GB2246794A - Process for preparing barrier packaging materials - Google Patents

Abstract

Metallic oxide or non-metallic oxide coatings are formed on plastics films (Y) by evaporating the metal or non-metal (E) in vacuo, reacting it with oxygen or an oxygen-containing gas or vapour, and then condensing the product in the continuously passing film (see Fig. 1). Vaporised metal from the heater F, supplied by wire feed E flows in a zone of evaporation towards drum B, the reactive gas being supplied by nozzle 10. The coated films so produced have barrier properties for use as packaging materials. Specific oxides deposited are SiO2 and Al2O3 and may have a weight of .03-0.5 gm/m<2>. The film may be formed of polyester such as polyethylene glycol terephthalate, polypropylene, polyethylene, polyamide such as nylon, regenerated cellulose, polystyrene, polyacrylonitrile, polyvinyl chloride, polyvinylidene chloride, polysulphone or certain papers or boards. Lamination of two oxide surface coated films with the two oxide coated surfaces adjacent can be carried out by any conventional lamination technique. A typical laminate is an oxide coated poly(ethylene glycol) terephthalate adhesive bound to an oxide coated polyethylene film. The coated material or laminates therefrom may be used as a container or component of a container for microwaveable foodstuffs. Such a component may be a sealing strip for cartons. <IMAGE>

Description

PRODUCTION OF OXIDE COATINGS ON PLASTICS FILMS This invention relates to the production of oxide coatings on plastics films and to the use of such films especially but not exclusively as packaging materials.

It is well known to package liquid or semi-solid foodstuffs sensitive to oxidative degradation in cartons fabricated from composites of thin cardboard, polyolefines and aluminium foil. Typical examples are the cartons sold under the trade names "Tetrapak" or "Combibloc" (trade mark). A typical construction of the composite used to produce such a carton is: a) polyethylene layer for surface protection or heat seal.

b) board to provide rigidity.

c) polyethylene bonding layer.

d) aluminium foil to provide barrier.

e) polyethylene heat sealable layer.

Although widely used for packaging of liquids like fruit juice or milk and semi solids like soups, meat products and desserts, such cartons have the disadvantage that they cannot be heated in a microwave oven because the aluminium foil web, which is essential for barrier to ingress of oxygen during storage, is not transparent to microwave radiation. Therefore, if it is required to microwave cook the product, it must be transferred to another microwave transparent container, which is inconvenient.

It is also well known to coat a plastic film, such as a film of poly (ethylene glycol) terephthalate, with a thin layer of metal such as aluminium, by vapour deposition in vacuo in order to achieve a good barrier of oxygen and water vapour, but such metallised films are also nontransparent to microwaves. US patent 3442686 describes coating of films of poly (ethylene glycol) terephthalate or polyolefines such as polyethylene or polypropylene with oxides of silicon or aluminium by vapour deposition in vacuo. Such oxides are transparent to microwaves, but the barrier of these materials, although enhanced significantly by the oxide coating, is still insufficient to allow their use in barrier cartons requiring long shelf life.

Existing techniques for the deposition of oxide coatings include electron beam evaporation and condensation in vacuo or sputtering in vacuo onto the film web. Both these techniques are well known and start with the oxide as feedstock.

We have now found that it is possible to form an oxide coating on the film by a process involving reactive evaporation, i.e. the element or metal rather than the oxide is used as the feedstock and this is reacted during evaporation with oxygen or a gas or vapour of a compound of oxygen (such as water vapour) such that the transparent oxide is deposited on the film.

According to the present invention, there is provided a process for the production of a coating of a metallic or non-metallic oxide on a plastic film which comprises evaporating a metal or non-metal in vacuo, reacting it with oxygen or an oxygen-containing gas or vapour, and condensing the product on the film.

It is possible to produce oxide-coated films by the process of the invention, which films have excellent barrier properties and are substantially transparent to microwave radiation. These coated films can also be used in the laminates claimed in our copending application no.

8724451 and in the film composites claimed in our copending application no. . The laminates comprise two layers of thermoplastic film coated with a metallic or non-metallic oxide, joined together (with the oxide-coated faces adjacent) by a layer of adhesive. Either or both of the coated films can be produced by the process of the present invention.

The plastic film onto which the oxide is condensed can be any suitable web substrate which is preferably substantially transparent to microwaves and which can be coated with oxide, and can include polyester, polypropylene, polyethylene, polyamide, regenerated cellulose, polystyrene, polyacrylonitrile, polyvinyl chloride, polyvinylidene chloride, polysulphone or certain papers or boards including any composites or coextrusions of the above materials, or variants coated with other plastics. In one embodiment of the invention, poly (ethylene glycol) terephthalate or nylon 6 films are preferred, since when used in the laminates or composites referred to above, they give exceptionally low oxygen permeability ( < 0.2 cc/metre2/24 hours), comparable to the barrier of aluminium foil.This product can then be laminated or extrusion coated with polyethylene to form the composite for the carton. In another embodiment of the invention, polyethylene is preferred, since when oxide coated and laminated or recoated as described above it gives an adequate oxygen permeability (typically 0.5 cc/metre2/24 hours) and is also of low cost and can be used to substitute both the aluminium and the polyethylene plies in the composite described above.

Oxide B can be any metal or non-metal oxide with good barrier to oxygen and adequate adhesion to the film A.

The oxides of silicon (six2) and aluminium (A1203) are preferred.

The thickness of each oxide coating should be in a range such that it provides good adhesion to the substrate, and has good mechanical strength and barrier. We prefer a coat weight of each oxide layer of 0.03 - 0.5 grams/metre2, with a coat weight of 0.15 - 0.3 grams/metre2 being especially preferred.

In order that the invention may be more fully understood, reference is made herein to the accompanying drawings in which: Figure 1 is a schematic illustration of a modified conventional film metallising apparatus; Figure 2 is a section through an embodiment of carton of the invention; and Figure 3 illustrates in section an embodiment of seal for a carton of the invention.

The reactive evaporation process of the invention can be carried out in a suitably modified conventional film metallising chamber. Figure 1 shows a diagrammatic representation of one embodiment of the apparatus, which comprises a vacuum chamber X for depositing a layer of metal such as aluminium on one surface of a continuous web of plastic film. Within the chamber is a supply reel A from which film Y passes over a cooled drum B to take up reel C.

An aperture and shutter assembly D controls the zone of evaporation of a metal supplied from a wire feed E to a ceramic heater block F maintained at a temperature above the evaporation temperature of the metal at the chamber pressure. The chamber is provided with viewing ports G and is connected at H to vacuum pumps. When used for conventional metallisation, vapourised metal from the heater flows in a zone of evaporation, indicated by broken lines Z, towards drum B and condenses on the film overlying the drum. In accordance with the present inventionjthere is provided a discharge nozzle 10 connected by a pipe ii to an inlet 12 for receiving a supply of a gas or vapour (not shown). The nozzle 10 is located within the zone Z and is directed in the direction of vapour flow from the heater to the drum.

Several such assemblies may be mounted across the chamber to facilitate coating of wide films. It is a crucial feature of the invention that the gas is introduced into the zone of vapourising metal. If the gas is merely introduced into the chamber outside this zone, this produces a pressure rise within the chamber causing unsatisfactory deposition and eventually the mean free path of the vapourised metal is so reduced that no deposition occurs. Unlike other known processes for reactive evaporation such as reactive sputtering, it is not necessary to use any electrical or plasma discharge to initiate the reaction.

Lamination of two oxide coated films with two oxide coated surfaces adjacent can be carried out by any conventional lamination technique. The nature of the adhesive used is not critical, and it does not need to have inherent barrier properties. Various curing and noncuring packaging grade adhesives, or hot melt resin adhesives or extrusion lamination with a polyolefine or similar thermoplastic resin can be used. We prefer to use a moisture resistant, two component polyurethane curing adhesive.

The improvement in oxygen and moisture barrier achievable by laminating two oxide coated films in accordance with the present invention is considerably higher than might be anticipated from known technology. Thus it is normally anticipated that when two conventional plastic films, each with gas permeability X, are laminated together, the gas permeability of the laminate is approximately 0.5 X.

In contrast, when two oxide coated films are laminated, in accordance with the present invention, improvements in barrier of typically 10 to 100 fold are achieved.

Conversion of the materials of the invention into laminates with board suitable for carton manufacture can be carried out using conventional lamination techniques such as adhesive lamination or extrusion lamination using polyethylene. Typical constructions can include those below. For the function of the layers a-e refer to the description for a typical-existing laminate given above.

1. a Polyethylene coating b Board c Polyethylene bonding layer { Poly (ethylene glycol) terephthalate film { Oxide coating d { Adhesive { Oxide coating { Poly (ethylene glycol) terephthalate film e Polyethylene heat sealable layer 2. a Polyethylene coating b Board c Polyethylene bonding layer Polyethylene film d { Oxide coating + { Adhesive e { Oxide coating Polyethylene film a a Polyethylene coating b Board c Adhesive ( Polyethylene film d { Oxide coating + { Adhesive e { Oxide coating Polyethylene film 4 a Polyethylene coating b Board c Adhesive { Poly (ethylene glycol) terephthalate film d { Oxide coating + { Adhesive e ( Oxide coating { Polyethylene film Other constructions are also possible.

Fabrication of cartons from these laminates can be carried out using conventional techniques. The laminate can be formed into a tube by sealing either surface e to itself (fin seal) or surface e to surface a (lap seal). In the latter case it is preferable to use the known technique of sealing a further strip of polyethylene or a polyethylene/ polyester/polyethylene composite along the longitudinal seal to avoid penetration of liquid from the carton into the board (see Figure 2). We have now found that a further improvement in the barrier of the carton is achieved if this strip is produced from a heat sealable barrier material, such as one of the materials of the invention, with a laminate of polyethylene film/oxide/ adhesive/oxide/polyethylene film being preferred.This is preferably applied in the form of an open tube, as shown in Figure 3 which will seal to itself and to the carton so that no exposed edge can come in contact with the carton contents. This technique can also be used to improve barrier of conventional (non-microwaveable) cartons by sealing the exposed edge with a tube of barrier laminate such as polyethylene/foil/polyethylene or polyethylene/ metallising/adhesive/metallising/polyethylene. Finally, the laminate tube can be transversely sealed and formed into a carton.

Whilst the materials of the invention are particularly useful for manufacture of microwave transparent cartons, they also have other uses. For example: a) They can be incorporated into flexible packaging laminates for the packaging of oxygen or moisture sensitive dry foods.

Typical constructions include: 1. { Poly (ethylene glycol) terephthalate film Oxide coating Adhesive Oxide coating Polyethylene film Many other constructions are possible according to the requirements of the end use.

b) They can be included into flexible packaging laminates for bag-in-box packaging of liquids sensitive to oxygen and/or moisture.

Typical constructions include: 1. ( Polyethylene film Oxide coating { Adhesive Oxide coating Poly (ethylene glycol) terephthalate film Adhesive Ethylene vinyl acetate film Many other constructions are possible according to the requirements of the erd use.

c) They can be used for lids or closures for packs of fresh foods packed in a controlled gas atmosphere to increase their shelf life (controlled or modified atmosphere packaging). For this application, in addition to the benefits of high barrier and transparency, certain products of the invention offer additional benefits. Many foodstuffs packed using controlled atmosphere packaging such as vegetables or bakery products, give off water vapour which will form a mist on the inner surface of a lid made from conventional plastic film and partially obscure the foodstuff, giving the pack an unattractive appearance. We have found that by using an oxide coated film with the oxide surface facing the foodstuff, moisture evolved from the foodstuff forms a transparent film rather than a mist.

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

EXAMPLE 1 A 2 metre wide web of 12 micron poly (ethylene glycol) terephthalate film (Melinex S ex ICI Films) was placed inside a metallising chamber similar to that shown diagrammatically in diagram 1 and threaded from the of find position A, over the drum B onto the wind up position C. 18 ceramic heaters F were arranged across the web and spaced 20 centimetres from drum B. A nozzle 10 was located between each heater and the drum, all the nozzles being connected to the inlet 12 to which was connected a controllable supply of oxygen gas. Each nozzle was positioned at the centre of the evaporation zone of its ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ associated heater and directed towards the drum.The chamber was evacuated to a pressure of 10-3 torr and aluminium wire fed to each heater at a rate of 30 grams/minute, the web being passed over the drum at a speed of 300 metres/minute.

Initially, and in the absence of oxygen, approximately 258 ot the aluminium evaporated was deposited on the web, giving a coating weight of 0.12 grams/metre6. Oxygen was then discharged through the nozzles and the flow rate increased until the coating on the film because just transparent, at which stage the flow rate was approximately 40 litres/minute. Chamber pressure was maintained at approximately 10-3 torr.

The aluminium oxide coated film was found to have a coat weight of 0.21 grams/metre2, an oxygen permeability of 5 cc/metre2/24 hours at 23 C/0% RH and an MVTR of 1.9 grams/ metre2/24 hours at 380C/908 RH. Surface energy of the oxide coated surface, determined by standard wetting tension pens, was greater than 72 dynes/cm, ie the film was wetted by water. Wetting tension of the uncoated poly (ethylene glycol) terephthalate film was 38-42 dynes/cm.

EXAMPLE 2 The aluminium oxide coated poly (ethylene glycol) terephalate film from Example 1 was coated on its oxide surface with a layer of a commercially available two component polyurethane adhesive and dried to give coat weight of 2.7 grams/metre2. The adhesive coated surface was then calendered to the oxide coated surface of a further ply of the same aluminium oxide coated poly (ethylene glycol) terephthalate film. After leaving for 1 week to allow the adhesive to cure, an oxygen permeability of 0.2 cc/metre2/24 hours at 230C, 0% RH and an MVTR of 0.4 gram/metre2/24 hours at 380C, 90% RH were measured on the laminate. The laminate could be formed into a container, filled with liquid, and heated effectively in a microwave oven without sparking.

A control sample in which the aluminium oxide coated film was laminated to uncoated poly (ethylene glycol) terephthalate film had an oxygen permeability of 3.8 cc/metre2/24 hours.

EXAMPLE 3 The experiment described in Example 2 was repeated using as substrate a 40 micron corona treated polyethylene film (Polyane CT ex Prosyn Polyane), which was coated on its treated side with 0.19 grams/metre2 of aluminium oxide by the process described in Example 1. Before lamination, this film had an oxygen permeability of 90 cc/metre2/24 hours at 230C, 0% RH and an MVTR of 1.5 grams/metre2/24 hours at 380C, 90% RH. After lamination, oxygen permeability was 0.5 cc/metre2/24 hours and MVTR 0.2 gram/metre2/24 hours.

Claims (13)

CLAIMS:

1. A process for the production of a coating of a metallic or non-metallic oxide on a plastic film which comprises evaporating a metal or non-metal in vacuo, reacting it with oxygen or an oxygen-containing gas or vapour, and condensing the product on the film.

2. An oxide-coated film produced by the process of claim 1.

3. An oxide-coated film according to claim 2, which film is substantially transparent to microwaves.

4. An oxide-coated film according to claim 3, with a barrier to oxygen of better than 10 times the barrier of the uncoated film.

5. An oxide-coated film according to any of claims 2 to 4, wherein the said film is poly (ethylene glycol) terephthalate.

6. An oxide-coated film according to any of claims 2 to 4, wherein the said film is a heat sealable polyolefine.

7. An oxide-coated film according to any of claims 2 to 6, wherein the oxide is aluminium oxide.

8. An oxide-coated film according to any of claims 2 to 6, wherein the oxide is silicon dioxide.

9. An oxide-coated film according to any of claims 2 to 8, wherein the oxide coat weight is 0.03 - 0.5 gram/metre2.

10. A plastics film laminate having low permeability to gases and moisture, which comprises two layers of thermoplastic film coated with a metallic or non-metallic oxide, joined together (with the oxide-coated faces adjacent) by a layer of adhesive, wherein at least one of the oxide-coated films is as claimed in any of claims 2 to 9.

11. The use as a packaging material or as a component of a packaging material of any of the materials as claimed in claims 2 to 10.

12. The use of a material claimed in any of claims 2 to 10, and having an oxygen permeability of less than 1 cc/metre2/24 hours, as a container or component of a container for microwaveable foodstuffs.

13. The use of a heat-sealable microwave transparent barrier laminate as claimed in claim 10 as a sealing strip for cartons, substantially as shown in Figure 3 of the accompanying drawings.