WO2010015402A1

WO2010015402A1 - Multilayer structures comprising a microlayer sequence

Info

Publication number: WO2010015402A1
Application number: PCT/EP2009/005695
Authority: WO
Inventors: Roberto Forloni
Original assignee: Cryovac, Inc.
Priority date: 2008-08-07
Filing date: 2009-08-06
Publication date: 2010-02-11

Abstract

Multi-layer cast structures particularly suitable for thermoforming applications comprise a coextruded polypropylene-containing microlayer sequence (a) of at least 16 microlayers, made of a number n of identical repeating units (a’), that in addition to a polypropylene microlayer (A) preferably comprise also a polyolefin microlayer (B) having a composition other than that of (A). In a preferred embodiment the microlayer sequence (a) is coextruded with an outer heat-sealable layer (b) and an outer abuse-resistant layer (c), preferably comprising a (co)polyamide or/and a (co)polyester.

Description

MULTILAYER STRUCTURES COMPRISING A MICROLAYER SEQUENCE

The present invention relates to multilayer structures suitable for packaging applications and in particular to cast co-extruded multilayer films and sheets comprising a microlayer sequence comprising polypropylene microlayers, to cast films and sheets obtained therefrom by addition of further layers via lamination and/or extrusion coating and to the films obtained from any of the above structures by a solid state orientation process that orient all the layers of the structure.

BACKGROUND OF THE INVENTION In the packaging area, and particularly in the food packaging area, cast, polypropylene-based structures are currently and widely used for thermoforming applications. The most interesting of these structures are those designed for use at customers on conventional thermoforming machines for the vacuum packaging of refrigerated or heat-treated foodstuff. These polypropylene-based structures generally include at least one bulk layer of polypropylene homopolymer, typically isotactic polypropylene (iPP or just PP), often a sealant lower melting polyolefin layer, if the thermoformed article needs to be closed by sealing a lid thereto, sometimes a core gas- barrier layer, typically comprising ethylene-vinyl alcohol copolymers and/or polyamides, optionally an outer abuse-resistant layer, and adhesive layers to improve the bonds between possible adjacent layers whenever this is necessary.

While the use of polypropylene homopolymer for the bulk layer brings about a number of advantages, in particular when the article must sustain high temperatures and aggressive environmental conditions, on the other hand this may create problems of poor impact resistance particularly at the low temperatures. An attempt to solve this problem foresees the inclusion in the overall structure, either as a replacement of the polypropylene homopolymer or in addition thereto, typically admixed in the same layer, of other propylene polymers, such as propylene copolymers, in particular propylene block copolymers, that are known to improve impact strength. Also the thermoformability of a structure comprising a bulk polypropylene homopolymer layer often needs to be improved and for the time being this problem is typically addressed by the addition of other polyolefin (polyethylene-based) or polyamide layers in the structure. This generally leads to very thick structures if the desired thermoformability combined with the required impact strength, particularly in the end thermoformed articles, has to be obtained.

There is therefore a continuous need for a propylene-containing cast sheet or film excellent in thermoformability and endowed with a high impact resistance even at low temperatures and at low gauges.

SUMMARY OF THE INVENTION

The present invention is directed to cast co-extruded multi-layer, films or sheets which comprise a microlayer sequence (a) comprising polypropylene microlayers. Said microlayer sequence (a) is of at least 16 microlayers and comprises a number n of identical sub-units (a') that are repeated.

In one embodiment the microlayer sequence contains only polypropylene microlayers. In another, preferred, embodiment each repeating unit (a') comprises in addition to a polypropylene microlayer at least one other polyolefin microlayer with a composition different from that of the polypropylene microlayer.

The present invention is also directed to the multilayer films and sheets that can be obtained from the above cast co-extruded ones by the addition of further layers via lamination or extrusion coating.

The cast films and sheets of the present invention comprising the co-extruded microlayer sequence (a) show improved thermoformability and stretchability properties with respect to the corresponding films and sheets containing single layers of the same resins of a thickness corresponding to the sum of the thicknesses of the layers in the sequence (a). Often this is coupled with improved mechanical properties. These structures therefore are particularly suitable for thermoforming packaging applications. The present invention is also directed to the films which are obtained from any of the above multi-layer cast structures by a solid state orientation process carried out at a temperature sufficiently low to provide for the solid state orientation of all the layers of the structure.

The films thus obtained may be heat-shrinkable or heat-set and may be employed in packaging application e.g., as wrapping films, lidding films, for the manufacture of bags or pouches, etc. The objects, advantages, and features of the present invention will be more readily understood and appreciated by reference to the detailed description of the invention.

DEFINITIONS

While the term "film"generally refers to plastic web materials having a thickness of 250 μm or less, and the term "sheet" to those with a thickness of more than 250 μm, for the sake of simplicity in the present description and claims the term "film" is used in a generic sense to include any plastic web, regardless of whether it is film or sheet.

As used herein the phrases "inner layer" and "internal layer" refer to any film layer having both of its principal surfaces directly adhered to another layer of the film. As used herein, the phrase "outer layer" refers to any film layer having only one of its principal surfaces directly adhered to another layer of the film

As used herein, the phrases "seal layer", "sealing layer", "heat seal layer", and "sealant layer", refer to the film outer layer which will be involved in the sealing of the film either to itself or to another film to close the package and that will thus be in contact with, or closer to, the packaged product.

As used herein, the phrase "adhesive layer" or "tie layer" refers to any inner film layer having the primary purpose of adhering two layers to one another.

As used herein, and widely accepted in the art, the term "cast co-extruded" refers to a film where the combined layered melt of all the simultaneously extruded (co-extruded) layers that exits from the co-extrusion die is immediately quenched, e.g. by contact with a chilled roll or with a chilled fluid, and thus solidified.

As used herein, the phrases "longitudinal direction" and "machine direction", herein abbreviated "MD", refer to a direction "along the length" of the film, i.e., in the direction of the film as the film is formed during extrusion and/or coating. As used herein, the phrase "transverse direction", herein abbreviated "TD", refers to a direction across the film, perpendicular to the machine or longitudinal direction.

As used herein, the term "solid state orientation" refers to the process of stretching of the cast film carried out at a temperature at which all the layers of the structure are solid state oriented by the stretching process, i.e., a temperature higher than the Tg (glass transition temperatures) of the resins making up the layers of the structure and lower than the temperature at which any of the layers of the structure is in the molten state. The solid state orientation may be mono-axial, either longitudinal or transversal, or, preferably, bi-axial.

As used herein the phrases "heat-shrinkable," "heat-shrink," and the like, refer to the tendency of the solid-state oriented film to shrink upon the application of heat, i.e., to contract upon being heated, such that the size of the film decreases while the film is in an unrestrained state. As used herein said term refer to solid-state oriented films with a free shrink in at least one of the machine and the transverse directions, as measured by ASTM D 2732, of at least 5 % at 95 ⁰C.

As used herein, the term "homo-polymer" is used with reference to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of mer, i.e., repeating unit.

As used herein, the term "co-polymer" refers to polymers formed by the polymerization reaction of at least two different monomers. When used in generic terms the term "co-polymer" is also inclusive of, for example, ter-polymers. The term "co-polymer" is also inclusive of random co-polymers, block co-polymers, and graft co-polymers.

As used herein, the terms "(co)polymer" and "polymer" are inclusive of homo- polymers and co-polymers.

As used herein, the phrase "heterogeneous polymer" refers to polymerization reaction products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, i.e., typical polymers prepared, for example, using conventional Ziegler-Natta catalysts.

As used herein, the phrase "homogeneous polymer" refers to polymerization reaction products of relatively narrow molecular weight distribution and relatively narrow composition distribution. This term includes those homogeneous polymers prepared using metallocene, or other single-site type catalysts.

As used herein, the term "polyolefin" refers to any polymerized olefin, which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted, including "modified polyolefin". More specifically, included in the term polyolefin are homo- polymers of olefin, co-polymers of olefin, co-polymers of an olefin and a non-olefinic co-monomer co-polymerizable with the olefin, such as vinyl monomers, modified polymers thereof, and the like. Specific examples include polyethylene homo-polymer, polypropylene homo-polymer, polybutene homo-polymer, ethylene-α-olefin co-polymer, propylene-α-olefin co-polymer, butene-α-olefin co-polymer, ethylene-unsaturated ester co-polymer, ethylene-unsaturated acid co-polymer, (e.g. ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate co-polymer, ethylene-methyl acrylate co-polymer, ethylene-acrylic acid co-polymer, and ethylene-methacrylic acid co-polymer), ethylene- vinyl acetate copolymer, ionomer resins, polymethylpentene, etc.

As used herein the term "modified polyolefin" is inclusive of modified polymer prepared by co-polymerizing the homo-polymer of the olefin or co-polymer thereof with an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like. It is also inclusive of modified polymers obtained by incorporating into the olefin homo-polymer or co-polymer, by blending or preferably by grafting, an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like. As used herein, the term "adhered", as applied to film layers, broadly refers to the adhesion of a first layer to a second layer either with or without an adhesive, a tie layer or any other layer therebetween, and the word "between", as applied to a layer expressed as being between two other specified layers, includes both direct adherence of the subject layer to the two other layers it is between, as well as a lack of direct adherence to either or both of the two other layers the subject layer is between, i.e., one or more additional layers can be imposed between the subject layer and one or more of the layers the subject layer is between.

In contrast, as used herein, the phrase "directly adhered" is defined as adhesion of the subject layer to the object layer, without a tie layer, adhesive, or other layer therebetween. When referred to an overall structure, the term "gas-barrier" is used herein to identify structures characterized by an Oxygen Transmission Rate (evaluated at 23 °C and 0 % R.H. according to ASTM D-3985) of less than 300 cm³/m².day.bar.

As used herein the terms "polypropylene layer", "polyolefin layer", "polyamide layer", "polystyrene layer", or "polyester layer" refer to layers comprising a major proportion, i.e., > 50 wt. %, such as > 60 wt.%, > 70 wt.%, > 80 wt.%, > 90 wt.%, > 95 wt. %, up to about 100 wt.%, of one or more of the corresponding resins, i.e., one or more polypropylene resins, one or more polyolefins, one or more (co)polyamides, one or more polystyrene resins, or one or more polyesters respectively, calculated on the overall weight of the layer considered.

DETAILED DESCRIPTION OF THE INVENTION A first object of the present invention is a cast co-extruded film comprising a propylene-containing sequence (a) of at least 16 microlayers, which sequence (a) comprises a number n of identical microlayer repeating units (a') each comprising the same sequence of microlayers and either only polypropylene layers (A) or in addition to a polypropylene microlayer (A), another polyolefin microlayer (B) having a composition different from that of microlayer (A).

In a preferred embodiment each microlayer repeating unit (a') comprises at least one polypropylene microlayer (A) and at least one polyolefin microlayer (B) having a composition different from that of microlayer (A)

The polypropylene resins that can be used for the microlayers (A) include polypropylene homopolymers, possibly nucleated, propylene-ethylene block copolymers, propylene-ethylene random copolymers, as well as any blend thereof. The polypropylene resins that can suitably be employed in the microlayers (A) have a melting point higher than 140 ⁰C, preferably higher than 141 ⁰C. Preferably however the microlayers (A) are polypropylene homopolymer layers for the advantages in terms of properties and costs of this resin. The MFI of the polypropylene resins suitable for the microlayers (A) is typically comprised between 1 and 10 g/10 min, preferably between 1.1 and 9 g/10 min, and more preferably between 1.2 and 8 g/10 min (at 230 °C/2.16 kg), and their flexural modulus is typically comprised between 800 and 2000 MPa, preferably comprised between 900 and 1800 MPa, and more preferably between 1000 and 1600 MPa. The microlayer (B), when present, will comprise one or more polyolefins having a density comprised between 0.870 and 0.950 g/cm³, a melting point not higher, and preferably lower, than 140 °C, and a MFI comprised between 1 and 15 g/10 min. Examples of suitable polyolefins include propylene co- and ter-polymers with ethylene and/or (C₄-Cg)-α-olefins, e.g., propylene-ethylene copolymers, both heterogeneous and homogeneous, with an amount of ethylene comonomer that is typically at least about 3 % by mole, and preferably at least about 3.5 % by mole; propylene-butene-ethylene terpolymers or propylene-ethylene-butene terpolymers where the combined amounts of the ethylene and butene comonomers is up to about 30 % by mole, preferably up to about 25 % by mole, more preferably up to about 20 % by mole; propylene-butene copolymers with an amount of butene comonomer that is up to about 30 % by mole, preferably up to about 25 % by mole, more preferably up to about 20 % by mole; ethylene homopolymers, such as, preferably, LDPE or MDPE; ethylene copolymers with one or more straight, branched or cyclic (C4-C₈)-α-olefins, both homogeneous and heterogeneous; ethylene copolymers with unsaturated esters or unsaturated acids, such as ethylene-ethyl acrylate co-polymer, ethylene-butyl acrylate co-polymer, ethylene-methyl acrylate co-polymer, ethylene-acrylic acid co-polymer, and ethylene-methacrylic acid co-polymer, ethylene- vinyl acetate copolymers and ionomer resins.

In a preferred embodiment the microlayer (B) will comprise propylene-butene- ethylene terpolymers, propylene-ethylene-butene terpolymers, ethylene copolymers with one or more straight, branched or cyclic (C₄-C₈)-α-olefins, ethylene copolymers with unsaturated esters or unsaturated acids, ethylene-vinyl acetate copolymers, or ionomer resins, and in a most preferred one it will comprise propylene-butene-ethylene terpolymers or propylene-ethylene-butene terpolymers.

In another embodiment the polyolefin microlayer (B) will comprise recycle material. In such a case the microlayer (B) may comprise recycle material blended with virgin resins in any proportion from 1 :99 to 99:1 or it may be formed only of recycle material. In any case the microlayer (B) will comprise at least 50 % by volume of polyolefin material, preferably at least 60 %, and more preferably at least 70 % by volume of polyolefins. In case said layer (B) comprises recycle material, said material preferably but not necessarily comes from the same production line where the end structure is manufactured. The recycling process would in fact be easier as any trimmed edges and/or any scrap in the manufacturing process may be pelletized and directly recycled. The recycle material besides at least 50 % of polyolefins may contain any other resin used in the manufacture of the end structures, such as for instance (co)polyamides, (co)polyesters, polystyrene, and the like resins. It has been found in fact that when the recycle material is introduced in a microlayer or in the form of a microlayer no problems of bond with the adjacent layers arise and no significant negative effects on the mechanical properties as well as on the thermoformability of the end structure are generated. If advisable, in case of recycle material, compatibilizers, as known in the art, can be employed. Preferably however the amount of recycle material in said microlayers (B) is limited to less than 50 % by volume, e.g., up to 40 %, up to 30 %, up to 20 % or up to 10 % by volume over the total volume of the microlayer.

The repeating unit (a') may comprise one or more (A) layers as well as one or more (B) layers. If more than one, the (A) layers as well as the (B) layers, may have the same composition or a different one, as defined above. In particular for instance the repeating unit may comprise one (A) layer and two (B) layers, where the (B) layers may have the same composition or on the contrary where e.g., one of said (B) layers, (B'), comprises only or mostly virgin polyolefin resins and the other, (B"), contains some recycle material, or the B' and B" layers may both be free of recycle material but have a different composition or still alternatively they may both contain recycle material but in a different proportion or of a different origin. If the unit (a') contains more than one (A) layers, the compositions of the different polypropylene layers (A) may be different. In a preferred embodiment however the composition of said layers (A) will be the same.

Besides the polypropylene layers (A) and the polyolefin layers (B), each repeating unit (a') may also contain additional layers, such as for instance adhesive layers (C), particularly if the bond between the adjacent layers is not sufficiently high. Said adhesive layers will typically comprise modified polyolefins, or blends of polyolefins with modified polyolefins.

Thus the sequence of microlayers in each of the n repeating units (a') may be for instance A, A/B (A/B' or A/B"), B/A/B (B7A/B' or B7A/B" or B"/A/B"), AJB/ A (A/B VA or AJB"/ A), B/C/A/B (B 7C/A/B ' or B 7C/A/B", B'VC/A/B ' or B'VC/A/B" ), A/C/B/A (A/C/B7A or A/C/B'VA), B/C/A/C/B (B7C/A/C/B', B'VC/A/C/B", B7C/A/C/B"), A/C/B/C/A (A/C/B7C/A or AJCIB" ICI A) where the polypropylene layers (A), when two (A) layers are present, and the adhesive layers (C), when two of said layers are present, may have the same or a different composition. While in its most basic structure, that represents a preferred embodiment of the present invention, each repeating unit of the microlayer sequence (a) comprises only layers A, optionally and preferably also layers B, and optionally also layers C, directly adhered one to the other in any of the sequences indicated above, it will be appreciated that said repeating unit may also contain one or more additional microlayers. Said additional microlayers can be internal layers of the sequence, being interposed between the A, B and/or C layers, and/or they may be positioned on one or both sides of the indicated sequences, i.e., on the outer surfaces of the A and/or B layers. The maximum number of microlayers that will compose each identical repeating unit will depend essentially on the extrusion equipment employed and repeating units composed of up to 9 or 10 microlayers may be easily foreseen. Non limitative examples are for instance repeating units composed of 5-, 6-, or 7-layers, wherein the polymers or polymer blends used for the additional layers will be suitably selected to further improve the properties of the end structure or to reduce its cost and provide for a sufficiently cohesive structure. Examples of possible microlayers that can be part of the repeating unit (a') are for instance polyamide microlayers that may further improve the thermoformability of the overall structure and also provide for some gas-barrier properties, polyesters microlayers that have effects similar to the polyamide ones, EVOH microlayers when high gas- barrier properties are desired, etc.

For the sake of simplicity in the following description and claims the microlayer sequence (a) will be indicated as (a')_n, wherein (a') indicates the repeating unit that comprises polypropylene layers (A) and possibly other microlayers as indicated above, and n is the number of repeating units (a') in the microlayer sequence (a). n, i.e., the number of repeating units (a') in the microlayer sequence (a), is at least 3, and preferably at least 4. The number of repeating units can however be much higher than 3 or 4 or 5 or 6 and preferably it is a multiple of 3 or 4 or 5 or 6, typically dictated by the particular technology used for the manufacture of these structures. As it will be described in more details later on, these structures are in fact generally obtained using the multiplier technology, where the multi-layer melt flow corresponding to the first unit which is coextruded, i.e., (a'), is splitted, longitudinally, into a number of packets, for example three or four, each having the same number and sequence of layers corresponding to that of the first unit; said packets are then recombined, stacked one on top of the other, to provide for an alternating sequence with three or four repeating units. Said combined melt flow, of a microlayer sequence (a')_{3 or} 4 can then be splitted once more for example into three or four packets that are then re-combined and stacked one on top of the other, thus giving, in this specific example, structures with 9, or 12, or 16 repeating units (a'). In their turn these can still be splitted and recombined one or more times. The number of packets in which each melt flow can be splitted is not limited to three or four, values that are given above only by way of example, but it can easily be higher. In particular the multiplier technology already available allows splitting a melt flow also in five or six packets that are then stacked, one on top of the other, and processed as described above where each further splitting step can foresee an equal or a different number of packets. In line of principle the number of multiplying steps can be as high as the equipment may allow and the resins may withstand. Typically said number is maintained between 1 and 6, preferably between 2 and 5, more preferably between 2 and 4, and the number of layers in the microlayer sequence (a) may comprise up to 1,000 microlayers, with a typical maximum number of 800 or 700 or 600 or 500 or 400 or 300 microlayers.

As in any coextrusion process, the polymers or polymer blends used in the microlayer sequence (a) will be selected and combined in the respective layers in such a way to give rheologically similar polymer streams in the co-extrusion process, i.e., polymer streams being sufficiently similar in viscosity at the temperatures chosen for the co-extrusion process to avoid significant interfacial instability. More particularly the polymers or polymer blends used in the microlayer sequence should preferably have a viscosity vs. shear rate value (determined through the Cross model equation) that, in the shear rates range comprised between 10 sec^"1 and 50 sec^"1, preferably does not differ by more than 100 % from the corresponding value of the polypropylene resin of layer (A) (i.e., the viscosity ratio between the resins for the (A) and (B) layers should be lower than 2 in order to have an excellent compatibility).

The thickness of the microlayers in the repeating units (a') of the sequence (a) may vary, depending on e.g., the total thickness desired for the overall structure, the number n of repeating units in the sequence, the number of microlayers in each repeating unit, and whether the end structure is solid state oriented or not, from about 0.01 μm up to about 20 μm, preferably up to about 15 μm, more preferably up to about 10 μm, and yet more preferably up to about 5 μm. When thick structures (i.e., structures from 500 to 1000 μm thick) are obtained, and particularly when said thick structures consist only of the sequence (a), the thickness of each microlayer in the sequence may be as high as 15-20 μim. In general however the thickness of the microlayers in the sequence (a) is more limited and preferably it is from about 0.03 to about 4.5 μm, more preferably from about 0.05 to about 4.0 μm, even more preferably from about 0.07 to about 3.5 μm, yet more preferably from about 0.09 to about 3.0 μm, and most preferably from about 0.1 to about 2.0 μm.

When, according to a preferred embodiment, the repeating unit (a') also comprises a layer (B), the relative volume between layer(s) (A) and layer(s) (B) (i.e., the sum of the volumes of the (A) or (B) layers, in case more than one layer (A) or (B) is present) in each repeating unit is preferably comprised between 1 : 10 and 10: 1 , more preferably between 1 :8 and 8: 1 , even more preferably between 1 :6 and 6: 1 and yet even more preferably between 1 : 1 and 5:1. The film according to the present invention may comprise more than one microlayer sequence. As an example it may comprise an additional microlayer sequence (d), composed of m repeating units (d'), wherein the number of microlayers, the thickness thereof, and the polymers for the various layers are as defined above for the microlayer sequence (a) but wherein m can be equal to or different from n, and the structure of (d') can be equal to or different from the structure of (a') in any of the following features : number of microlayers, composition thereof, thickness and relative thickness of the microlayers. When one or more additional microlayer sequences are present they may be directly adhered one to the other or preferably they may be separated by one or more layers serving different purposes, such as tie layers (e), used to increase the bond between the microlayer sequences, bulk layers (f), to increase the thickness of the overall structure, or gas-barrier layers (g), such as for instance an EVOH or a polyamide layer, to provide the structure with gas-barrier properties or anyway to increase the gas-barrier properties of the structure.

In one embodiment of the present invention the multi-layer film comprising the microlayer sequence (a) may additionally comprise either one or two outer layers. Said outer layers are indicated in this description as outer heat-sealable layer (b) and outer abuse-resistant layer (c). This is done just to distinguish between the two outer surfaces of the end structure, even if not necessarily the two layers, if both are present, have a different composition and even if not necessarily the film comprising an outer layer (b) has to be heat-sealed. The thickness of said outer layer (b), when present, can be up to about 70 % of the overall thickness of the structure, preferably up to about 50 % and more preferably up to about 40 %. When present, said outer heat-sealing layer (b) is typically adhered to the outer surface of the microlayer sequence, either directly or through an adhesive layer, and preferably its thickness is higher than about 5 %, and more preferably higher than about 8 % of the overall thickness of the film or sheet, e.g., typically comprised between about 10 and about 35 %.

In one preferred embodiment the outer heat-sealable layer (b) will comprise one or more heat-sealable polyolefins as polyolefins are particularly suitable in those applications where the film is heat-sealed to itself or in those many applications where the film is sealed to a polyolefin surface.

Preferred polyolefins for said layer (b) will then be selected from the group of ethylene homopolymers, ethylene co-polymers, propylene homopolymers, propylene copolymers and blends thereof.

Ethylene homo- and co-polymers particularly suitable in such a case are selected from the group consisting of ethylene homo-polymers (polyethylene), heterogeneous or homogeneous ethylene-α-olefin copolymers, ethylene-cyclic olefin copolymers, such as ethylene-norbornene copolymers, ethylene-vinyl acetate co-polymers, ethylene-(Ci-C4) alkyl acrylate or methacrylate co-polymers, such as ethylene-ethyl acrylate co- polymers, ethylene-butyl acrylate co-polymers, ethylene-methyl acrylate co-polymers, and ethylene- methyl methacrylate co-polymers, ethylene-acrylic acid co-polymers, ethylene-methacrylic acid co-polymers, ionomers, and blends thereof in any proportion.

Propylene polymers suitable for said outer heat-sealable layer (b) are selected from the group consisting of propylene homo-polymer and propylene co- and ter-polymers with up to about 30 % by mole, preferably up to about 25 % by mole, more preferably up to about 20 % by mole of ethylene and/or a (C₄-Ci₀)-α-olefin, and more preferably from the group consisting of polypropylene, propylene-ethylene co-polymers, propylene-ethylene- butene co-polymers and propylene-butene-ethylene copolymers with a total ethylene and butene content up to about 30 % by mole, preferably up to about 25 % by mole, and more preferably up to about 20 % by mole; and blends thereof in any proportion.

Said outer heat-sealable polyolefin layer (b) may also comprise a blend of a major proportion of one or more polymers of the group of ethylene homo- and copolymers and propylene homo- and co-polymers, with a minor proportion of one or more other polyolefins and/or modified polyolefins, such as polybutene homo-polymers, butene-(Cs- Cιo)-α-olefin copolymers, anhydride grafted ethylene-α-olefin copolymers, anhydride grafted ethylene-vinyl acetate copolymers, rubber modified ethylene-vinyl acetate copolymers, ethylene/propylene/diene (EPDM) copolymers, and the like.

The composition of said outer layer (b) will however mainly depend on the final application foreseen for the end structure and polymers other than polyolefins may as well be employed therein. For instance when the film of the present invention is used in thermoformed applications where a given lidding film has to be heat-sealed to the thermoformed tray rim, it may be convenient or necessary to have an outer heat-sealable layer (b) of a polymer or polymer blend suitably chosen to be heat-sealable to the material making up the sealing surface of the lidding film. Or if the film according to the present invention is used for tray lidding applications, it may be convenient or necessary to have an outer heat-sealable layer (b) of a polymer or polymer blend suitably chosen to be heat-sealable to the material making up the tray outer surface. As an example, when a film according to the present invention has to be heat-sealed to a polyester support, such as a rigid or foamed PET tray or a rigid or foamed PLA tray, a suitable outer heat- sealable layer (b) will preferably comprise a polyester, such as a PETG, or an amorphous PLA optionally blended with, e.g., a minor proportion of PETG, or a polyglycolic acid. Depending on the final use of the film of the present invention the second outer layer (c) may have a composition identical or similar to that of the first outer heat- sealable layer (b), when for instance lap-sealable films are desired, or it may comprise any thermoplastic material that might be adapted to function as an abuse layer when for instance thermoformable sheets or pouches/bags are desired. Thus suitable resins for the second outer layer (c) include for instance polyolefins, modified polyolefins, polyesters, copolyesters, polyamides, copolyamides, and polystyrene polymers. Suitable polyolefins that can be used for the second outer layer (c) are ethylene homo-polymers, ethylene co-polymers, propylene homo-polymers, propylene copolymers and blends thereof as described for the first outer heat-sealable layer (b). Preferred in said class are ethylene homopolymers, such as MDPE and HDPE, ethylene- α-olefin copolymers, particularly those with a density of from about 0.895 to about 0.935 g/cm³, and more preferably of from about 0.900 to about 0.930 g/cm³, ethylene-vinyl acetate copolymers, particularly those with a vinyl acetate content of from about 4 to about 14 % by weight, ionomers, polypropylene homopolymers, propylene-ethylene copolymers, propylene-ethylene-butene copolymers, propylene-butene-ethylene copolymers, and their blends.

The second outer layer (c) may also comprise polystyrene polymers. Preferred polymers for the outer abuse-resistant layer (c) are however (co)polyamides and (co)polyesters.

Polyamides and co-polyamides that could suitably be employed for the second outer layer (c) are for instance those (co)polyamides characterised by a high crystalline melting point, such as certain aliphatic or partially aromatic polyamides or copolyamides, e.g. polyamide 6, MXD6, polyamide 66, copolyamide 6/66, copolyamide 6/12, copolyamide MXD6/MXD1, etc. They can be used alone or in blends thereof. They can also be used blended with other polyamides such as for instance amorphous polyamides, e.g. copolyamide 61/6T, polyamide 61, etc.

In a most preferred embodiment however the second outer layer (c), if present, will comprise one or more thermoplastic polyesters. Thermoplastic polyesters may include those obtained from an acid component comprising an aromatic dibasic acid, such as terephthalic acid or isophthalic acid, and a glycol component comprising an aliphatic glycol, an alicyclic glycol or an aromatic glycol, such as ethylene glycol, diethylene glycol or cyclohexane dimethanol. Co-polyesters are formed starting from two or three species of acid component or/and glycol component. Useful polyesters include those made by condensation of polyfunctional carboxylic acids with polyfunctional alcohols, polycondensation of hydroxycarboxylic acid, and polymerization of cyclic esters (e.g., lactones). Exemplary polyfunctional carboxylic acids (and their derivatives such as anhydrides or simple esters like methyl esters) include aromatic dicarboxylic acids and derivatives (e.g., terephthalic acid, isophthalic acid, dimethyl terephthalate, dimethyl isophthalate) and aliphatic dicarboxylic acids and derivatives (e.g., adipic acid, azelaic acid, sebacic acid, oxalic acid, succinic acid, glutaric acid, dodecanoic diacid, 1 ,4-cyclohexane dicarboxylic acid, dimethyl- 1,4-cyclohexane dicarboxylate ester, dimethyl adipate). As is known to those of skill in the art, polyesters may be produced using anhydrides and esters of polyfunctional carboxylic acids.

Exemplary polyfunctional alcohols include dihydric alcohols (and bisphenols) such as ethylene glycol, 1 ,2- propanediol, 1 ,3-propanediol, 1 ,3 butanediol, 1,4-butanediol, 1 ,4- cyclohexanedimethanol, 2,2-dimethyl-l ,3-propanediol, 1,6-hexanediol, poly(tetrahydroxy-l, l '- biphenyl, 1 ,4-hydroquinone, and bisphenol A.

Exemplary hydroxycarboxylic acids and lactones include 4-hydroxybenzoic acid, 6- hydroxy-2-naphthoic acid, pivalolactone, and caprolactone. Useful polyesters include homopolymers and copolymers. These may be derived from one or more of the constituents discussed above. Exemplary polyesters include polyethylene terephthalate) ("PET"), poly(butylene terephthalate) ("PBT"), and poly(ethylene naphthalate) ("PEN"). If the polyester includes a mer unit derived from terephthalic acid, then such mer content (mole %) of the diacid of the polyester may be at least about any the following: 70, 75, 80, 85, 90, and 95 %.

The thickness of the second outer layer (c), when said layer is present, is typically comprised between about 2 and about 50 % of the overall structure, preferably between about 4 and about 45 %, more preferably between about 6 and about 40 %, and yet more preferably between about 8 and about 35 %. If necessary or advisable one or more other layers may be positioned between the microlayer sequence and the outer layers (b) and/or (c). As seen above, suitable layers may include tie or adhesive layers (e), used to increase the bond between the microlayer sequence and any of the outer layers, or between the microlayer sequence and another layer positioned between said microlayer sequence and any of the outer layers, or between any of the outer layers and another layer positioned between said outer layer and the microlayer sequence; bulk layers (f), to increase the thickness of the overall structure; a gas-barrier layer (g) to provide the structure with gas-barrier properties or to improve them; a seal-assistant layer (h), directly adhered to the heat-sealable outer layer (b), to improve sealability of the structure particularly in difficult conditions; a cohesive failure layer (i), comprising poorly compatible resins, directly adhered to the heat-sealable outer layer (b) to provide a film or sheet suitable for the manufacture of an easy-openable package; shrink layers (j), to induce compatible shrinkage of the overall multilayer film structure, if solid state oriented structures are obtained; etc.

The thickness of any of these layers will vary depending on the particular purpose of the layer and on the thickness of the overall structure : tie or adhesive layers (e) will typically have a limited thickness, in the order of few μm, while bulk (f) and shrink (j) layers will typically be reasonably thicker and the other types of layers will have an intermediate thickness.

The resins used for the adhesive layers (e) preferably comprise one or more modified polyolefins, possibly blended with one or more polyolefins. Specific, not limitative, examples thereof may include: ethylene-vinyl acetate copolymers, ethylene-

(meth)acrylate copolymers, ethylene-α-olefin copolymers, any of the above modified with carboxylic or preferably anhydride functionalities, elastomers, and a blend of these resins.

In the following there are reported non limitative examples of possible sequences for the end structure of the presente invention, wherein the letters identify the layers or the microlayer sequences as indicated above :

(a); (b)/(a); (b)/(a)/(c); (a)/(d); (b)/(a)/(d); (b)/(e)/(a); (b)/(a)/(d)/(c); (a)/(e)/(d);

(b)/(a)/(e)/(d); (b)/(a)/(e)/(d)/(c); (a)/(f)/(d); (b)/(a)/(f)/(d); (b)/(a)/(f)/(d)/(c);

(b)/(e)/(a)/(c); (b)/(e)/(a)/(e)/(c); (b)/(a)/(e)/(g)/(e)/(c); (a)/(e)/(g)/(e)/(d); (a)/(e)/(f)/(e)/(d); (b)/(a)/(e)/(g)/(e)/(d); (b)/(a)/(e)/(f)/(e)/(d); (b)/(a)/(e)/(g)/(e)/(d)/(c);

(b)/(a)/(e)/(f)/(e)/(d)/(c); and the like sequences.

In one preferred embodiment, the film of the present invention comprises a microlayer sequence (a), one or both outer layers (b) and (c), and at least one tie layer between the microlayer sequence (a) and one of the outer layers to provide for a sufficient adhesion between said layers. In another embodiment the film of the present invention comprises, a microlayer sequence (a), outer layers (b) and (c), and tie layers between the microlayer sequence (a) and both outer layers (b) and (c), wherein the adhesive resins used for these tie layers may be equal or different.

In all the film layers, the polymer components may contain appropriate amounts of organic or inorganic additives normally included in such compositions. Some of these additives are preferably included in the outer layers or in one of the outer layers, while some others are preferably added to inner layers. These additives include processing aids, such as slip and anti-block agents such as talc, waxes, silica, and the like, antioxidants, stabilizers, plasticizers, fillers, pigments and dyes, cross-linking inhibitors, cross-linking enhancers, oxygen scavenging compositions, UV absorbers, antistatic agents, anti-fog agents or compositions, and the like additives known to those skilled in the art of packaging films.

Other additives that can be employed are nanoclay particles, i.e., nanometer scale finely dispersed clay, such as, natural or synthetic phyllosilicates, preferably of the smectite group, e.g., montmorillonite clay. These nanoclay particles can be added to the resins to increase their gas-barrier properties or/and their mechanical properties. These nanoclay particles can be added to the resin composition in an amount up to about 8 % by weight, generally comprised between about 0.1 and about 5 % by weight.

The end films will generally have a total thickness that may be comprised between about 15 μm and about 1 ,000 μm, depending on whether the structure is solid-state oriented or not and depending on the particular use foreseen. As an example, films with a thickness comprised between about 15 and about 30 μm will typically be solid state oriented, heat- shrinkable, annealed, or heat-set structures, suitable for use in shrink wrapping or tray lidding applications; films with a thickness comprised between about 30 and about 150 μm, can be either non oriented or solid state oriented, heat-shrinkable, annealed, or heat-set films, suitable for many different applications including the manufacture of bags, casings, pouches, or in flow-wrap, thermoform, or thermoform-shrink applications; films or sheets with a thickness higher than 150 μm are generally cast non-oriented structures, mainly used in thermoforming applications or in the manufacture of pouches.

Representative examples of films according to the present invention are illustrated in Figures 1 to 3. Figure 1 illustrates a first embodiment of multilayer film where 10 represents the microlayer sequence (a) (also indicated as (a')_n), 12 is the first outer heat- sealing layer (b), 14 is the second outer layer (c) and 1 1 and 13 represent two tie layers that can be equal or different and are used to increase the adhesion of the outer layers to the core microlayer sequence (a')_n. In said embodiment n is typically an integer from 3 to 300, preferably at least 4, at least 5, at least 6, at least 7, at least 8, from 9 to 273, more preferably from 16 to 256. Figure 2 illustrates a second preferred embodiment of a multi-layer film where 22 is the first outer heat-sealable layer (b), 20 is the microlayer sequence (a), and 21 is a tie layer adhering the first outer heat-sealable layer (b) to the microlayer sequence (a). Figure 3 illustrates a third preferred embodiment of a multilayer film where 33 is the first outer heat-sealable layer (b), 30 is the core microlayer sequence (a), 31 is a cohesive failure layer directly adhered to the first outer heat-seal layer (b) that in this case would be fairly thin, typically thinner than 10 μm, and preferably thinner than 8 μm, 32 is a tie layer binding the cohesive failure layer 31 to the core microlayer sequence (a) 30, 35 is the second outer layer (c) and 34 is a second tie layer, that may be equal or different from tie layer 32, and is used to increase the bond between the second outer layer 35 and the core microlayer sequence 30.

As indicated above the microlayer sequence (a) is obtained by cast co-extrusion using a multiplier device, a device that comprises a series of multiplying elements which extend from a co-extrusion block connected to the extruders of the resins for the layers of the repeating unit (a'), to a final discharge die where the melt laminate forms a film with a number of layers dictated by the number of layers in the repeating unit, the number of multiplying elements and the number of ramps in each of these elements. An example of multiplier device is illustrated in Figure 4. In said Figure 4, 100 is a die element disposed in the melt flow passageway from the device for the coextrusion of the resins of the repeating unit, device which is not illustrated in Figure 4. The die element 100 divides the melt flow passage into four passages, 1 10a, 1 10b, 1 1 Oc, and 1 1Od, leading the divided melt flows, each containing the microlayer sequence of the repeating unit, to the expansion platform 120 where the split melt flows are stacked one on top of the other and the obtained melt laminate is then expanded transversally and conveyed to a second die element 101. The melt flow in said second die element will contain a sequence of four repeating units as the melt flow has been divided in four packets in the first die element. The process is then repeated through the second multiplier element where four separated melt flows, each containing the sequence of 4 repeating units, (a')₄, are formed and conveyed through their respective passageways 1 1 Ia to 1 1 Id to a second expansion platform 130, where they are stacked one on top of the other giving a melt laminate with an alternating sequence comprising 16 repeating units, i.e., (a')i₆. In said Figure 4, 102 represents the discharge die through which the multilayer film of the present invention is then finally extruded. If desired, one or more additional multiplying element can be interposed between the second expansion platform 130 and the final extrusion die 102.

If a multilayer structure is desired which also comprises other layers in addition to the microlayer sequence (a), said additional layers can be cast co-extruded with the microlayer sequence (a) by passing the molten flow corresponding to the desired sequence (a')_n into a suitable feedblock and then through a suitable coextrusion die with said other layers and finally quenching the overall co-extruded structure thus obtained.

Alternatively said additional layers can be pre-formed and then heat- or glue-laminated to one or both outer surfaces of the cast co-extruded microlayer sequence (a); or they can be extrusion coated on one or both of the outer surfaces of the obtained cast co-extruded microlayer sequence (a); or they may be obtained by any suitable combination of the above methods. The co-extrusion is however the preferred manufacturing method for the cast structures of the present invention.

Cast structures with any desired number of repeating units n can be obtained by suitably combining the process steps and devices. The first coextrusion step in fact not necessarily is limited to a sequence of layers corresponding to a single unit, but it is possible to start with a coextruded sequence of layers corresponding to e.g., two or three repeating units; the multiplying devices can suitably be combined and not necessarily need to be multiple of the same number; and it is possible to foresee at the end of the multiplying process a further step where a coextruded repeating unit (a') is either co-extruded, extrusion-coated or laminated to the precursor structure where the microlayer sequence comprises n -I repeating units.

The cast structures of the present invention may be cross-linked if desired. Cross- linking is typically obtained by passing the film or sheet through an irradiation vault where it is irradiated by high-energy electrons. Depending on the characteristics desired, this irradiation dosage can be up to about 200 kGy, preferably up to about 150 kGy, more preferably up to about 130 kGy. Typically for the irradiated structures the irradiation dosage will be comprised between 10 and 200 kGy, preferably between 20 and 150 kGy and more preferably between 30 and 130 kGy.

The cast structures according to the first and second object of the present invention are particularly suitable for use in thermoforming applications for the packaging of food products and the packages obtained by enclosing a product between two pieces of thermoplastic packaging materials where at least one of said pieces is obtained by thermoforming a cast structure according to the present invention represent a further specific object of the present invention. Typical thicknesses of the cast films suitable for thermoforming applications will vary from at least about 70 μm, when used as flexible bottom webs for the thermoforming of very shallow profile trays, to about 1 ,000 μm mainly suitable for the manufacture of pre-made trays, preferably comprised from about 90 to about 700 μm, more preferably from about 1 10 to about 650 μm, and even more preferably from about 1 30 to about 500 μm, and still even more preferably from about 1 50 to about 400 μm. For thermoforming applications a cast structure including an outer abuse resistant layer (c) would be preferred as the contact of a very thin outer layer with the thermoforming mold may negatively affect the appearance of the outer surface. Preferably for said applications a suitable outer layer (c) will be a (co)polyamide or a (co)polyester layer.

As most of the applications will require a sealing step, e.g., when a closing film is sealed to the thus formed container, either as a lid sealed to the tray rim or as an upper skin sealed to the surface of the lower container or support under vacuum, the structure will preferably comprise also an outer heat-sealable layer (b). The Young's or tensile modulus of the end, non oriented, cast structures of the present invention, determined according to the standard method ASTM D-882, will be typically comprised between about 4,000 and about 12,000 kg/cm², depending on the specific resins employed in the sequence (a) and in the optional additional layers.

Cast, non solid-state oriented films according to the present invention may also be used, in a thinner version, as lidding or wrapping films or for the manufacture of pouches in HFFS or VFFS processes. In such a case a suitable thickness can be comprised between about 30 and about 150 μm, preferably between about 35 and 130 μm, more preferably between about 40 and 1 10 μm, and even more preferably between about 45 and about 100 μm.

The cast structures according to the present invention are generally obtained, by coextrusion optionally followed by lamination and/or extrusion coating, as webs of a width comprised between 1 and 4 m. The thus obtained webs are then longitudinally slit down to the customer specified width, generally comprised between 250 and 1500 mm depending on the particular packaging application foreseen, and rewinded in rolls. If desired the cast structures of the present invention, which comprise a cast co- extruded microlayer sequence (a), may then be solid-state oriented. If solid-state oriented, they may be uniaxially oriented or bi-axially oriented, i.e., oriented in both the MD and TD directions. The solid-state orientation process is carried out at a temperature sufficiently low to bring about the orientation of all the layers of the structure, thus a temperature sufficiently low not to produce the melting of any layer therein. In case of co-extruded structures solid-state orientation thereof is typically an in-line step. In such a case the cast co-extruded multilayer structure, quenched immediately after extrusion, is then reheated, in either an oven using hot air or infrared heaters or by passing it over a series of heated rolls, and then stretched still under heating at a temperature higher than the Tg of all the resins making up the structure but lower than the melting temperature of any of its layers, either in only one or preferably in both directions. In case the cast structure to be solid state oriented is not co-extruded, orientation can be an in-line or an off-line step but in any case it will be necessary to carry it out soon after the manufacture of the cast structure to control as much as possible the crystallization of the resins.

The orientation or stretching ratios that can be applied in this step may be different depending on the scope of the orientation. As an example a light orientation in MD only, with stretching ratios of from 1.1 : 1 to 1 .6: 1 , followed by heat setting, may be applied to improve the mechanical properties of the structure in the longitudinal direction and mainly to improve the film flatness.

Higher stretching ratios, e.g., up to 5: 1 or even more in either only one or both directions can be applied particularly if a heat-shrinkable film is desired or if the mechanical properties of the end structure need to be remarkably improved. The solid- state oriented films of the present invention will have in fact a tensile modulus typically comprised between about 8,000 and 20,000 kg/cm².

When solid-state oriented, the films of the present invention can be heat-shrinkable, i.e., characterised by a free shrink in at least one of the two directions of at least 5 % at 95 ⁰C, preferably at least 10 %, more preferably at least 15 %, and even more preferably at least 20 %; or they may be heat-set, i.e., characterised by a free shrink lower than 3 %, preferably lower than 2 %, in both directions at 140 ⁰C; or annealed, i.e., characterized by a free shrink in at least one of the two directions of at least 15 %, preferably at least 20 % and even more preferably at least 25 % at a temperature of 120 ⁰C but a free shrink lower than 5 % at 95 ⁰C. The free shrink of the film is determined by measuring the percent dimensional change in a 10 cm x 10 cm film specimen when subjected to selected heat in a suitable liquid according to ASTM D2732.

Longitudinal orientation is usually carried out by drawing the sheet over rolls which rotate at different speeds where at least one of the first pairs of rolls is heated, for example by inner circulation of hot oil. Biaxial orientation can be obtained using a simultaneous tenterframe, such as for instance a LISIM™ line by Bruckner or a pantograph line such as a Dornier line, but also, easily and effectively, using a more conventional sequential tenterframe. It has been found in fact that a structure with a microlayer sequence (a) can be solid-state oriented much more easily than a structure with a single layer of a thickness corresponding to the sum of the thickness of the plurality of the microlayers of the same resin. In the sequential tenterframe the stretching in MD is obtained by drawing the sheet between rolls moving at different speeds, with the downstream set moving at a higher speed and the stretching in TD is accomplished in a heated area using two continuous chains mounted on each side of the sheet and bearing clamps that grip the edges of the sheet. The two side chains gradually move apart and as they do they drive the sheet in the transverse direction between them, until the end of the transverse stretch section where the clamps open and the chains turn around a wheel and return to the beginning of the transverse stretching section. If a heat-shrinkable structure is desired having a controlled shrink at low temperatures or if an annealed or heat-set oriented structure is desired, at the end of the TD stretching section the side chains are maintained parallel or slightly converging and the oriented film, still clamped to the chains, is allowed either to relax or is heat-set at the suitably selected temperature, typically comprised between 40-50 °C and 190-210 ⁰C (these values refer to the temperature of the air and not of the film itself).

The solid-state oriented films of the present invention, either heat-shrinkable, annealed, or heat-set may be employed and are preferably employed for some of the packaging applications seen above in which thin cast non-oriented structures can also be used, such as lidding or wrapping films or for the manufacture of pouches in HFFS or VFFS processes. Furthermore heat-shrinkable or annealed solid-state oriented films according to the present invention, with a thickness typically comprised between about 40 and about 160 μm can suitably be employed in the so-called "thermoform-shrink" processes. These are processes that involve the thermoforming of a solid-state oriented heat-shrinkable film to form a flexible container. In these methods the product to be packaged is loaded in the container thus obtained, and the package is then closed, once air is evacuated from the inside, with a lid, which may be e.g., a flat film, another thermoformed flexible container, or a stretched film, that is sealed to the flange of the loaded container. Shrinkage of the packaging material, induced by a heat-treatment, then provides the desired tight appearance to the end vacuum package. In such a case the optimum thickness will depend on the depth desired for the formed container. For medium depths a preferred thickness will be generally in the range between 50 and 100 μm, while for high depths a preferred thickness will be typically in the range between 70 and 160 μm.

The films of the present invention, particularly in the embodiments where the second outer layer c) comprises a high melting resin that is adapted to be in contact with a sealing bar during a heat sealing operation without sticking, can be used also as the lidding film that closes the package. If also the lid is deep-drawn, then the same thickness range will be appropriate, while if the film is sealed to the flange of the deep-drawn container as a flat lid, a thickness comprised between about 20 and about 35 μm will be sufficient and if it has to be stretched to a certain extent, because the product loaded into the deep-drawn container slightly protrudes therefrom, then a thickness of e.g., from about 25 to about 40 μm, will be preferred. The solid state oriented heat-shrinkable films of the present invention can be employed also for other packaging applications, in particular for any packaging application where a shrink thermoplastic material can be employed, such as shrink wrapping, shrink bags, etc. For these uses the solid state oriented shrink film may have a thickness ranging from about 15 to about 120 μm, preferably between 15 and 40 μm for shrink film applications and between 40 and 120 μm for shrink bag or seamed casing applications.

The following examples are presented for the purpose of further illustrating and explaining the present invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

In the following examples the resins indicated in Table I below have been employed:

TABLE I

> Melt Flow Indexes (Mi's) are measured by ASTM D- 1238 and are reported in grams/10 minutes. Unless otherwise indicated the conditions used are 190°C/2.16 kg.

> Unless otherwise specifically indicated, all percentages are by weight.

EXAMPLE 1

A melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDl. The thickness of each of the PPl layers was 1.1 μm while that of the EC3 layer was about 1.0 μm. The 48-layer melt stream was then passed as the core layer in a five- layer feedblock apparatus together with an EC 1 with 1 % MB 1 outer sealant layer (b) of 50 μm, an outermost abuse resistant layer (c) of a blend of 98 % PET and 2 % MB2 of 55 μm, and two intermediate adhesive layers of ADl of 10 μm each, positioned between the outer layers and the core sequence (a). The total thickness of the end cast co-extruded structure was 175 μm.

In the microlayer sequence (a), PP2 or PP3 could replace PPl in microlayers (A), EXAMPLE 2

The procedure of Example 1 was repeated by increasing the thickness of each of the PPl layers of the repeating unit (a') to 1.6 and that of the EC3 layer to 1.4 μm, that of the outer sealant layer (b) to 65 μm, of the outer abuse layer to 82 μm, and of each of the adhesive intermediate layers to 15 μm. The total thickness of the end cast co-extruded structure was 250 μm.

EXAMPLES 3 TO 7 AND 8 TO 12

The structures of Examples 3 to 7 have been prepared by following exactly the same procedure as in Example 1 but replacing EC3 in layer (B) with the resin indicated in Table 11 below. Analogously Examples 8 to 12 have been prepared by following exactly the procedure of Example 2 but replacing EC3 with the resin indicated in Table II below.

The total thickness of the end cast co-extruded structures of Examples 3 to 7 was 175 μm, while that of the structures of Examples 8 to 12 was 250 μm.

TABLE II

EXAMPLES 13 TO 15

A melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EPC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDl. The thickness of each of the PPl layers was 1.1 μm while that of the EPC3 layer was about 1.0 μm. The 48-layer melt stream was then passed as the core layer in a five- layer feedblock apparatus together with the outer sealant layer (b) and the outer abuse resistant layer (c) indicated in Table III below, and two intermediate adhesive layers of ADl of 10 μm each, positioned between the outer layers and the core sequence (a). The total thickness of the end cast co-extruded structures was 175 μm.

TABLE III

EXAMPLES 16 TO 18

A melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EPC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDI. The thickness of each of the PPl layers was 1.6 μm while that of the EPC3 layer was about 1.4 μm. The 48-layer melt stream was then passed as the core layer in a five- layer feedblock apparatus together with the outer sealant layer (b) and the outer abuse resistant layer (c) indicated in Table IV below, and two intermediate adhesive layers of ADl of 15 μm each, positioned between the outer layers and the core sequence (a). The total thickness of the end cast co-extruded structures was 250 μm.

TABLE IV

EXAMPLES 19 TO 25

All the examples 19 to 25 have the same structure but differ in the thickness of the various layers and in the total thickness of the end structure. For their preparation a melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EPC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDl. The 48-layer melt stream was then passed as the core layer in a five-layer feedblock apparatus together with the outer sealant layer (b) of 99 % ECl + 1 % MBl and the outer abuse resistant layer (c) of 98 % PET + 2 % MB2 and two intermediate adhesive layers of AD2 positioned between the outer layers and the core sequence (a). The thickness of the various layers and the total thickness of the end structures are reported in Table V below.

TABLE V

EXAMPLE 26

A melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is REC was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDI. The thickness of each of the PPl layers as well as of the REC layer was 1.56 μm. The 48-layer melt stream was then passed as the core layer in a five-layer feedblock apparatus together with an ECl with 1 % MBl outer sealant layer (b) of 70 μm, an outer abuse resistant layer (c) of a blend of 99 % PAl and 1 % MB3 of 80 μm, and two intermediate adhesive layers of ADl of 12.5 μm each, positioned between the outer layers and the core microlayer sequence (a). The total thickness of the end cast co- extruded structure was 250 μm.

EXAMPLE 27 The procedure of Example 26 was repeated by replacing the resin used for the outer abuse layer (c) with 98 % PET + 2 % MB2. The thickness of said outer layer (c) and thus the overall thickness of the end cast co-extruded structure were as in Example 26.

EXAMPLE 28 A melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EPC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDI. The thickness of each of the PPl layers as well as of the EPC3 layer was 1.56 μm. The 48-layer melt stream was then passed as the core layer in a five-layer feedblock apparatus together with an ECl with 1 % MB l outer sealant layer (b) of 70 μm, an outer abuse resistant layer (c) of a blend of 98 % PET and 2 % MB2 of 80 μm, and two intermediate adhesive layers of a blend of 50 % ADl and 50 % EVA, of 12.5 μm each, positioned between the outer layers and the core microlayer sequence (a). The total thickness of the end cast co-extruded structure was 250 μm.

EXAMPLES 29 TO 35

All the examples 29 to 35 have the same structure but differ in the thickness of the various layers and in the total thickness of the end structure. For their preparation a melt stream of a total of 48 microlayers repeating 16 times the sequence (A)/(B)/(A) where (A) is PPl and (B) is EPC3 was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying devices by EDI. The 48-layer melt stream was then passed as the core layer in a five-layer feedblock apparatus together with the outer sealant layer (b) of 99 % ECl + 1 % MB 1 and the outer abuse resistant layer (c) of 99 % PAl + 1 % MB3 and two intermediate adhesive layers of ADl positioned between the outer layers and the core sequence (a). The thickness of the various layers and the total thickness of the end structures are reported in Table VI below.

TABLE VI

EXAMPLES 36 TO 38

A melt stream of a total of 48 microlayers repeating 16 times the sequence A/B/A, where the composition of the A and B layers and their thickness are reported in Table VlI below, was obtained by first co-extruding the resins through a three layer coextrusion feedblock apparatus and then feeding the resulting first composite stream through a series of two four-channel multiplying device by EDI. No other layer was coextruded and the structure was extruded and quenched as such.

TABLE VII

EXAMPLES 39 TO 41

By following the procedure described in the foregoing examples but submitting the 48 layer melt stream to a third multiplication step still through a four-channel multiplying device before extruding and quenching, structures corresponding to those of Examples 36 to 39 but with 192 layers in the microlayer sequence (a) instead of 48 and with a correspondingly reduced thickness of the microlayers, as indicated in Table VIII below, would have been obtained.

TABLE VIII

If, following the final coextrusion and the quenching step, the cast structures of the preceding examples are re-heated at a temperature which is above the glass transition temperature of the resins employed and at which none of the layers is in the molten state, and stretched at this temperature, the corresponding solid state oriented films are obtained. The orientation in such a case may be mono-axial, i.e., either longitudinally or transversally, or, preferably, biaxial. The stretching ratios that can be applied, will depend on the particular structure and will generally be comprised between 1.5: 1 and 5: 1. The obtained solid state oriented films will thus be endowed with heat-shrinkable properties unless they are then submitted to a heat-setting step.

Representative cast co-extruded structures of the present invention were submitted to a series of tests to evaluate the optical and mechanical (puncture resistance) properties, the machinability, and the thermoformability in comparison with representative flexible thermoformable structures presently on the market and proved to have comparable optical properties (haze and gloss) and comparably good machinability but higher puncture resistance and better thermoformability.

The puncture resistance was measured at 30 °C by an internal test method that is described shortly in the following : a sample (6.5 x 6.5 cm) of the film is fixed in a specimen holder connected to a compression cell mounted on a dynamometer (an Instron tensile tester), when the dynamometer is started, a punch (a punching sphere, 5-mm in diameter soldered on a plunger) is brought against the film sample at a constant speed (30 cm/min) at a temperature of 30 °C, and the force needed to puncture the sample is thus determined. In this test for instance the structure of Example 3 showed a puncture resistance comparable to that of a commercial polypropylene-based thermoformable structure 35 % thicker and the structure of Example 15 showed a puncture resistance higher than that of said much thicker commercial polypropylene-based thermoformable structure.

All the structures of the Examples that have been tested, including in particular those of Examples 2, 8, 9, 10, 1 1 , 12, 16, 17, and 18, gave a better formability, in terms of maximum thermoformable depth, than the commercial polypropylene-based thermoformable structure used as standard, in spite of their reduced thickness. In particular the structures tested were at least 20 % thinner than said commercial comparative structure and gave a maximum depth of thermoforming at least 15 % higher. The structure of example 38 has also been compared with a 10% thicker 3-layer structure:

80%PP2 + 20%EPC l/80%PP2 + 20%EPC l/80%PP2 + 20%EPC l 270 μm /130 μm /270 μm and proved to have better mechanical properties (better impact resistance particularly in the longitudinal direction and better results in a drop test where trays obtained from the respective structures were sealed, stored at 0⁰C for 48 hours and then dropped from 60 cm) and improved formability giving trays with a much more uniform thickness distribution.

Also the structures of examples 36 and 37, in spite of the reduced thickness, showed improved mechanical properties over the above comparative 3-layer structure.

Claims

1. A cast coextruded film comprising a polypropylene-containing sequence (a) of at least 16 microlayers, which sequence (a) comprises a number n of identical microlayer repeating units (a') each comprising the same sequence of microlayers and either only polypropylene layers (A) or in addition to a polypropylene microlayer (A) another polyolefin microlayer (B) having a composition different from that of microlayer (A).

2. The cast co-extruded film of claim 1 wherein each repeating unit (a') comprises at least one polypropylene microlayer (A) and at least one polyolefin microlayer (B) having a composition different from that of microlayer (A).

3. The cast co-extruded film of claims 1 or 2 wherein the polypropylene resins that can be used for the microlayers (A) include polypropylene homopolymers, propylene- ethylene block copolymers and propylene-ethylene random copolymers, with a melting point higher than 140 ⁰C, as well as any blend thereof.

4. The cast co-extruded film of claim 2 wherein the microlayer (B) will comprise one or more polyolefins having a density comprised between 0.870 and 0.950 g/cm , a melting point not higher, and preferably lower, than 140 ⁰C, and a MFI comprised between 1 and 15 g/10 min.

5. The cast co-extruded film of claim 4 wherein the polyolefins for the microlayer (B) are selected from the group consisting of propylene co- and ter-polymers with ethylene and/or (C₄-C₈)-α-olefins; ethylene homopolymers; ethylene copolymers with one or more straight, branched or cyclic (C₄-Cs)-α-olefins, both homogeneous and heterogeneous; ethylene copolymers with unsaturated esters or unsaturated acids; ethylene-vinyl acetate copolymers; and ionomer resins.

6. The cast co-extruded film of claim 5 wherein the polyolefins for the microlayer (B) are selected from the group consisting of propylene ter-polymers with ethylene and/or (C₄-C₈)-α-olefins.

7. The cast co-extruded film of claim 2 wherein the polyolefin microlayer (B) will comprise recycle material.

8. The cast co-extruded film of claim 7 wherein the recycle material may additionally comprise (co)polyamides, (co)polyesters, and/or polystyrene.

9. The cast film of any of the preceding claims which in addition to the microlayer sequence (a) also comprises an outer heat-sealable layer (b).

10. The cast film of any of the preceding claims which also comprises an outer abuse- resistant layer (c).

1 1. The cast film of claim 10 wherein the outer abuse-resistant layer (c) is a (co)polyamide or (co)polyester layer.

12. The cast film of any of claims 9, 10 or 1 1 which is coextruded.

13. A solid-state oriented film obtained by stretching the cast film of any of the preceding claims, either mono-axially or biaxially, at a temperature higher than the Tg of all the resins of the structure and lower than the melting temperature of any of the cast structure layer.

14. A package comprising a product enclosed between two pieces of thermoplastic materials where at least one of said two pieces is obtained by thermoforming a cast film of any of preceding claims 1 to 12 or a solid-state oriented film of claim 13.