of the sterilization process, package Y made of the multilayer film 1 according to the invention (under temperature conditions similar to those described in the previous paragraphs): package Y will be very smooth and regular because the multilayer film 1 according to the invention is thermally stable, that is, it does not undergo thermal deformation at the normal temperature for sterilization.

[0064] Similarly, Figure 3 The sealing strip W of a stand-up pouch type package X made of a standard type of BOPP film is shown: this seal W is achieved by heat-sealing two overlapping flaps of the standard film, performed by sandwiching them between a pair of heat-sealing strips pressed against each other (typically maintained at a temperature between 130°C and 160°C). It is clear that the heat-sealing process inevitably creates wrinkles and creases in the material, which impairs the appearance and quality of the package X, making it unacceptable for many different applications.

[0065] In comparison, Figure 5 A sealing strip K for heat sealing a stand-up pouch type package Y made of a multilayer film 1 according to the invention (similar to those types described in the previous paragraphs, operating under the same operating conditions) is shown: it can be seen that, in this case, the heat sealing process does not result in the formation of any wrinkles or creases, because the film 1 is thermally stable, that is, it does not undergo thermal deformation at the normal heat sealing temperature if applied for the standard time envisioned for such operation.

[0066] The invention will now be described with reference to the following non-limiting embodiments:

[0067] Example 1: Preparation of a multilayer thin film

[0068] Multilayer films are produced by the following steps:

[0069] (i) The following three films, A, B and C, are co-extruded at a temperature of 255°C:

[0070] A: 96% by weight of polypropylene homopolymer (HP522H sold by LyondellBasell Industries Holdings BV), 1% by weight of anti-blocking agent (AB06001PP sold by CONSTAB Polyolefin Additives Ltd.), and 3% of slip agent (ABVT34SC sold by LyondellBasell Industries Holdings BV).

[0071] B: 100% by weight of polypropylene homopolymer (HP522H sold by LyondellBasell Industries Holding GmbH), and

[0072] C: 98% by weight of polypropylene copolymer (ADSYL3C30FHP sold by LyondellBasell Industries Ltd.), 2% by weight of antiblocking agent (AB06001PP sold by CONSTAB Polyolefin Additives Ltd.

[0073] This results in a multi-layer sheet with a thickness of 1.065 mm;

[0074] (ii) Cool the multilayer sheet to a temperature of 45°C;

[0075] (iii) The sheet is then reheated to 125°C for 60 seconds.

[0076] (iv) The sheet is then heated to 165°C and maintained at 165°C during the following stretching and orientation steps:

[0077] (iv.a) Perform the first deformation operation over a distance of 3 meters from the start of the stretching, during which a longitudinal stretching ratio equal to MD = 300% and a transverse stretching ratio equal to TD = 750% are applied.

[0078] (iv.b) The second deformation operation, over a distance of 10 meters from the start of the stretching, involves performing a longitudinal stretching ratio equal to MD = 360% and a transverse stretching ratio equal to TD = -20%.

[0079] (iv.c) The third deformation operation, over a distance of 25 meters from the start of the stretching, involves performing a longitudinal stretching ratio equal to MD = -80% and a transverse stretching ratio equal to TD = -20%.

[0080] (v) The longitudinal (MD) and transverse (TD) stretch ratios are stabilized by performing a "final heat setting" using clamps fastened to the opposing edges of the multilayer sheets, while simultaneously holding the stretched multilayer sheets at 170°C, thereby producing a multilayer film in which layer A is 1 μm thick, layer B is 23 μm thick, and layer C is 1 μm thick. The resulting multilayer film is characterized by a longitudinal stretch ratio (MD) of 6.0 and a transverse stretch ratio (TD) of 7.1.

[0081] Example 2: Evaluation of the dimensional stability of the thin film according to Example 1 of the present invention

[0082] The dimensional stability of the film preparation of Example 1, the standard BOPP film with a thickness of 25 µm, and the PET film with a thickness of 12 µm were evaluated by drying in a ventilated oven at 150 °C for 5 minutes. The results are shown in Table 1 below:

[0083]

[0084] Table 1

[0085] Therefore, it can be concluded that the multilayer film (25 μm thick) of Example 1 has a minimum dimensional shrinkage of -3.1% in the longitudinal direction due to thermal shrinkage. This value is on the same order of magnitude as the shrinkage of a 12 μm thick film made of polyethylene terephthalate (1.1% in the longitudinal direction), and is on the order of magnitude lower than that of a standard type BOPP sheet (11.6% shrinkage in the longitudinal direction). This means that the film does not exhibit the delamination shown in films A, B, and C, and is not subjected to the stretching procedures and processing parameters listed in the applications described above.

[0086] In the transverse direction, due to thermal shrinkage, the film of Example 1 has a minimum dimensional shrinkage of -1.8% in the transverse direction. This is an order of magnitude lower than the transverse shrinkage (-13%) achievable with a standard BOPP film of the same thickness (25 μm); it is still a close and consistent value even compared to the transverse shrinkage of a 12 μm thick sheet made solely of polyethylene terephthalate (-0.5%).

[0087] Therefore, it has been shown that the film according to Example 1 exhibits surprising thermal stability when placed in a ventilated oven at 150°, and thus it is perfectly suited to undergo sterilization operations.

[0088] To verify that the film according to Example 1 is suitable for heat sealing using a heat-sealing strip heated to 180°C, the corresponding dimensional changes were verified while a pressure of 32 psi was applied and the film was in contact with the heat-sealing strip heated to that temperature for a period of 2 seconds. The data from such tests are given in Table 2 below:

[0089]

[0090] Table 2

[0091] In this case, it can be seen that the layered film according to Example 1 presents a mid-range shrinkage percentage compared to the shrinkage percentage achievable with a standard BOPP film of the same thickness and a 12 µm thick polyethylene terephthalate film, thus ensuring a completely satisfactory level of performance for use, for example, in the packaging industry (and in many other industrial sectors).

[0092] Example 3: Preparation of multilayer thin films

[0093] Multilayer films are produced by the following steps:

[0094] (i) Three types of films, A, B, and C, were extruded at a temperature of 255°C:

[0095] A: 98% by weight of polypropylene homopolymer (HP522H sold by LyondellBasell Industries Ltd.), 2% by weight of antiblocking agent (AB06001PP sold by CONSTAB Polyolefin Additives Ltd.

[0096] B: 100% by weight of polypropylene homopolymer (HP522H sold by LyondellBasell Industries Holding GmbH), and

[0097] C: 98% by weight of polypropylene copolymer (ADSYL3C30FHP sold by LyondellBasell Industries Ltd.), 2% by weight of antiblocking agent (AB06001PP sold by CONSTAB Polyolefin Additives Ltd.

[0098] This results in a multi-layer sheet with a thickness of 0.8875 mm;

[0099] (ii) Cool the multilayer sheet to a temperature of 45°C;

[0100] (iii) The sheet is then reheated to 130°C for 120 seconds.

[0101] (iv) The sheet is then heated to 165°C and maintained at 165°C during the following stretching and orientation steps:

[0102] (iv.a) Perform the first deformation operation over a distance of 3 meters from the start of the stretching, during which a longitudinal stretching ratio equal to MD = 560% and a transverse stretching ratio equal to TD = 750% are applied.

[0103] (iv.b) The second deformation operation, over a distance of 10 meters from the start of the stretching, involves performing a longitudinal stretching ratio equal to MD = -20% and a transverse stretching ratio equal to TD = -20%.

[0104] (iv.c) The third deformation operation, over a distance of 15 meters from the start of the stretching, involves performing a longitudinal stretching ratio equal to MD = -40% and a transverse stretching ratio equal to TD = -20%.

[0105] (v) The longitudinal (MD) and transverse (TD) stretch ratios are stabilized by performing a "final heat setting" using clamps fastened to the opposing edges of the multilayer sheets, while simultaneously holding the stretched multilayer sheets at 178°C, thereby producing a multilayer film in which layer A is 1 μm thick, layer B is 23 μm thick, and layer C is 1 μm thick. The resulting multilayer film is characterized by a longitudinal stretch ratio (MD) of 5.0 and a transverse stretch ratio (TD) of 7.1.

[0106] Example 4: Evaluation of the dimensional stability of the thin film according to Example 3 of the present invention

[0107] The dimensional stability of the film preparation of Example 1, the standard BOPP film with a thickness of 25 µm, and the PET film with a thickness of 12 µm were evaluated by drying in a ventilated oven at 150 °C for 5 minutes. The results are shown in Table 3 below:

[0108]

[0109] Table 3

[0110] Therefore, it can be concluded that the multilayer film (25 μm thick) of Example 1 exhibits the same minimum dimensional shrinkage in the longitudinal direction of a film only 12 μm thick made of polyethylene terephthalate due to thermal shrinkage, and is orders of magnitude lower than that of standard type BOPP films (shrinkage in the longitudinal direction of 11.6%). That is, the film does not have the delamination shown in films A, B, and C, and is not subjected to the process of the above-mentioned type of stretching and the processing parameters listed in the application.

[0111] In the transverse direction, the film of Example 1 has a minimum dimensional shrinkage that is orders of magnitude lower than that of a standard BOPP film of the same thickness (25µm) (-13%); it is still very similar to the transverse shrinkage of a sheet that is only 12µm thick and made solely of polyethylene terephthalate (-0.5%).

[0112] Therefore, it has been shown that the film according to Example 2 exhibits surprising thermal stability when placed in a ventilated oven at 150°, and thus it is perfectly suited to undergo sterilization operations.

[0113] To verify the suitability of the film according to Example 1 for heat sealing using a heat-sealing strip heated to 180°C, the corresponding dimensional changes were verified while a pressure of 32 psi was applied and the film was in contact with the heat-sealing strip heated to that temperature for a period of 2 seconds. The data from such tests are given in Table 4 below:

[0114]

[0115] Table 4

[0116] In this case, it can be seen that the layered film according to Example 2 (in accordance with the invention) presents a mid-range shrinkage percentage compared to the shrinkage percentage achievable with a standard BOPP film of the same thickness and a 12 µm thick polyethylene terephthalate film, thereby ensuring a completely satisfactory level of performance for use, for example, in the packaging industry (and in many other industrial sectors).

[0117] In practice, it has been found that the method and film according to the invention fully achieve the intended goals and objectives because the film of the invention minimizes dimensional changes in the packaging during heat sealing and subsequent product sterilization operations. In particular, the multilayer film according to the invention minimizes the formation of wrinkles or creases on the final packaging compared to BOPP films produced using techniques and processes different from those of the invention.

[0118] Therefore, the precise modulation of film dimensional stability offers surprising advantages during finished product packaging, resolving application problems that compromise the appearance and function of the packaging. In particular, this improvement is evident in the use of the film according to the invention in the production of stand-up pouches and lid films for thermoforming pallets, where, in some cases, the thermoforming pallets can withstand a “retort” process. Specifically, dimensional stability values ​​of MD < -3.5% and TD < -2% measured for 5 minutes in a ventilated oven at 150°C, according to OPMA CT 4(a) standard (described below), resolve the deformation problem of packaging subjected to a “high-temperature retort” process. “High-temperature retort” refers to sterilization in a steam autoclave at 135°C for 60 minutes.

[0119] Furthermore, it has been observed that, measured using the “IRPLAST 36” method (described below), dimensional stability values ​​of MD < -6% and TD ≈ -6% after 2 seconds of contact with a heat-sealed strip heated to 180°C at 32 psi pressure resolve the issue of wrinkles generated in the heat-sealed packaging area.

[0120] IRPLAST 36 method:

[0121] Measurement of contact heat seal

[0122] 1. Purpose

[0123] The purpose of this method is to establish a dimensional change in a portion of the film after it has undergone contact heat sealing.

[0124] 2. Required equipment

[0125] -SENTINEL SENCORP heat sealer, which is equipped with a smooth single upper heat seal and a lower pad in rubber.

[0126] -Hirlinger Precision Ruler

[0127] - Calibrated 40x40mm template

[0128] 3. Test Execution

[0129] - Cut and measure a 15x10 (length x height) sheet from the sample to be inspected;

[0130] - Using a calibrated 40x40mm template, locate the center of the sheet and draw the outline with a black felt-tip marker;

[0131] - On the MD and TD, use a Hirlinger ruler to measure the distance between the edges and record the initial measurements;

[0132] - Introduce the sample into the heat sealer and set the following conditions on the heat sealer at the required temperature for the product type:

[0133] Pressure = 32 psi (2.2 bar)

[0134] Contact time = 2 seconds

[0135] - Position the component so that the smooth strip of the heat sealer affects the entire portion of the film being inspected;

[0136] - At the end of the test, measure and record the distance between the edges on the MD and TD using a Hirlinger ruler.

[0137] 4. Calculation

[0138] Shrinkage, expressed as a percentage, is calculated using the following formula:

[0139] Percentage shrinkage rate = ((AB) * 100) / A

[0140] A = Initial measurement value of the test piece

[0141] B = Final measurement value of the test piece

[0142] For both directions, the calculation is repeated for longitudinal (MD) and transverse (TD).

[0143] accuracy

[0144] The accuracy of the measurement method is estimated using the standard deviation of repeatability and reproducibility, according to the calculation method indicated in UNI-ISO 5725-2 standard. The starting data used to perform the calculations are given in the following three tables:

[0145] Table 1

[0146]

[0147] Table 2

[0148]

[0149] Table 3

[0150]

[0151] For each level, three variances are calculated. These are the repeatability variance, the between-laboratory variance, and the reproducibility variance:

[0152]

[0153]

[0154] The precision of this method is expressed as:

[0155]

[0156] Repeatability standard deviation Sr: 0.175

[0157] Reproducibility standard deviation Sr: 0.155

[0158]

[0159] OPMA TC 4(a) method

[0160] A method for determining a shrink film

[0161] 1. Introduction

[0162] The shrinkage rate of the film is determined by measuring the dimensional change of the film strip after it has been exposed to circulating air in an oven under preset time and temperature conditions.

[0163] 2. Safety

[0164] The test requires the use of blades and an oven at temperatures above 100°C.

[0165] It is recommended to wear protective gloves.

[0166] 3. Methods

[0167] Take a section of film from the reel and place it on the worktable. Adhesive tape can be used to keep the film, which has slight tension, flat.

[0168] Mark two parallel lines spaced 250 mm apart in the longitudinal (MD) and transverse (TD) directions.

[0169] - Measure the length of each strip, slightly tensioned, with a tolerance of 0.1 mm.

[0170] Place the film in an oven for 300 seconds at the required test temperature. Remove and allow to cool.

[0171] - Measure the length of each strip, slightly tensioned, with a tolerance of 0.1 mm.

[0172] 4. Calculation

[0173] Calculate the shrinkage using the following formula:

[0174] Percentage contraction = ((AB) * 100) / A

[0175] A = Initial measurement value of the test piece

[0176] B = Final measurement value of the test piece

[0177] (Two values ​​indicating direction, MD and TD)

[0178] Area shrinkage factor: MD + TD

[0179] 5. Results

[0180] The results are expressed as percentage contraction in both directions, specifying the test conditions. The contraction factor is defined as MD + TD.

[0181] accuracy

[0182] The precision of this method is expressed as:

[0183] Repeatability standard deviation Sr: 0.167

[0184] Reproducibility standard deviation Sr: 0.135

[0185] Therefore, the conceived method and multilayer thin film are susceptible to many modifications and variations, all of which are within the scope of the appended claims; furthermore, all details may be replaced by other technically equivalent elements.

[0186] In fact, the materials and dimensions used can be any that meet the requirements and existing technology.

[0187] The disclosure of Italian patent application No. 102023000025677, which claims priority to this application, is incorporated herein by reference.

[0188] Where reference numerals follow any technical feature mentioned in a claim, these reference numerals have been included to increase the comprehensibility of the claim, and therefore, such reference numerals have no limiting effect on the interpretation of each element identified by way of example through such reference numerals.

Claims

1. A multilayer thin film, comprising a first layer (A), a third layer (C), and a second layer (B) situated between the first layer (A) and the third layer (C), characterized in that, - Each of the layers (A, B, C) includes at least one propylene polymer selected from propylene homopolymers, multiphase polymers, propylene copolymers having at least one comonomer selected from ethylene and linear or branched C4-C12 α-olefins, and mixtures thereof; - In the second layer (B), the propylene polymer has a melt mass flow rate (MFR) of no more than 5 g / 10 min, wherein the melt mass flow rate is determined according to ISO 1133 standard.

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

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

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

5. The multilayer thin film according to claim 4, characterized in that, The propylene copolymer is characterized in that the content of the comonomers in the copolymer is 0.5-20% by weight, preferably 5-20% by weight, and more preferably about 8-15% by weight of the total weight of the copolymer.

6. The multilayer thin 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 of 1.5 to 10 g / 10 min, preferably 2 to 8 g / 10 min, wherein the melt mass flow rate is determined according to ISO 1133.

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

8. The multilayer thin 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 selected from slip agents and anti-blocking agents, in an amount greater than or equal to 20% by weight, preferably from 1% to 20% by weight, preferably from 3% to 15% by weight, based on the total weight of the layers, wherein the compound is selected from the group consisting of: -Organic reagents, -Organic-inorganic elements, -Inorganic reagents, and its mixtures.

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

10. The multilayer thin film according to any one of the preceding claims, characterized in that, The layer (B) further includes a hydrocarbon resin selected from the group consisting of resins obtained by polymerization of styrene, methylstyrene, vinyltoluene, indene, pentadiene, cyclopentadiene, and mixtures thereof, preferably amorphous, and in the total amount of the layer, it is less than or equal to 20% by weight, preferably 1-20% by weight, and more preferably 5-10% by weight.

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

12. A method for obtaining a multilayer thin film according to any one of the preceding claims, characterized in that, The method includes the following steps: (i) At a temperature between 200°C and 270°C, the layers (A, B, C) according to any one of the preceding claims are co-extruded to obtain a multilayer sheet; (ii) Cool the multilayer sheet obtained in step (i) to a temperature between 30-60°C; (iii) Heating the multilayer sheet previously cooled in step (ii) to a temperature between 30°C and 140°C; (iv) The sheet heated in step (iii) is stretched and oriented in an oven at a temperature between 150°C and 190°C by clamping the edges of the sheet with a series of clamps that can move according to the corresponding divergent motion rules, resulting in a longitudinal stretch ratio (MD) between 5.0 and 8 and a transverse stretch ratio (TD) between 5.5 and 8. (v) The multilayer film is obtained by means of clamps fastened to the opposite edges of the multilayer sheet and holding the stretched multilayer sheet at a temperature of about 130°C-180°C, thereby thermally stabilizing the stretch ratio of both the longitudinal (MD) and transverse (TD) directions by performing final heat setting.

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

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

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

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