WO2007014431A1

WO2007014431A1 - Multilayer metallised film

Info

Publication number: WO2007014431A1
Application number: PCT/AU2006/001097
Authority: WO
Inventors: Frank Farchione
Original assignee: Amcor Limited
Priority date: 2005-08-02
Filing date: 2006-08-02
Publication date: 2007-02-08

Abstract

A multilayer metallised film and a method of forming the film are disclosed. The method comprises a step of extruding a first polymer between a metallised layer and a film of a second polymer to form the multilayer metallised film. The first extruded polymer includes one or more than one polymer additive that is selected to control the adhesion of the extruded first polymer or polymer blend to the metallised layer such that an adhesion force between the first extruded polymer and the metallised layer is lower than an adhesion force between the metallised layer and the supporting film.

Description

MULTILAYER METALLISED FILM

The present invention relates to multilayer metallised films having improved flex crack properties and to a method of preparing the films .

The multilayer metallised films include a layer that is a metallised film.

The multilayer metallised films are suitable for use in the packaging industry, particularly for food and beverages.

There is a wide range of known packaging materials for food and beverages. The packaging materials include multilayer films that have a range of different characteristics which are not available from single layer films. The range of characteristics for multilayer films includes oxygen barrier, light barrier, moisture barrier, and other characteristics. The multilayer films can be sealed together during production to form packaging that prevents liquid and food escaping from the packaging. Tri-layer films, with a metallised film sandwiched between two clear films, are known. These films are usually prepared by cold adhesive lamination and have strong inter-layer bonding between a metal surface of the metallised film and a surface of an adjacent film. The use of cold adhesives, which are more brittle than the polymers that they bond, leads to tri-layer films which are prone to cracking, especially when tested by the Gelbo-flex test, which flexes the structure 100 times, and is designed to replicate flexing during the normal usage of the flexible material. Tri-layer films with poor flexing behaviour show more cracking and creasing, which can lead to product failure, and greatly reduced oxygen barrier performance and leakage. The use of a combination of polymers and adhesives also gives lower durability than pure polymer film structures.

Hot extrusion of tri-layer films has not hitherto been used because of concerns about thermal shock and degradation of the metal surface of the metallised films. Specifically, hot extrusion coating thermo-adhesive resins onto a thin metal film can cause fine cracks by expansion and contraction through heat stresses and mechanical tensions during production. This leads to the gas barrier properties of the film being greatly lowered.

The present invention is based on a discovery that it is possible to prepare multilayer metallised films, i.e. multilayer films that include a layer of a metallised film by hot extrusion. More particularly, the present invention is based on a discovery that multilayer metallised films with improved flex crack properties can be prepared by hot extrusion if polymers with a suitable affinity for the metallised layer are used for hot extrusion. The term "metallised" used in respect of films herein refers to a film formed of a metal or a metal compound. The metallised film comprises a metal/metal compound layer on a supporting film of a polymer or polymer blend.

SUMMARY OP THE INVENTION

The present invention provides, in general terms, a method of forming a multilayer metallised film by hot extrusion in which one or more polymer film additives are selected to control the adhesion of an extruded polymer or polymer blend to a metallised film and another film so as to provide improved properties to the resulting multilayer metallised film.

The term "polymer" is understood herein to mean polymer or polymer blend.

In particular, the invention provides a method of preparing a multilayer metallised film comprising the steps of : a) providing a metallised film that includes a metallised layer on a supporting film, the metallised layer having gas and/or moisture barrier properties; and b) extruding a first polymer ("the first extruded polymer") between the metallised layer of the metallised film and a film of a second polymer ("the second polymer film") to form the multilayer metallised film, wherein the first extruded polymer includes one or more than one polymer additive that is selected to control the adhesion of the first extruded polymer to the metallised layer of the metallised film such that an adhesion force between the first extruded polymer and the metallised layer is lower than an adhesion force between the metallised layer and the supporting film.

The present invention maintains the integrity of the metallised film by ensuring that the affinity between the first extruded polymer with the additive (s) and the metallised layer of the metallised film is less than the affinity between the metallised layer and the supporting film. Accordingly, thermal stresses by extruding the first extruded polymer onto the metallised layer result in the extruded polymer layer delaminating from the metallised layer in preference to the supporting film delaminating from the metallised layer, without damaging the metallised layer. Additionally, the first extruded polymer with additive (s) will delaminate from the metallised layer when a force is applied, such as by flexing the laminate. Delamination of the first extruded polymer with additive (s) from the metallised layer ensures that the metallised layer is not damaged during use of the multilayer metallised film. Hence, the multilayer metallised film retains its gas and light impermeable properties.

It is preferable that the first extruded polymer be selected so as not to damage the material of the metallised layer by chemical or physical mechanisms. Preferably the method includes a further step (c) of extruding a third polymer ( "the third extruded polymer") between a film of a fourth polymer ("the fourth polymer") and the supporting film of the multilayer metallised film produced in step (b) to laminate the fourth polymer film and the third extruded polymer onto the multilayer metallised film.

The supporting film of the metallised film may comprise a polymer film such as bi-axially oriented polypropylene film (BOPP) , polyester film, and polyethylene film.

Preferably the supporting film is polyethylene terephthalate .

The metallised layer of the metallised film may be a metal or a metal compound. By way of example, the metallised layer may be one of the following metals: aluminium, titanium, indium or magnesium.

Preferably the extruded polymer is polytheylene or a polymer blend including polytheylene . Preferably the one or more than one polymer additive includes acid copolymers, most preferably ethylene-methacrylic acid- acrylate terpolymer or ethylene acrylate maleic anhydride. Preferably the second polymer film used in step (a) is a polyethylene film.

More preferably the second polymer film is a blend of linear low density polyethylene/low density polyethylene (LLDPE/LDPE) , although low density polyethylene (LDPE) alone can be used.

If a blend is used it is preferred that the blend contains about 20% LDPE.

Preferably step (b) includes extruding the first extruded polymer between the metallised layer of the metallised film and the second polymer film in a first laminator so that the multilayer metallised film formed in the first laminator includes the following layers, from top to bottom: the supporting film - PET, the metallised layer, the first extruded polymer - a polyethylene blended with one or more polymer additives, the second polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE. Preferably the fourth polymer film used in step (c) is a polyethylene film.

More preferably the step (c) includes extruding a modified ethylene acrylate resin onto the surface of the supporting film and further laminating a polyethylene film on the modified ethylene acrylate resin in a second laminator so that the multilayer metallised film includes the following layers, from top to bottom: the fourth polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE, the third extruded polymer - a modified ethylene acrylate resin, the supporting film - PET, the metallised layer, the first extruded polymer - a polyethylene blended with one or more polymer additives, and the second extruded polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE.

Two in-line extrusion lamination steps are preferred to make the film in a one pass process. The present invention also provides a multilayer metallised film comprising from top to bottom: a supporting film, a metallised layer, a first extruded polymer - a polymer blend with one or more than one polymer additive, and a second polymer film - a polyethylene .

The present invention also provides a multilayer metallised film comprising from top to bottom: a fourth polymer film - a polyethylene, a third extruded polymer - a modified ethylene acrylate resin, a supporting film - PET, a metallised layer, a first extruded polymer - a polyethylene with one or more polymer additives, and a second polymer film - a polyethylene.

Preferably the polymer additive is an acid copolymer such as ethylene-tnethacrylic acid-acrylate terpolymer and/or ethylene acrylate maleic anhydride.

Preferably the polyethylene film is a blend of linear low density polyethylene/low density polyethylene (LLDPE/LDPE) , although low density polyethylene (LDPE) can be used. If a blend is used it is preferred that the blend contains about 20% LDPE.

The invention allows for the construction of multilayer metallised films as fully polymeric structures because the two extruded polymers for example, the extruded modified ethylene acrylate resin and the extruded polyethylene with polymer with one or more polymer additives, are polymeric in nature. This gives a fully flexible structure with superior flex crack properties as well as aseptic packaging performance. The present invention also provides improved packaging for the packaging industry, particularly the food and beverage packaging industries . Packaging materials made from the metallised film structures of the present invention have improved oxygen and light barrier characteristics and also have improved flexing behaviour.

DETAILED DESCRIPTION

Suitable metallised films and other thin-film layers provide an oxygen and carbon dioxide barrier and are preferably based on a clear polymer film.

Metallised films are useful because they provide improved gas and water barrier properties. These are necessary when the films are used to contain food and beverage products including wine, potato and other snacks, and chocolate bars. They can also be used for bulk packaging of liquids including fruit juices, tomato and orange juice, milk fats and other milk products.

An embodiment of a multilayer metallised film in accordance with the present invention comprises a metallised layer formed on a supporting film disposed between upper and lower hot extruded polymers that, in turn, are disposed between top and bottom polymer films.

The supporting film includes films made from a range of possible polymers including bi-axially oriented polypropylene films (BOPP) , PET, polyethylene and other polymer films. However, the supporting film is preferably PET.

The metallised layer comprises a layer of metal, metal compound or an inorganic compound. Suitable metals for a metallised layer include magnesium, indium, aluminium and titanium. Oxides and nitrides of these metals are suitable for forming a metal compound thin-film layer. Compounds of other metals may be suitable for forming a thin-film layer provided the formed layer has the required gas and light barrier properties for a laminate used in food packaging and in other packaging applications. Suitable inorganic compounds for the thin- film layer may be formed with silicon. Again, a range of compounds may be used provided the compound provides the required gas and light barrier properties as dictated by the intended application of the laminate.

The metallised layer is formed on the supporting film, i.e. a substrate layer, by usual processes such as vacuum deposition, ion plating, sputtering, plasma deposition, electron beam processes or chemical vapour deposition.

The thickness of the metallised layer will vary depending upon the exact nature of the compound used, the characteristics required and the need to keep the layer as thin as possible to ensure the required flexibility.

The top and bottom films comprise any film that can be cast or blown such as PET, LDPE, HD/LD (either blends or coextruded) , LLDPE/LDPE (either blends or co- extruded) , BOPP (either stretched and therefore oriented or unstretched) or nylon films such as and including polyamides or nylon EVOH/nylon coextrusion.

The extruded polymer with additive for contacting the metallised layer and the top or bottom film includes ionomers (low density resins with ionic groups throughout the chain), LLDPE, LDPE, HDPE and blends thereof. Any suitable additive or combination of additives can be selected so long as the adhesion force between it and the thin-film layer must be lower than the adhesion force between the thin-film layer and it supporting film. The polymer additives can be selected from compositions such as ethylene acrylic acid copolymers, ethylene acrylic acid resin. More specifically ethylene-methacrylic acid- acrylate terpolymer rein or ethyl acrylate maleic anhydride copolymer/terpolymer resin can be used. An alternative polymer additive is polyolefine plastomer (POP) .

The extruded polymer for contacting the supporting film and the other of the top or bottom film includes ionomers, LLDPE, LDPE, HDPE and blends thereof. The polymer additive (s) can be selected from thermo- adhesive resins and polymers having functional groups such as anhydride modified LDPE, polyolefine plastomer. The main purpose of this polymer additive is to ensure the resultant film has the desired properties as dictated by the selected application for the laminate. The additives in the second blend, therefore, can be selected by those skilled in the art. An embodiment of the method of the present invention commences in a first laminator with a selection of a polymer additive (s) , such as either ethylene- methacrylic acid-acrylate terpolymer, or ethylene acrylate maleic anhydride. These create a soft flexible bond with the metallised layer. The polymer additive is blended with a mixture of low density, high density and/or linear low density polyethylene, in an addition level of 15 to 50%, typically 20 to 30% when it is ethylene-metacrylic acid-acrylate terpolymer, while the ethylene acrylate maleic anhydride is blended with a mixture of low density, high density and/or linear low density polyethylene, in an addition level of 15 to 60%, typically 40 to 50%. Polyethylene forms of HDPE, LDPE and LLDPE are all polymers suitable for this process as the rheology of the polyethylene is compatible with the additives disclosed above .

This polymer is then extruded out of the first laminator between two films at a level of 8 to 12 grams per square meter, typically 10 gsm, at typical polyethylene temperatures and settings. The first of the two films is the metallised layer on the supporting film of PET as described above. The second film is the polyethylene blend film, typically linear low density polyethylene/low density polyethylene (LLDPE/LDPE) film, although LDPE can be used alone. The ratio of LDPE to LLDPE is typically 20% LDPE, as this gives a good combination of flexibility, strength, sealability, and proσessability. Other additives can be added to the

LDPE/LLDPE blend including slip additives and anti-blocks. Low tensions are maintained at all stages to avoid MD banding and tension stresses on the metal surface during the laminating step. For example, tension forces of 7-9 kg/metre wide of product are appropriate but can be varied by the skilled operator as necessary.

The polymer can optionally be treated on extrusion and/or pre-treated with a corona treatment which ionizes the surface, so as to increase the surface energy of the film and hence raise its adhesion. Chemical treatments of the LLDPE surface are also possible to improve adhesion. The thin-film layer surface of the supporting film faces the extrusion, and can be corona treated prior to laminating, again to raise the surface energy to improve the adhesion. Alternatively it is possible to use pretreated films, where the pretreated film is either corona treated, or chemically treated using the likes of an acrylic binder, again to raise the surface energy and improve the adhesion. Alternatively it is possible to run the extrusion at a lower temperature to typical polyethylene temperatures which reduce metal crazing, where the metal surface fragments due to shrinkage during cooling. Metal crazing occurs quite commonly with excessive tensions, heat shock and or physical contact with rollers. The crazing needs to be limited so that gas barrier properties are not severely affected. Metal crazing can show up as faint fractures of the thin-film layer running in the machine direction. The lines are noted by placing the structure up to light and noting passage of light through the damaged thin-film layer.

The preferred structure coming out of the first laminator is from top to bottom: PET support film / metallised layer / extruded polyethylene and acid copolymer blend / LLDPE or LDPE blend or LDPE film.

The laminated structure continues in-line on to the second laminator in which another film of LDPE/LLDPE blend or LDPE alone, similar to that described above, is joined to the exposed PET film of the laminated structure using an extrusion of a modified ethylene acrylate resin. This resin has acrylate or methacrylate functional groups attached to low density polyethylene back bone, giving it greater affinity to the PET surface (the backing of the metallised film) and polyethylene films. The amount of extruded film is around 8 to 12 gsm, typically 10 gsm. Other settings are typical for polyethylene extrusions. The ratio of LDPE to LLDPE is typically 20% LDPE, as this gives a good combination of flexibility, strength, sealability, and processability. Other additives can be added to the LDPE/LLDPE blend including slip additives and anti-blocks. The polymer film can be pretreated with a corona or chemical treatment to improve surface energy to improve adhesion, as above.

The full structure is now from top to bottom: a LDPE/LLDPE blend or LDPE film / extruded modified ethylene acrylate resin / PET supporting film / metallised layer / extruded polyethylene and acid copolymer blend / LDPE or LLDPE blend or LDPE film.

It is thought that the strong affinity polymers that are typically considered for use to bond a polymer to the metallised layer are too strong for this purpose. The strong affinity combined with the damaging effect of hot polymer on the metal surface (heat shock) , means that on separation of the layers, the metallised layer/PET interface is the weakest point and comes away easily in preference to the polymer coming away from the thin-film layer. This leads to a significantly inferior bond so that the resulting product does not meet the needs of the food and beverage industry for which it is intended. By variation of the polymer blends used and polymer extrusion temperatures run, variation in affinity to the thin-film layer can be achieved. Polymers with varying degrees of affinity can be used at different temperatures and run speeds such that the successful interface separation can be controlled. Strict control of process variables and formulation control allows for a repeatable result in terms of interlayer bond control . The key is to create a balance in affinity of extruded polymer to thin-film layer such that it releases the thin- film layer with a degree of force that does not strip the thin-film layer from the surface that the thin-film layer is bound to. This balance can be achieved using a multitude of polymer and extrusion machine settings.

The present invention has been developed based upon the selection of the polymer additive to adhere to the metallised layer. The additives were selected after an extensive trialling program which included testing a range of polymers specifically designed to adhere to a thin-film layer formed of metal (which ended up stripping the metal as above) , and a range of other polymers, none of which were able to achieve the target bond strength of 400 gram per 25 mm strip, a standard industry test. It was found that the preferred extrusion polymer additives adhere to both surface layers, i.e. metallised layer and PET supporting film, during the bond testing process.

In another embodiment, the invention utilises the application of ethylene methacrylic acid-acrylate terpolymer resin or ethylene acrylate maleic anhydride (as above) towards the low density film side during the first extrusion process so that it is kept slightly away from the metallised film, and given a chance to cool slightly, hence reducing the stresses on the thin-film layer and supporting film as there is a lower temperature difference during extrusion.

Other advantages are provided by the application of the second extruded polymer towards the supporting film (PET) side so the bonding properties onto the PET side are maximised. A variation of the invention is to co-extrude low density polyethylene with modified methyl acrylate polymer using an A/B feedblock in the second laminator at a 60/40 ratio. This alternative better stabilises the polymer melt and directs the PET affinity resin directly onto the PET surface. The A/B feedblock allows two different polymers to be extruded side by side effectively forming a two layered film with different qualities on either side. The side facing the PET surface would contain the modified methyl acrylate polymer and would be the 40% portion of the feedblock. This is instead of running a monolayer of pure methyl acrylate polymer mixture. This has the advantages of running higher temperatures on the modified methyl acrylate polymer and increase bonding strength to PET as well as maintaining good film integrity and control .

Various alternatives and additions to the processes described above are also contemplated based upon the existing knowledge of the skilled worker.

EXAMPLE

METAL ADHESION TEST

Objective/Scope of Test This test method is designed to measure the adhesion or bond strength between the metallised layer and the polymer film, of metallised polymer film eg metallised

BOPP, metallised polyester etc. This test is referred to as the AM Surlyn Text .

Sampling/Test Specimens

Metallised polymer film sample. Preparation of test specimen is described in the procedure.

Equipment/Reagents

• A tensile strength tester eg Instron, Shimadzu

• Surlyn film (source of Surlyn is Surlyn 1702 ex Dupont)

• 25mm wide template

Procedure

• Heat-seal the metallised surface of the test sample of the Surlyn film. Surlyn test heat seal condition:

135°C 275kPa

1 second

• For the Surlyn test, cover both sides of the test sample with paper, for example in paper towel, before sealing.

Using the 25mm template cut a sample for the tensile tester at 90° to the heat seal. Cut to a length that allows 50mm on one side of the seal only so there is no ^λtail' when placed in the tensile tester.

Open the two layers of the test sample and place in the clamps of the tensile strength tester. Use a crosshead speed of 200mm/min. to pull the seal apart .

Interpretation/Recording of Result

Observe the chart recording and take the mean value of the trace in grams. This is the metal adhesion strength of the metallised polymer film, recorded with the unit g/25mm. Also record the amount of metal loss or transfer, by comparing the difference between the amount of metal on the two halves of the test sample .

Typical Values of Properties

+ Non-metallised side to non-metallised side

Results

The following table sets out the metal adhesion results for a number of materials tested

The above results show that additives 1, 2 and 3 provided insufficient metal adhesion even if there was not metal stripping (additives 2 and 3) . The additive 4 shows very good bonds with the metal surface without stripping the metal surface.

OXYGEN TRANSMISSION RATES

Objective/Scope of Test This test method is designed to simulate flexing which laminates encounter during usage so as to determine the extent of any resulting degradation of the oxygen barrier.

Samples and Testing Metallised polymer film samples were tested for oxygen transmission rate using MOCON Oxtran 1000 equipment before and after Gelbo flex testing for 100 cycles.

Results

The following tables sets out the oxygen transmission rates before and after Glebo flexing for a number of materials.

A reasonable increase in OTR after Glebo flexing s <300. The above tables shows that equivalent laminates have an unsatisfactory increase in OTR after Glebo flexing

(4,5). In contrast, laminates of the invention (1,3) show low rates of increase whilst a comparable laminate (2) not using the polymer additive (s) as required by the invention shows an unacceptable rate increase.

Claims

CLAIMS :

1. A method of preparing a multilayer metallised film comprising the steps of: a) providing a metallised film that includes a metallised layer on a supporting film, the metallised layer having gas and/or moisture barrier properties; and b) extruding a first polymer ("the first extruded polymer") between the metallised layer of the metallised film and a film of a second polymer ("the second polymer film") to form the multilayer metallised film, wherein the first extruded polymer includes one or more than one polymer additive that is selected to control the adhesion of the first extruded polymer to the metallised layer of the metallised film such that an adhesion force between the first extruded polymer and the metallised layer is lower than an adhesion force between the metallised layer and the supporting film.

2. The method defined in claim 1 wherein the first extruded polymer is selected so as not to damage the material of the metallised layer by chemical or physical mechanisms .

3. The method defined in claim 1 or claim 2 includes a further step (c) of extruding a third polymer ("the third extruded polymer") between a film of a fourth polymer ("the fourth polymer") and the supporting film of the multilayer metallised film produced in step (b) to laminate the fourth polymer film and the third extruded polymer onto the multilayer metallised film.

4. The method defined in any one of the preceding claims wherein the supporting film of the metallised film may comprise a polymer film such as bi-axially oriented polypropylene film (BOPP) , polyester film, and polyethylene film.

5. The method defined in claim 4 wherein the supporting film is polyethylene terephthalate.

6. The method defined in any one of the preceding claims wherein the metallised layer of the metallised film is a metal or a metal compound.

7. The method defined in claim 6 wherein the metallised layer may be one of the following metals: aluminium, titanium, indium or magnesium.

8. The method defined in any one of the preceding claims wherein the extruded polymer is polyethylene or a polymer blend including polyehythlene .

9. The method defined in any one of the preceding claims wherein the one or more than one polymer additive includes acid copolymers, most preferably ethylene- methacrylic acid-acrylate terpolymer or ethylene acrylate maleic anhydride.

10. The method defined in any one of the preceding claims wherein the second polymer film used in step (a) is a polyethylene film.

11. The method defined in claim 10 wherein the second polymer film is a blend of linear low density polyethylene/low density polyethylene (LLDPE/LDPE) , although low density polyethylene (LDPE) alone can be used.

12. The method defined in any one of the preceding claims wherein step (b) includes extruding the first extruded polymer between the metallised layer of the metallised film and the second polymer film in a first laminator so that the multilayer metallised film formed in the first laminator includes the following layers, from top to bottom: the supporting film - PET, the metallised layer, the first extruded polymer - a polyethylene blended with one or more polymer additives, the second polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE.

13. The method defined in any one of the preceding claims when dependent directly or indirectly on claim 3 wherein the fourth polymer film used in step (c) is a polyethylene film.

14. The method defined in any one of the preceding claims when dependent directly or indirectly on claim 3 wherein step (c) includes extruding a modified ethylene acrylate resin onto the surface of the supporting film and further laminating a polyethylene film on the modified ethylene acrylate resin in a second laminator so that the multilayer metallised film includes the following layers, from top to bottom: the fourth polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE, the third extruded polymer - a modified ethylene acrylate resin, the supporting film - PET, the metallised layer, the first extruded polymer - a polyethylene blended with one or more polymer additives, and the second extruded polymer film - a polyethylene such as a LLDPE/LDPE blend or LDPE.

15. A multilayer metallised film comprising from top to bottom: a supporting film, a metallised layer, a first extruded polymer - a polymer blend with one or more than one polymer additive, and a second polymer film - a polyethylene .

16. A multilayer metallised film comprising from top to bottom: a fourth polymer film - a polyethylene, a third extruded polymer - a modified ethylene acrylate resin, a supporting film - PET, a metallised layer, a first extruded polymer - a polyethylene with one or more polymer additives, and a second polymer film - a polyethylene.

17. The film defined in claim 15 or claim 16 wherein the polymer additive is an acid copolymer such as ethylene-methacrylic acid-acrylate terpolymer and/or ethylene acrylate maleic anhydride .

18. The film defined in any one of claims 15 to 17 wherein the polyethylene film is a blend of linear low density polyethylene/low density polyethylene (LLDPE/LDPE) , although low density polyethylene (LDPE) can be used. If a blend is used it is preferred that the blend contains about 20% LDPE.