HK1135652A - Five-layer barrier film, sealing process and use thereof for packaging food - Google Patents

Abstract

The invention relates to a barrier film made up of five stacked layers comprising: a central barrier layer comprising a first blend of EVOH and of PA, two outer layers comprising a second blend, and two adhesive layers between the central layer and the two outer layers. The second blend only contains a first copolymer of ethylene and of an a-olefin having a density less than or equal to 0.925 g/cm3 and greater than or equal to 0.915 g/cm3 and a second metallocene copolymer of ethylene and of an a-olefin having a density less than or equal to 0.910 g/cm3. According to the invention, the proportion of the second copolymer relative to the weight of its outer layer is greater than or equal 20% and less than or equal to 45%, the seal initiation temperature for a seal breaking strength greater than or equal to 15 N/30 mm is below 120 C, the seal breaking strength of the film is greater than or equal to 10 N/30 mm after passing through a water bath at 85 C for one and half hours.

Description

Five-layer barrier film, sealing method and use thereof for packaging food

The invention relates to a barrier film consisting of five layers including a central barrier layer based on EVOH.

Barrier films are used in the field of packaging food products under modified atmospheres.

For this purpose, EVOH (ethylene vinyl alcohol copolymer) is known, which has good impermeability to gases, in particular oxygen.

To ensure the tightness of the package, the barrier film should be capable of being sealed by heat.

Thus, one or both of the outer layers of the film is a sealant layer.

The function of the intermediate layer is to make each outer layer adhere to the barrier layer, thus ensuring that the film is resistant to delaminating.

Such a five-layer structure enables thinner films to be made with lower cost materials than films having six, seven, and more layers.

Different five-layer films are known.

Document EP-B-561428 describes a film comprising:

-a core layer comprising EVOH mixed with polyamide,

-a bonded intermediate layer comprising a polyolefin modified with an acid or an anhydride,

-an outer layer comprising a blend of LLDPE and VLDPE.

Document EP-B-217596 relates to a film comprising:

-a core layer comprising a blend of EVOH and polyamide,

-two inner adhesive layers comprising a polymer modified with an acid or an anhydride,

-two outer layers comprising a blend of three components of LLDPE, LMDPE and EVA.

Document EP-B-881966 relates to a film comprising:

-a core layer comprising EVOH,

-two outer layers, at least one of which comprises a blend of three components:

component A is a homogeneous or heterogeneous copolymer of ethylene and an alpha-olefin having a density of from 0.915 to 0.925 g/cc,

component B is a homogeneous or heterogeneous copolymer of ethylene and an alpha-olefin having a density greater than or equal to 0.925 g/cc,

component C is a homogeneous or heterogeneous copolymer of ethylene and of an alpha-olefin having a density less than or equal to 0.915 g/cc,

-two adhesive layers between the core layer and the outer layer.

Document EP-B-369808 relates to a film comprising:

-a heat-sealable VLDPE layer having a density of less than 920 kg/cc,

-a barrier layer of PVDC or EVOH,

-an outer thermoplastic layer selected from LLDPE having a density greater than 920 kg/m, high-density linear PE, EVA copolymers, copolymers of ethylene with alkyl acrylates (containing from 1 to 8 carbon atoms), copolymers of ethylene with acrylic acid or ionomers,

an intermediate VLDPE layer between the barrier layer and the thermoplastic outer layer,

-an additional thermoplastic layer between the VLDPE heat-seal layer and the barrier layer, selected from LLDPE having a density of more than 920 kg/m, high-density linear PE, EVA copolymers, copolymers of ethylene with alkyl acrylates (containing 1 to 8 carbon atoms), copolymers of ethylene with acrylic acid or ionomers.

Barrier films must meet a number of requirements.

The seal should have some mechanical strength to enable the packaged product to be handled and stored.

It is desirable that these seals can be made by machine at relatively low temperatures, thereby increasing the rate of sealing stations on the packaging line and saving energy.

When the film must enter the shrinking oven, the free shrinkage of the film should be greater at low temperatures to preserve the packaged food, shorten the time used in these ovens and increase productivity.

For pasteurization, the seal should be heat resistant so that the film can enter the 85 ℃ water bath for one and a half hours without weakening the strength of the seal.

The film should be able to be peeled off at the seal when the food product is removed.

Thus, the seal must be resistant to peeling, but can be peeled by the user opening.

The known films described above do not meet or only partially meet one of these requirements.

The present invention relates to obtaining a barrier film that meets these requirements, in particular the sealing requirements.

To this end, a first object of the invention is a barrier film consisting of five superposed layers comprising:

a central barrier layer comprising a first blend of ethylene-vinyl alcohol copolymer (based on the majority by weight) and polyamide,

-two outer layers comprising a second blend of ethylene and an alpha-olefin copolymer,

two adhesive layers between the central layer and the two outer layers respectively,

it is characterized in that

The second blend of ethylene and alpha-olefin copolymer of at least one outer layer comprises only a first copolymer of ethylene and alpha-olefin having a density less than or equal to 0.925 g/cc and greater than or equal to 0.915 g/cc and a second metallocene copolymer of ethylene and alpha-olefin having a density less than or equal to 0.910 g/cc.

According to other features of the invention:

-the first copolymer of the second blend is LLDPE.

The proportion of the first copolymer, for example formed by LLDPE, with respect to the weight of the outer layer is greater than or equal to 40%.

The proportion of the second copolymer relative to the weight of the outer layer is greater than or equal to 20% and less than or equal to 45%, in particular greater than or equal to 35%.

-the density of the second copolymer of the second blend is less than or equal to 0.905 g/cc.

-each outer layer comprises said second blend of ethylene and alpha-olefin copolymer.

-the thickness of the barrier layer is greater than or equal to 4 microns.

For example, the film is symmetrical with respect to the central barrier layer.

In a particular embodiment, the first blend of barrier layers comprises at least 80% by weight of EVOH, preferably at least 90% by weight of EVOH.

In one embodiment, the first blend of barrier layers comprises copolyamide 6/6-6. For example, the first blend is 90 weight percent EVOH and 10 weight percent copolyamide 6/6-6.

In another embodiment, the first blend of barrier layers comprises terpolyamide 6/6-6/12. The proportion of terpolyamide 6/6-6/12 in the first blend of the barrier layer may be less than 9%. For example, the first blend is 92 wt.% EVOH and 8 wt.% terpolyamide 6/6-6/12.

LLDPE is a low density linear polyethylene, i.e., having a density less than or equal to 0.925 grams per cubic centimeter and greater than or equal to 0.915 grams per cubic centimeter.

LLDPE-g-MAH is LLDPE grafted with maleic anhydride.

LDPE is a low density ethylene homopolymer.

One and/or the other of the intermediate layers comprises polyethylene grafted with maleic anhydride, for example LLDPE grafted with maleic anhydride. Each intermediate layer has a thickness of at least 2 microns.

In one embodiment, the percentage of the thickness of the layer relative to the total thickness of the film is as follows:

30％/10％/20％/10％/30％。

the invention will be better understood by reading the following description of non-limiting specific embodiments, given only by way of example, and with reference to the accompanying drawings, in which:

FIG. 1 shows a plot of the strength of the seal performed on the film of the invention against temperature, an

FIG. 2 shows the shrinkage rate vs. temperature of the film according to the invention.

An exemplary embodiment of a film according to the present invention, hereinafter referred to as example 1, has the following structure:

a barrier layer having a thickness of 5 microns formed by 90% by weight of EVOH having a density of 1.14 g/cc and a melting point of 161 ℃ -165 ℃ and 10% by weight of copolyamide 6/6-6 having a density of about 1.13 g/cc and a melting point of 192 ℃,

two intermediate layers, each layer having a thickness of 2.5 μm, each layer being formed from LLDPE-g-MAH having a density of 1.14 g/cc and a melting point of 161 ℃,

-two outer layers, each having a thickness of 7.5 microns, each outer layer being formed by 46% by weight of a LLDPE with a density of 0.916 g/cc and a melting point of 122 ℃, 40% by weight of a metallocene copolymer of ethylene and an α -olefin with a density of 0.902 g/cc and a melting point of 95 ℃, 8% by weight of an antifogging composition and 6% by weight of an antiblocking agent.

The anti-fog composition was a masterbatch with 10 wt% anti-fog agent on an LDPE carrier, i.e. in the layer 8 wt% of the added anti-fog composition provided 0.8 wt% active anti-fog agent, and 7.2 wt% supported LDPE with a density of 0.920 g/cc.

The antiblock composition is a masterbatch having 5 weight percent synthetic silica on an LDPE support, i.e., in a layer, 6 weight percent of the added antiblock composition provides 0.3 weight percent synthetic silica, and 5.7 weight percent supported LDPE having a density of 0.920 grams per cubic centimeter.

For example, the film is produced by a double bubble process during which a cylindrical sheath (sheath) is produced, as is known to the person skilled in the art. The double bubble process occurs as follows: a first cylindrical jacket, called the main jacket, is immersed in water for cooling and then conveyed to a height of, for example, 20 meters. Then, it was heated in an oven to obtain a determined orientation temperature. Next, the sheath is inflated with air injected from below. The thus obtained bubble was closed by pinching from below and cut at both sides with two blades to obtain two separate films, which were wound on a roll. The film has a longitudinal or axial direction (which is the direction in which the film is wound onto the roll during its manufacture), and a transverse direction parallel to the axis of the roll.

Exemplary film 2 for comparison is a prior art barrier film related to BDF 8050 available from Cryovac.

The method for testing the seal strength of the film was accomplished by sealing two components of the film and evaluating the peel strength. The sealing was performed with the same known Brugger type machine at different temperatures, the force exerted on the film by the sealing bar being 120 newtons for 0.7 seconds. The tensile break force of a specimen consisting of two thin film assemblies sealed perpendicular to their length was measured at a constant rate (150 mm/min). The width of the test specimen was 30 mm + or-0.5 mm. The breaking force must be expressed in relation to the width of the sealed specimen. The values indicated in newtons are in particular in newtons/30 mm.

Fig. 1 shows the seal strength in N on the ordinate measured at different sealing temperatures in C on the abscissa for example 1 of a film according to the invention according to curve C1 (consisting of squares) and for example 2 of a film according to the prior art according to curve C2 (consisting of triangles).

The seal strength of the exemplary film 1 of the present invention is large from 115 ℃ to 135 ℃ and higher than the seal strength of the known film of example 2 at these temperatures, thus forming a very low seal window W (defined by the seal strength above the initial threshold of 15-20N) at these temperatures, which enables high rates on the packaging machine. In this window, the seal is peelable. The initial sealing temperature is less than 120 ℃.

Above 135 deg.C, the film is delaminated.

The sealing window of the known example 2 is between 140 ℃ and 160 ℃ and 170 ℃. In this window, example 2 was systematically delayered.

T._SITSeal initiation temperature was 115 ℃ for inventive example 1.

T._SITThe seal initiation temperature was 140 ℃ for inventive example 2.

Other seals were made on a Flowpack type (heat seal) machine from Ulma, their strength was measured according to the method previously shown, and the results are shown in the table below.

SL is longitudinal sealing direction. SL is in the transverse sealing direction. Because of the high shrinkage rate at low temperatures, inventive example 1 was able to seal at very low temperatures and pass very quickly in a shrinkage oven, as shown in fig. 2 (25% at 93 ℃ and 12% for example 2). The temperature of the sealing strip can thereby advantageously be reduced by a value of between 35 ℃ and 40 ℃. The temperature of the shrinking oven used for the film of the present invention may be 160 ℃ and the throughput rate in the oven is increased by 70%. The shrinkage rate for example 1 was 13-15%/second, compared to 9.5% for example 2.

The seal strength of the Flowpack type was also measured under pasteurization conditions as shown in the table below.

The seal of the Flowpack package was not impaired by one hour and thirty minutes in the water bath.

Unlike example 2, the seal of example 1 according to the invention is peelable at low temperature and there is no delaminating of the film.

According to a feature of the invention:

-a seal initiation temperature of less than 120 ℃ for a seal rupture force of greater than or equal to 15N/30 mm,

-the sealability window comprises a range extending from 115 ℃ to 135 ℃ for a seal rupture force greater than or equal to 10 newtons/30 mm;

-the sealability window comprises a range extending from 115 ℃ to 135 ℃ for a seal rupture force of greater than or equal to 15 newtons/30 mm;

-the seal breaking force of the film is greater than or equal to 10 newtons/30 mm after entering a water bath at 85 ℃ for one and a half hours;

-a modulus of elasticity greater than or equal to 700 mpa;

-an oxygen permeability at a humidity rate of 0% and at 23 ℃ of less than or equal to 20 cubic centimetres per square metre per 24 hours;

-an oxygen permeability of less than or equal to 25 cubic centimeters per square meter per 24 hours at a humidity rate of 80% and at 23 ℃;

-the total thickness of the film is less than or equal to 30 microns.

The modulus of elasticity of example 1 was sufficiently large at 700 to 800 mpa, and the film was sealable at low temperature.

The Oxygen Transmission Rate (OTR) was measured according to the following table for a barrier layer having a thickness of 4.7 microns measured in example 1 and a barrier layer having a thickness of 3.2 microns measured in example 2.

Oxygen permeability (in cubic centimeter/square meter/24 hour meter)	At a humidity rate of 0% and at 23 deg.C	At a humidity rate of 80% and 23 deg.C
			Example 1	16.6	20.8
Example 2	23.8	28.5

In the case where the barrier layer of example 1 contained 8% of the terpolymer 6/6-6/12 instead of 10% of the copolyamide (as described above), the oxygen transmission rate for the barrier layer measured at a thickness of 4.1 microns is shown in the following table.

Oxygen permeability (in cubic centimeter/square meter/24 hour meter)	At a humidity rate of 0% and at 23 deg.C	At a humidity rate of 80% and 23 deg.C
			Example 1 Using terpolyamide 6/6-6/12	16.4	20

By adding a terpolyamide at a lower rate than the copolyamide (8% for terpolyamide and 10% for copolyamide), it is possible to obtain better stretchability (improved processability) for the film without degradation of barrier properties.

The object of the present invention is also a method for sealing at least two walls of a package, said walls being constituted by a film as described above, comprising at least one step of applying at least one heating fin assembly on the wall for sealing of the wall,

it is characterized in that

The temperature of the seal assembly is less than or equal to 130 ℃.

According to a feature of the invention:

-at least one step following the step of applying the sealing assembly is entering at least one shrinking oven, and the residence time in the shrinking oven at a temperature of greater than or equal to 150 ℃ is less than or equal to 4 seconds.

-the shrinkage rate of the film is greater than or equal to 10%/second.

An exemplary application of the above method is sealing a package of a food product under a modified atmosphere.

Claims (25)

1. A barrier film comprised of five superposed layers comprising:

-a central barrier layer comprising a first blend of a majority by weight of ethylene vinyl alcohol copolymer and polyamide,

-two outer layers comprising a second blend of ethylene and an alpha-olefin copolymer,

two adhesive layers between the central layer and the two outer layers respectively,

it is characterized in that

The second blend of ethylene and alpha-olefin copolymer of at least one outer layer comprises only a first copolymer of ethylene and alpha-olefin having a density less than or equal to 0.925 g/cc and greater than or equal to 0.915 g/cc and a second metallocene copolymer of ethylene and alpha-olefin having a density less than or equal to 0.910 g/cc,

the proportion of the second copolymer relative to the weight of the outer layer is greater than or equal to 20% and less than or equal to 45%,

a seal initiation temperature for a seal rupture force of greater than or equal to 15 newtons/30 millimeters is less than 120 c,

the seal breaking force of the film is greater than or equal to 10 newtons/30 mm after entering a 85 ℃ water bath for one and a half hours.

2. The film of claim 1 wherein the first copolymer of the second blend is an LLDPE.

3. Film according to any one of the preceding claims, characterized in that the proportion of the first copolymer with respect to the weight of the outer layer is greater than or equal to 40%.

4. A film according to any one of the preceding claims, characterized in that the proportion of the second copolymer with respect to the weight of the outer layer is greater than or equal to 35%.

5. The film according to any of the preceding claims, characterized in that the density of the second copolymer of the second blend is less than or equal to 0.905 g/cc.

6. The film according to any of the preceding claims, characterized in that each outer layer comprises a second blend of said ethylene and α -olefin copolymer.

7. Film according to any one of the preceding claims, characterized in that the first blend of barrier layers comprises at least 80% by weight of EVOH.

8. The film according to any of the preceding claims, characterized in that the first blend of barrier layers comprises copolyamide 6/6-6.

9. The film according to any of claims 1 to 7, characterized in that the first blend of barrier layers comprises terpolyamide 6/6-6/12.

10. The film of claim 9 wherein the proportion of terpolyamide 6/6-6/12 in the first blend of the barrier layer is less than 9%.

11. A film according to any of the preceding claims, characterized in that at least one intermediate layer comprises polyethylene grafted with maleic anhydride.

12. A film according to any of the preceding claims, characterized in that at least one intermediate layer comprises LLDPE grafted with maleic anhydride.

13. A film according to any one of the preceding claims, characterized in that the thickness of each intermediate layer is at least 2 microns.

14. A film according to any of the preceding claims, wherein the sealability window comprises a range extending from 115 ℃ to 135 ℃ for a seal fracture force of greater than or equal to 10 newtons/30 mm.

15. A film according to any of the preceding claims, wherein the sealability window comprises a range extending from 115 ℃ to 135 ℃ for a seal fracture force of greater than or equal to 15 newtons/30 mm.

16. Film according to any one of the preceding claims, characterized in that the modulus of elasticity is greater than or equal to 700 MPa.

17. The film according to any of the preceding claims, characterized in that the oxygen permeability is less than or equal to 20 cubic centimeters per square meter per 24 hours at a humidity rate of 0% and 23 ℃.

18. The film according to any of the preceding claims, characterized in that the oxygen permeability is less than or equal to 25 cubic centimeters per square meter per 24 hours at a humidity rate of 80% and 23 ℃.

19. The film according to any of the preceding claims, characterized in that it is symmetrical with respect to the central barrier layer.

20. The film according to any of the preceding claims, characterized in that the total thickness of the film is less than or equal to 30 microns.

21. Film according to any one of the preceding claims, characterized in that the thickness of said barrier layer is greater than or equal to 4 microns.

22. A method for sealing at least two walls of a package, said walls being constituted by a film according to any one of the preceding claims, the method comprising at least one step of applying at least one heating fin assembly on a wall for sealing of the wall,

it is characterized in that

The temperature of the seal assembly is less than or equal to 130 ℃.

23. The sealing method of claim 22, wherein the step of applying the sealing assembly is followed by at least one step of entering at least one shrinking oven, and wherein the residence time in the shrinking oven at the temperature of the shrinking oven is greater than or equal to 150 ℃ is less than or equal to 4 seconds.

24. The method of sealing according to claim 23, wherein the film has a shrinkage rate of greater than or equal to 10%/second.

25. Use of the method according to any one of claims 22 to 24 for sealing food packaging under a modified atmosphere.