The present invention relates to a film based on polypropylene, in particular biaxially oriented polypropylene, which is usable for the fabrication of packagings for the containment of food products thanks to its particular chemical and physical characteristics, among which are low density, good mechanical strength and transparency. However, many of these food products require that the associated fats be protected from oxidation which can be .caused by the presence of oxygen within the packaging. Such 'presence is caused by the low barrier properties exhibited by bi-oriented polypropylene film. In particular and depending on its thickness, the oxygen permeability value of a film of this type lies in a range between 1800 and 2200 cm3/m2*d*bar (measured according to ASTM D3985) which is not compatible with the normal requirements of use.

Consequently it is conventional to improve the barrier properties of a polypropylene-based film by adding to it materials suitable to allow it to obtain values of oxygen permeability of the order of 100 cm3/m2*d*bar, again measured according to ASTM D3985. Typically, ethylene-alcohol vinyl copolymers (EVOH) are used for this purpose.

For example, EP-A-311 293 describes the use of an EVOH copolymer in the form of a base inner layer enclosed between outer layers of polyolefin material.

US-A-4 650 721 on the other hand describes the use of an EVOH copolymer in the form of an outer layer of cladding over a base layer constituted essentially of polypropylene. Both these arrangements have disadvantages. In fact, the EVOH copolymer is decidedly more expensive and mechanically less strong than polypropylene. Therefore, its use to form the base layer, which constitutes the major part of the film, results in an increase in the costs and a reduction in the mechanical properties in comparison with films containing only polypropylene. On the other hand, EVOH copolymer is strongly subject to hydrolysis in a moist environment. Therefore it degrades rapidly when it is utilised to form an outer cladding layer which is exposed to environmental moisture.

For the purpose of overcoming these disadvantages the subject of the present invention is a film having the characteristics indicated in the following Claim 1. Preferential characteristics of the film of the invention are indicated in the dependent Claims.

The fact that the EVOH copolymer is present in an intermediate layer and therefore not exposed to the outside, avoids it being damaged by environmental moisture with consequent degradation and loss of its barrier properties. At the same time the EVOH copolymer is not present in the inner base layer, which is that which constitutes the majority as far as mass and thickness of the film is concerned and to a large measure determines its mechanical properties. Consequently, the film of the invention is decidedly more economical and mechanically stronger than prior art films in which the EVOH copolymer is the essential component of the inner base layer.

By indicating the base layer as A, the at least one intermediate layer as B and the outer covering layer as C, the minimum configuration of the film of the invention is therefore of the type CBAC. This configuration can be completed with the addition of a desired number of layers. For example, by inserting a further intermediate layer a configuration of type CBABC is achieved, in which the further layer B may or may not have barrier properties against oxygen.

The first intermediate layer must also contain, as well as the EVOH copolymer, a compatibility agent of such copolymer with polyolefin, for example a polypropylene chemically modified by means of grafting with maleic anhydride. In order to improve the adhesion of the various layers, and preferably, but not necessarily for the purpose of the structure, this compatibility agent may be present also in the base layer and in the covering layers in contact with an intermediate layer containing the EVOH copolymer. This compatibility agent is however decidedly more expensive than polypropylene which is the principal component of the base layer constituting the greater part of the film. Therefore, for reasons of economy, it is possible to form other configurations of film in which the base layer is constituted essentially of polypropylene and, between this and the intermediate layer containing the EVOH copolymer, there is interposed an auxiliary compatibility layer of reduced thickness, which is formed for example of polypropylene and a component for ensuring compatibility of EVOH copolymer with polyolefin. By indicating such an auxiliary layer as D it is thus possible to have configurations of type CBDADBC and CBDAC. For reasons of process simplicity it can be convenient to have an auxiliary layer adjacent both sides of the base layer even in the presence of a single intermediate layer, and therefore achieve a configuration of the type CBDADC.

Advantageously the base layer has a thickness lying between 7 and 70 μm and is composed of polypropylene homopolymer or a mixture of polypropylene homopolymer - for example with a melting temperature lying between 150 and 17O0C and melt flow index (MFI) lying between 1 and 5 g/lθ min at T of 23O0C and 2.16kg - in proportion lying between 50 and 100%, preferably between 60 and 80%, by weight of the layer and by a polypropylene homopolymer chemically modified by means of grafting on the polymer chains of molecules of maleic anhydride in ratio lying between 50 and 0% by weight of the layer. This chemically modified polypropylene homopolymer usable for the base layer may, for example, be Fusabond M613- 05 product of DuPont. Smaller quantities of conventional additives can likewise be added to this layer, such as, for example, antistatic, slip and anti-UV agents, the purpose of which is that of modifying specific properties of the film, which do not have a direct influence on the barrier properties.

Advantageously, the first intermediate layer has a thickness lying between 1 and 6 μm and preferably between 2 and 5 μm and is composed of a copolymer of ethylene and vinyl alcohol - having a percentage of ethylene lying between 5 and 70%, preferably between 20 and 60%, more preferably between 30 and 50%, and MFI lying between 3 and 10 g/lθmin at 19O0C and 2.16kg - in percentages lying between 30 and 90%, preferably between 50 and 80%, by weight of the layer, and by a polypropylene homopolymer chemically modified by grafting on the polymer chains of molecules of maleic anhydride in quantities complementary to 100%. Such chemically modified polypropylene homopolymer usable for this layer may for example be Fusabond M613-05 product of DuPont, whilst the ethylene and vinyl alcohol copolymer may for example be Eval Gl56b product of Kuraray.

Advantageously, the first covering layer has a thickness lying between 0.5 and 3 μm, preferably between l and 2 μm, and is composed of a ternary copolymer of propylene-ethylene- butylene - for example with a melting point lying between 120 and 140 0C and MFI lying between 2 and 10 g/lθmin at 230 0C and 2.16kg, preferably between 3 and 8 g/lθmin - in percentages lying between 40 and 100%, preferably between 60 and 80% by weight of the layer, and by a polypropylene homopolymer chemically modified by grafting on the polymer chains of molecules of maleic anhydride in quantities complementary to 100%. Such chemically modified polypropylene homopolymer usable for this layer may for example be Fusabond M613-05 product of DuPont. The ternary copolymer can possibly be replaced by a polypropylene homopolymer or by a mixture of polypropylene homopolymer, with a melting temperature between 150 and 170°C and MFI lying between 1 and 5 g/10 min at T of 230'C at 2.16 kg in proportions lying between 50 and 100% of the weight of the layer. The first covering layer may likewise have smaller quantities of anti¬ blocking additives.

Advantageously, the second intermediate layer has a thickness lying between 1 and 6μm, preferably between 1.5 and 4μm, and has a composition similar to that of the first intermediate layer. Alternatively, it can be entirely composed of polypropylene homopolymer with a melting point lying between 150 and 170 0C and MFI lying between 1 and 8 g/lθmin, preferably between 2 and 6 g/10 min, at 230°C at 2.16 kg. Advantageously, the second covering layer has a thickness lying between 0.5 and 3μm, preferably between 1 and 2μm, and has a composition similar to that of the first intermediate layer. Alternatively, it can be substantially entirely composed of a ternary propylene-ethylene-butylene copolymer with a melting point lying between 120 and 140°C and MFI lying between 2 and 10 g/10min at 2300C at 2.16 kg and preferably between 3 and 8 g/10min. Smaller quantities of anti-blocking additives may moreover be added to it.

Advantageously, the first and second auxiliary layer may have a thickness lying between 0.5 and 5μm and a composition of the type indicated for the base layer.

The film of the invention can be produced by means of an extrusion and biaxial orientation procedure, the general characteristics of which are known. Advantageously, the base layer and the first intermediate layer can be extruded by means of respective twin screw co-rotating extruders to permit an easy mixing of components, whilst the remaining layers can be extruded by means of twin screw or single screw extruders.

The extruded film is then advantageously subjected to a sequential biaxial orientation in which, in a first phase, it is stretched in the longitudinal direction with respect to the machine sense (MDO) in a draw ratio lying between 4 :1 and 6:1, and in a second phase is stretched transversely of the machine sense (TDO) in a draw ratio lying between 6:1 and 8:1.

After biaxial orientation the film can be subjected to electric discharge treatment (corona treatment) or with flame, for surface activation whereby to render it suitable for printing and /or for further treatments such as coupling with other films of plastics material, paper or aluminium.

In particular, a film of the present invention having any of the configurations described above can further comprise a metallised layer, which may be applied by conventional metallisation techniques. Such a film has further improved barrier properties against oxygen and water vapour in the same way as the application of a further metallised layer improves the barrier properties of ■ films based, on biaxially oriented polypropylene of conventional type. In more detail, the presence of a metallised layer makes it possible to improve the barrier properties of the film against oxygen and water vapour by an order of magnitude even if this is to the detriment of its transparency. This improvement makes it possible to use metallised films of the present invention in those applications where very much more expensive films and/or structures are usually used, comprising a sheet of aluminium on its own or in combination.

Further advantages and characteristics of the present invention will be evident from the following examples of film compositions provided by way of non-limitative example, in which all the percentages are to be considered as percentages by weight if not otherwise indicated, and the values expressed in μm refer to the respective thicknesses of the various layers.

EXAMPLE 1

Layer A -, polypropylene homopolymer MFI 2 68% 21.8μm Fusabond M613-05 30% Ampacet 400687 (antistatic additive) 2%

Layer Bl - Eval G156B 70% 3μm Fusabond M613-05 30%

Layer Cl - Propylene-ethylene-butylene 69.5% 1.5μm terpolymer Fusabond M613-05 30% Schulman AB PP 10 (antiblocking 0.5% additive)

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 2

Layer A - polypropylene homopolymer MFI 2 68% 21.8μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - Propylene-ethylene-butylene terpolymer 99.5% 1.5μm Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer- C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 3

Layer A - polypropylene homopolymer MFI 2 68% 21.8μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3pm Fusabond M613-05 30%

Layer Cl - Polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 4

Layer A - polypropylene homopolymer MFI 2 68% 21.8μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - Polypropylene homopolymer MFI 3 99.5% l.5μm Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 5

Layer A - polypropylene homopolymer MFI 2 98% 21.8μm Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - Propylene-ethylene-butylene terpolymer 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 6

Layer A - polypropylene homopolymer MFI 2 88% 21.8μm Fusabond M613-05 10% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - Propylene-ethylene-butylene terpolymer 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 7

Layer A - polypropylene homopolymer MFI 2 98% 21.8μm Ampacet 400687 2%

Layer Bl - Eval G156B 80 % 3μm Fusabond M613-05 20%

Layer Cl - Polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 8

Layer A - polypropylene homopolymer MFI 2 98% 21.8μm Ampacet 400687 2%

Layer Bl - Eval G156B 80 % 3μm Fusabond M613-05 20%

Layer Cl - Polypropylene homopolymer MFI 3 99.5% 1.5μm Schulman AB PP 10 0.5%

Layer B2 - Polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 9

Layer A - polypropylene homopolymer MFI 2 68% 21.3μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - Propylene-ethylene-butylene terpolymer 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer C2 - Propylene-ethylene-butylene terpolymer 69.5% 1.2μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

EXAMPLE 10

Layer A - polypropylene homopolymer MFI 2 68% 21.3μm Fusabond M613-05 30% Ampacet 400687 2% Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer C2 - polypropylene homopolymer MFI 3 69.5% 1.2μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

EXAMPLE 11

Layer A - polypropylene homopolymer MFI 2 68% 21.3μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer C2 - Propylene-ethylene-butylene terpolymer 69.5% 1.2μm Fusabond M613-05 30% Schulman AB PP 10 0.5% EXAMPLE 12

Layer A - polypropylene homopolymer MFI 2 68% 21.3μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 99.5% 1.5μra Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 13

Layer A - polypropylene homopolymer MFI 2 98% 21.3μm Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer C2 - Propylene-ethylene-butylene terpolymer 69.5% 1.2μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

EXAMPLE 14

Layer A - polypropylene homopolymer MFI 2 98% 21.3pm Ampacet 400687 2%

Layer Bl - Eval G156B 80 % 3μm Fusabond M613-05 20%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer B2 - Eval G156B 80 % 3μm Fusabond M613-05 20%

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 15

Layer A - polypropylene homopolymer MFI 2 68% 24.3μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene-ethylene-butylene terpolymer 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer C2 - polypropylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 16

Layer A - polypropylene homopolymer MFI 2 68% 24.3μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene-ethylene-butylene terpolymer 99.5% 1.5μm Schulman AB PP 10 0.5%

Layer C2 - Propylene homopolymer MFI 3 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 17

Layer A - polypropylene homopolymer MFI 2 98% 17.8μm Ampacet 400687 2%

Layer Dl - polypropylene homopolymer MFI 2 68% 2μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer D2 - polypropylene homopolymer MFI 2 68% 2μm Fusabond M613-05 30% Ampacet 400687 2%

Layer B2 - polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 18

Layer A - polypropylene homopolymer MFI 2 98% 17.3μm Ampacet 400687 2%

Layer Dl - polypropylene homopolymer MFI 2 68% 2μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 99.5% 1.5μm Schulman AB PP 10 0.5%

Layer D2 - polypropylene homopolymer MFI 2 68% 2μm Fusabond M613-05 30% Ampacet 400687 2%

Layer B2 - Eval G156B 70 % 3μm Fusabond M613-05 30% Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 19

Layer A - polypropylene homopolymer MFI 2 98% 17.8μm Ampacet 400687 2%

Layer Dl - polypropylene homopolymer MFI 2 68% 2μm Fusabond M613-05 30% Ampacet 400687 2%

Layer Bl - Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Fusabond M613-05 30% Schulman AB PP 10 0.5%

Layer D2 - polypropylene homopolymer MFI 2 98% 2μm Ampacet 400687 2%

Layer B2 - polypropylene homopolymer MFI 3 100% 2.5μm

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

EXAMPLE 20

Layer A - polypropylene homopolymer MFI 2 98% 20.3μm Ampacet 400687 2%

Layer Dl - polypropylene homopolymer MFI 2 68% 2μm Pusabond M613-05 30% Ampacet 400687 2%

Layer Bl Eval G156B 70 % 3μm Fusabond M613-05 30%

Layer Cl - polypropylene homopolymer MFI 3 69.5% 1.5μm Pusabond M613-05 30% Schulman AB PP 10 0.5%

Layer D2 - polypropylene homopolymer MFI 2 98% 2μm Ampacet 400687 2%

Layer C2 - Propylene-ethylene-butylene terpolymer 99.5% 1.2μm Schulman AB PP 10 0.5%

Table 1 which follows plots the oxygen permeability values of films produced starting from the compositions of the above- indicated examples. These values are measured according to the ASTM D3985 method at 23 0C and 0% relative humidity.

Table 1 — values of oxygen permeabililty OTR - 23 °C 0% RH Method Value

Example 1 cm3/m2*d*bar ASTM D3985 25 Example 2 cm3/m2*d*bar ASTM D3985 33 Example 3 cm3/m2*d*bar ASTM D3985 67 Example 4 cm3/m2*d*bar ASTM D3985 46 Example 5 cm3/m2*d*bar ASTM D3985 53 Example 6 cm3/m2*d*bar ASTM D3985 80 Example 7 cm3/m2*d*bar ASTM D3985 13 Example 8 cm3/m2*d*bar ASTM D3985 52 Example 9 cm3/m2*d*bar ASTM D3985 15 Example 10 cm3/m2*d*bar ASTM D3985 18 Example 11 cm3/m2*d*bar ASTM D3985 22 Example 12 cm3/m2*d*bar ASTM D3985 35 Example 13 cm3/m2*d*bar ASTM D3985 28 Example 14 cm3/m2*d*bar ASTM D3985 16 Example 15 cm3/m2*d*bar ASTM D3985 58 Example 16 cm3/m2*d*bar ASTM D3985 72 Example 17 cm3/m2*d*bar ASTM D3985 65 Example 18 cm3/m2*d*bar ASTM D3985 34 Example 19 cm3/m2*d*bar ASTM D3985 75 Example 20 cm3/m2*d*bar ASTM D3985 69

From these values it can be seen that the films of the invention have an oxygen permeability less than or equal to 80 cm3/m2*d*bar whilst remaining transparent. The films of the invention further have a water vapour permeability equivalent to that of conventional films, that is to say less than or equal to 6g/m2*d*bar.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments can be widely varied with respect to what has been described purely by way of example, without by this departing from its scope.