GB2210899A

GB2210899A - High barrier metallised film

Info

Publication number: GB2210899A
Application number: GB8723913A
Authority: GB
Inventors: Kenneth Martin Revell
Original assignee: Bowater Packaging Ltd
Current assignee: SCA Packaging Britain Ltd
Priority date: 1987-10-12
Filing date: 1987-10-12
Publication date: 1989-06-21
Anticipated expiration: 2007-10-12
Also published as: GB2210899B; GB8723913D0

Abstract

Metallised plastics film composites having low permeability to gases, moisture and light, comprise a thermoplastic film e.g. polyester, polyamide e.g. nylon 6 or polyolefine, metallised on at least one face, overcoated on said metal face with a substantially continuous thin plastics coating, e.g. polyester, of thickness less than 10 microns and remetallised on the coated face. The composites are useful for packaging. Preferred composites comprise:- films of poly(ethylene glycol) terephthalate or nylon 6 and have an oxygen permeability of less than 0.2 cc/metre<2>/24 hours at 23 DEG C/0% RH films of heat sealable polyolefine and have an oxygen permeability of less than 5cc/metre<2>/24 hours at 23 DEG C/0% RH. The metal used for the metallising is preferably aluminium. Further coatings are thermoplastic lacquers or inks based on nitrocellulose, acrylic, vinyl or vinylidene chloride, hot melt coatings, extrusion coated thermoplastic resins and curing resin systems.

Description

High barrier metallised film This invention relates to metallised, coated and remetallised plastics films which are especially, but not exclusively, useful for packaging materials, particularly materials which are sensitive to oxygen and/or water vapour.

It is well known to package materials in plastics films or plastics film laminates. Where the materials are sensitive to oxygen and/or water vapour (eg, materials such as certain foodstuffs, beverages, chemicals, pharmaceuticals, seeds, electrical components etc) a plastics packaging material is chosen which will provide a barrier against ingress of oxygen and/or water vapour. It is also well known to package certain foodstuffs in an atmosphere of gas contained within a plastics material chosen for its low permeability to that gas (controlled atmosphere packaging).

One such plastics film which has the property of low permeability to oxygen, other gases and water vapour, is metallised poly (ethylene glycol) ;terephthalate (hereafter PET), a single web of which may typically provide an oxygen barrier of about 1 cc/metre2/24 hours (at 230C, 0% RH) and a moisture barrier of about 1 gram/metre2/24 hours (at 380C, 908 RH). By laminating metallised PET to a further web of metallised film, either another ply of metallised PET or another metallised thermoplastic polymer film, further reductions in permeability can be achieved. UK patent application No 2103999A describes such a packaging material comprising of two layers of metallised thermoplastic polymer film, at least one of which is metallised PET.Other plastics films which have the property of low permeability to moisture (but not oxygen) are metallised polyolefines, such as metallised oriented polypropylene (hereafter OPP) or metallised polyethylene (hereafter PE), single webs of which may typically provide a moisture barrier of about 1 gram/metre2/24 hours (at 380C, 90% RH), and laminates of such metallised films to unmetallised polyolefine films, suitable for packaging of moisture sensitive materials are described in UK patent specification No 1566925. By laminating such a metallised polyolefine film to a further web of a metallised polyolefine film, further reductions in permeability can be achieved and a material with both good moisture barrier and good oxygen barrier produced. European patent specification No 15At28 describes such laminates.

Whilst such laminates of two metallised films are highly advantageous in packaging moisture and/or oxygen sensitive materials, because of their excellent barrier properties, they also have some disadvantages, for example: a) PET film is expensive relative to many other thermoplastic films used in packaging, so for some end uses, use of laminates of two metallised films is precluded on cost grounds.

b) PET film has high stiffness compared with many other thermoplastic films used in packaging. The high stiffness of laminates of two metallised PET films can give rise to handling difficulties and slower line speeds on some high speed packaging machinery. For some high barrier packaging applications a flexible laminate is desirable, eg bag-in-box packaging of liquids, where the bag is required to collapse as the liquid is dispensed to avoid oxygen ingress.

c) High barrier laminates of metallised PET to metallisd PE can be expensive to produce, especially if the PE is thick, because film metallisation is a batch process and cost is proportional to film thickness.

d) Special grades of some films, eg polyolefine films are required for metallisation. Such grades are usually more expensive than grades commonly used, eg for lamination, and some performance properties may have to be sacrificed, eg it is normally desirable to omit migratory additives, such as certain types of slip additives, from polyolefine films designed for metallisation. In consequence laminates of such metallised films may have poorer handling properties.

We have now found that, in contrast to prior teachings, it is not necessary to combine together two metallised films as a laminate in order to achieve very high barrier properties.

Rather, we have found that by applying to the metallised surface of.a metallised film a thin layer of a plastic coating, and remetallising over the coating, very high barriers can still be achieved, generally at least a factor of ten better than the barrier of the single metallised film. The presence of the thin coating is crucial to the invention, and it is not necessary for said coating to have inherently good barrier properties. If the metallised film is merely remetallised without the intermediate stage of coating, the barrier improvement is limited to a factor of 2-3 at best. The actual barrier achieved will depend on the substrate, the metallising conditions, the metal thicknesses and the coating material type and thickness, but using optimum conditions, the following are achievable.

Substrate Oxygen permeability MVTR (cc/metre2/24 hours at (gram/metre2/24 hours 230C/ 0% RH) at 380C/ O% RH) PET (typically 12 micron) < 0.1 < 0.4 OPP (typically 20 micron) < 1 < 0.2 PE (typically 40 micron) < 0.5 < 0.2 Such double metallised films can have lower cost and weight and better flexibility than the laminates of two metallised films previously described and by achieving the high barrier properties in a single film which can then be combined with another web of heat sealable film (rather than laminating a metallised film to a metallised heat sealable film), the constraints on the composition and performance properties of the heat sealable film are avoided.

In accordance with the present invention, there is provided a flexible plastic film A, metallised on one or both faces, preferably with aluminium, coated on one or both metallised faces with a thin coating B, and remetallised over one or both coated surfaces. The invention also provides a process for packaging a material, in which process a double metallised film as defined above, or a laminate of such a film to other films, is used.

Film A can be any web substrate of any thickness which can be metallised, and can include polyester, polypropylene, polyethylene, polyamide, regenerated cellulose, polystyrene, polyacrylonitrile, poly vinyl chloride or certain papers, including any composites or coextrusions of the above materials, or variants coated with other plastics. Polyester, polyamide and polyolefine films are especially preferred.

Coating B can be any plastic resin coating, with a thickness of less than 10 microns, which is not required to have any inherent barrier properties, but which will adhere to and provide cover for a substantial percentage of the metallised surface of the substrate, and give good adhesion to both metallised layers.

Such coatings include water based, solvent-based or solventless thermoplastic lacquers or inks based on resins such as polyester, nitrocellulose, acrylic, vinyl or polyvinylidene chloride. hot melt coatings, extrusion coated thermoplastic resins and curing resin systems (cured by chemical cross-linking, ultraviolet or electron beam irradiation or any other system). We prefer to use so3vent-based polyester lacquers with a coating thickness of between 0.5 and 3 microns.

The thickness of each metal layer should be such that at its minimum thickness it provides a largely continuous metal layer and at its maximum thickness it still has adequate adhesion to the substrate or coating. Thickness of thin vacuum deposited metal layers is normally, and most conveniently, quoted in terms of their light transmission or optical density. An optical density of each metal layer in the range 0.5 - 4.0 is preferred, with the range 1.8 - 2.5 being especially preferred. Any metal which on vacuum deposition gives a barrier layer is satisfactory with aluminium being preferred.

In order that the invention may be more fully understood, the following examples are given by way of illustration only.

EXAMPLE 1 A 12 micron commercially available polyester film (Melinex S from ICI Films) was metallised with aluminium on the treated surface by vapourisation/condensation in vacuo, in accordance with known technique, so as to deposit an aluminium coating of optical density 2.2. After metallisation, oxygen permeability of this film was 0.75 cc/metre2/24 hours at 23C, 0% RH and MVTR 0.94 gram/metre2/24 hours at 380C/90% RH. The metallised surface of the film was coated with a layer of a commercially available polyester-based lacquer and dried to give a coat weight of 1.4 grams/metre2, and a coat thickness of 1.4 microns.

After lacquering, oxygen permeability was 0.62 cc/metre2/24 hours at 230C, 08 RH and MVTR 0.91 gram/metre2/24 hours at 380C/90% RH. The lacquered surface was remetallised as above with a further coating of optical density 1.8 giving an overall optical density of 4.0. Oxygen permeability was 0.02 cClmetre2/24 hours at 230C, 0% RH and MVTR 0.12 gram/metre2/24 hours at 380C/90% RH.

A control in which the same metallised polyester film of optical density 2.2 was remetallised to a final optical density of 4.0 without applying an intermediate lacquer coating gave an oxygen permeability of 0.3 cc/metre2/24 hours at 230C, 0% RH and an MVTR of 0.78 gram/metre2/24 hours at 380C/90% RH. Both products werelaminated to a standard commercially available high slip laminating grade of 50 micron low density polyethylene using a standard commercially available two part polyurethane adhesive.

After curing, the product of the invention had an interply bond strength of greater than 200 grams/25 mm, whereas the double metallised control sample had an interply bond strength of only 40 grams/25 mm with failure at the polyester metal interface.

EIL4MPLE 2 A 20 micron commercially available oriented polypropylene film (Shorko M from Shorko Films) was metallised with aluminium on the corona treated surface by the technique described above to an optical density of 2.3. After metallisation, oxygen permeability of this film was 85 cc/metre2/24 hours at 230C, 0% RH and MVTR 1.1 gram/metre2/24 hours at 380C/90% RH. The metallised surface was coated with a commercially available polyester-based lacquer and dried to give a coat weight of 1.6 grams/metre2. The lacquered surface was remetallised to give an overall optical density of 4.2.Oxygen permeability was 1.3 cc/metre224 hours at 230C, 0% RH and MVTR 0.16 gram/metre2/24 hours at 380C/90% RH.

A control in which the same metallised polypropylene film of optical density 2.3 was remetallised to a final optical density of .2 without applying an intermediate lacquer coating gave an oxygen permeability of 45 cc/metre2/24 hours at 230C, 0% RH and an MVTR of 0.82 gram/metre2/2Z. hours at 380C, 90% RH.

EXAMPLE 3 A 40 micron commercially available low density polyethylene film (Polyane CT from Prosyn Polyane) was metallised with aluminium on the corona treated surface by the technique described above to an optical density of 2.1. After metallisation, oxygen permeability of this film was 75 cc/metre2/24 hours at 230C, 0% RH and MVTR 1.8 gram/metre2/24 hours at 380C/90% RH. The metallised surface was coated with a commercially available polyester-based lacquer and dried to give a coat weight of 1.6 grams/metre2. The lacquered surface was remetallised to give an optical density of 4.0. Oxygen permeability was 0.6 cc/metre2/ 24 hours at 23 C/08 RH, and MVTR 0.14 gram/metre2/24 hours at 380C/90% RH.

The invention includes: 1. A metallised plastics film composite having low permeability to gases, moisture and light, which comprises a thermoplastic film metallised on at least one face, overcoated on said metal face with a substantially continuous thin plastic coating of thickness less than 10 microns and remetallised on the coated face.

2. A metallised plastics film composite according to paragraph 1 wherein the said film is poly (ethylene glycol) terephthalate, the composite having an oxygen permeability of less than 0.2 cc/metre2/24 hours at 23 C/0% RH.

3. A metallised plastics film composite according to paragraph 1 wherein the said film is nylon 6, the composite having an oxygen permeability of less than 0.2 cc/metre2/24 hours at 23 C/0% RH.

t. A metallised plastics film composite according to paragraph 1 wherein the said film is a heat sealable polyolefine, the composite having an oxygen permeability of less than 5 cc/metre2/24 hours at 230C/0% RH and an MVTR of less than 0.5 gram/metre2/24 hours at 380C/90% RH.

5. A metallised plastics film composite according to any of the preceding paragraphs wherein the said metal is aluminium.

6. A metallised plastics film composite according to any of the preceding paragraphs wherein the said coating is polyester.

7. The use as a packaging material or as a component of a packaging material, of a metallised plastics film composite, as defined in any preceding paragraph.

Claims

CLAIMS:

1. A metallised plastics film composite having low permeability to gases, moisture and light, which comprises a thermoplastic film metallised on at least one face, overcoated on said metal face with a substantially continuous thin plastic coating of thickness less than 10 microns and remetallised on the coated face.

2. A composite according to claim 1, wherein the said film is poly (ethylene glycol) terephthalate, the composite having an oxygen permeability of less than 0.2 cc/metre2/24 hours at 230C/O RH.

3. A composite according to claim 1, wherein the said film is nylon 6, the composite having an oxygen permeability of less than 0.2 cc/metre2/24 hours at 230C/O RH.

4. A composite according to claim 1, wherein the said film is a heat sealable polyolefine, the composite having an oxygen permeability of less than 5 cc/metre2/24 hours at 23 C/0% RH and an MVTR of less than 0.5 gram/metre2/24 hours at 38 C/90% RH.

5. A composite according to any of claims 1 to 4, wherein the said metal is aluminium.

6. A composite according to any of claims 1 to 5, wherein the said coating is polyester.

7. The use as a packaging material or as a component of a packaging material, of a metallised plastics film composite, as claimed in any of claims 1 to 6.