GB2236278A - Microwave interactive barrier packaging material - Google Patents

Abstract

A microwave susceptor comprises a laminate of: i) a thermoplastic or regenerated cellulose film vacuum deposited with a thin layer of a microwave absorbing metal, laminated to ii) a thermoplastic or regenerated cellulose film coated with a microwave transparent, barrier, metal or non-metal oxide, characterised in that the said laminate has an oxygen permeability of less than 5cc/metre<2>/24 hours at 23 DEG C, 50% RH and/or a moisture vapour transmission rate (MVTR) of less than 5 grams/metre<2>/24 hours at 38 DEG C, 90% RH, which oxygen permeability and MVTR are significantly less than the sum of the respective individual barriers of the two films.

Description

Microwave interactive barrier packaging material This invention relates to flexible packaging laminates which have both good barrier to oxygen, other gases and water vapour and also the capability of interaction with microwave radiation. Such laminates are useful for packaging foodstuffs which are sensitive to oxygen, other gases or water vapour which are intended to be cooked and browned in microwave ovens, so that they can be stored under ambient conditions of temperature and humidity for extended periods, without significant deterioration.

It is known eg from UK patent 2046060, to produce a food preparation receptacle for surface browning of food cooked in a microwave oven, by coating a plastic film of good heat stability with a thin layer of metal capable of interacting with microwave energy, and laminating the metallised film to a stock material such as paper or board to prevent it shrinking or distorting when the receptacle heats up.

Such devices are commonly termed microwave susceptors and are widely used commercially for browning or crisping foodstuffs such as pizzas, meats and potato products.

Such laminates do not have good barrier to oxygen, other gases or water vapour (in the context of this specification, good barrier means an oxygen permeability of less than about 5 cc/metre2/24 hours, and preferably less than 1 cc/metre2/24 hours, at 23 C/50 % relative humidity (RH) and/or a moisture vapour transmission rate (MVTR) of less than about 5 grams/metre2/24 hours, and preferably less than 1 gram/metre2/24 hours at 38 e C/90 % RH).

In consequence, foodstuffs intended for microwaving/browning which are oxygen or moisture sensitive have to be stored under refrigeration or be overwrapped with a barrier packaging material which is removed before the foodstuff is microwaved.

In theory, one way to produce a material which combines the properties of a microwave susceptor and a barrier packaging material in one material is to laminate a film with microwave susceptor properties of the type known in the art, such as a metallised plastic film, to another film which is microwave transparent and has good heat stability and barrier properties, which is in turn then laminated to a stock material such as paper. However, existing known microwave transparent, heat stable, barrier films are specialist products and prohibitively expensive, so this approach has hitherto been considered uneconomic.

It is known (eg from European patent number 312333) that the effectiveness of microwave susceptors can be improved by producing a composite comprising two or more zones of material capable of absorbing microwave energy, each set of said zones being separated by a zone of material transparent to or transmissive of microwave energy. This specification also teaches that in addition to metals, certain metal oxides, such as aluminium oxide, iron oxide and tin oxide can be used as microwave absorbing zones, but that aluminium oxide, which is not electrically conductive, can only be used in conjunction with electrically conductive material. Although they have improved microwave susceptor properties, such composities do not have sufficiently high barrier to be suitable for shelf stable packaging.

It is known (eg from UK patent number 2210826) to produce a material having high barrier (suitable for shelf stable packaging) by laminating together two films each coated with a metal or non-metal oxide, such as silicon dioxide (SiO2) or aluminium oxide (A1203). However, such laminates are substantially transparent to microwave radiation and do not function as microwave susceptors.

We have now found that by laminating a metallised film with microwave susceptor properties to a film coated with a microwave transparent metal or nonmetal oxide, such as silicon dioxide (SiO2) or aluminium oxide (A1203), neither of which films individually have good barrier, it is possible to achieve a synergistic combination, such that the resultant laminate has a high barrier, significantly better than the sum of the barriers of the individual component films, combined with the microwave susceptor properties of the metallised film component.

In contrast to the laminates described in European patent number 312333, the aluminium oxide coating is not used in conjunction with an electrically conductive material.

Preferred materials and combinations are illustrated by way of example only in the accompanying drawing, where each figure is a schematic diagrammatic cross section of a film or laminate. Like reference letters indicate like parts.

Referring to the drawing, figure 1 shows the primary film, A, which is coated with metal B. A can be any thermally stable film which is capable of being metallised and giving good metal adhesion such as polyesters, polyamides, polysulphones, polyolefines, regenerated cellulosics or other relatively stable plastic films, which will not degrade during the operation of the microwave oven at the temperature selected for cooking the desired food. Polyester films are preferred with poly (ethylene glycol) terephthalate film being especially preferred.

Thickness of the film is not critical.

B can be any metal or metal alloy having suitable microwave susceptor and barrier properties, such as aluminium, iron, nickel, silver, stainless steel, zinc, tin, tungsten or titanium, with aluminium being preferred. The critical parameter is the thickness of the metal, which should be sufficient to cause the metal to heat up via interaction with microwave radiation to a temperature which will cause browning of the foodstuff, but not so thick that its interaction with microwave radiation causes it to overheat and crack or otherwise break down.

It should also be sufficiently thick to give the metallised film a permeability to oxygen of less than about 50 cc/metre2/24 hours at 23 C, 50 % RH and/or an MVTR of less than 20 grams/metre2/24 hours at 38 ' C/90 % RH. We prefer to use a poly (ethylene glycol) terephthalate (polyester) film metallised with aluminium to an optical density of 0.2 - 0.3.

To facilitate manufacture of a barrier package for the foodstuff, film A is preferably provided with a heat sealable surface, C, on the non-metallised side, although this is not essential. Commercially available heat-sealable polyester films are particularly suitable.

Figure 2 shows a secondary thermoplastic film, D, which is coated with a microwave transparent, barrier, metal or non-metal oxide E. D can be any thermally stable thermoplastic film which is capable of being coated with a microwave transparent, barrier, metal or non-metal oxide and giving good oxide adhesion, such as polyesters, polyamides, polysulphones, polyolefines or other relatively stable films, with polyester films being preferred and non-heat sealable poly (ethylene glycol) terephthalate film being especially preferred.

Thickness of the film is not critical.

E is any oxide which has good oxygen and/or moisture barrier. We prefer to use silicon dioxide (Sio2) or aluminium oxide (A1203). The oxide coating can be produced by any known technique, including evaporation/condensation of the oxide in vacuo, for example using an electron beam, sputtering in vacuo, reactive evaporation of a metal with oxygen in vacuo to produce the corresponding oxide, electrolytic oxidation (anodising) of a metallised film to produce the corresponding oxide coated film etc. Thickness of the oxide coating is not critical provided it is sufficient to give the film a permeability to oxygen of less than about 10 cc/metre2/24 hours at 23 " C, 50 % RH and/or an MVTR of less than 10 grams/ metre2/24 hours at 38 C/90 % RH.

Figure 3 shows an alternative composition for the secondary film wherein the film F is a regenerated cellulose film, which is preferably coated on one or both surfaces with a lacquer which prevents it distorting due to absorption of moisture from the environment. Suitable lacquers include polyvinylidene cloride and nitrocellulose. We prefer to use for film F commercially available regenerated cellulose films such as Cellophane MXXT/W or Cellophane MS from British Cellophane Ltd. Thickness of the film is not critical.

Film F is coated with a microwave transparent, barrier, metal or non-metal oxide E, as previously described.

Figure 4 shows a laminate of the primary metallised film (ABC) to the secondary oxide coated thermoplastic film (DE), which is preferably further laminated to a ply of a stock material G, such as paper, board, glassine or plastic film which serves to prevent the packagage shrinking or distorting when it heats up. We prefer to use a thin (40-100 micron) Kraft paper.

The adhesive, H, used to laminate the primary film to the secondary film, is not critical, provided it does not react with the metallised or oxide coating and has sufficient stability to withstand the temperature reached during use of the laminate as a microwave susceptor (typically about 190-230 " C). The adhesive does not need to have barrier properties.

We prefer to use proprietary temperature resistant curing adhesives of the type known in the art.

Figure 5 shows a laminate of the primary metallised film (ABC) to the secondary oxide coated regenerated cellulose film (FE). The adhesive, H, is as previously described. This combination does not require further lamination to a ply of stock material such as paper etc, as the regenerated cellulose film to prevent shrinkage or distortion.

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

Example 1 A reel of 15 micron co-extruded polyester film, having one heat sealable surface (commercially available as Melinex 850, from ICI Films) and having an oxygen permeability of 95 cc/metre2/24 hours at 23 ' C, 50 % RH and an MVTR of 34 grams/metre2/24 hours at 38 C, 90 % RH was metallised on the non-heat sealable surface with aluminium to an optical density of 0.24.

This metallised film had an oxygen permeability of 6.9 cc/metre2/24 hours at 23 C, 50 % RH and an MVTR of 6.4 grams/metre2/24 hours at 38 e C g C, 90 % RH.

A second reel of 12 micron polyester film (commercially available as Melinex 800 from ICI Films), having an oxygen permeability of 120 cc/metre2/24 hours at 23 " C, 50 % RH and an MVTR of 40 grams/metre2/24 hours at 38 C, 90 % RH was metallised on one side with aluminium to an optical density of 0.60. The aluminium layer was then converted to aluminium oxide by the technique of

anodising /described in our UK patent appllcatlon number 8914881.1. The resultant aluminium oxide coated polyester film had an oxygen permeability of 4.1 cc metre2/24 hours at 23 C, 50 % RH and an MVTR of 3.8 grams/metre2/24 hours at 38 ' C, 90 % RH.

The metallised surface of the 15 micron metallised coextruded polyester film was coated with a proprietary commercially available polyester polyurethane curing adhesive (2525/2526 ex Holdens Surface Coatings), dried to give a dry coat weight of 2.7 grams/metre2, and laminated to the oxide coated surface of the 12 micron oxide coated polyester film.

The resultant laminate had an oxygen permeability of 0.4 cc/metre2/24 hours at 23 o C, 50 % RH and an MVTR of 0.32 gram/metre2/24 hours at 38 " C, 90 % RH.

A control experiment using unmetallised Melinex 850 laminated under identical conditions to unmetallised Melinex 800 produced a laminate with an oxygen permeability of 50 cc/ metre2/24 hours at 23 C, 50 % RH and an MVTR of 18 grams/metre2/24 hours at 38 C, 90 % RH, in line with what would be predicted using current theory (Fick's law). This laminate did not attenuate microwave radiation.

The metal/oxide laminate described above was heat sealed to produce an open pouch, in which was placed uncooked processed meat product (sausage). This package was placed in a 600 watt domestic microwave oven.

Within a few seconds the laminate cracked, shrivelled and distorted.

The metal/oxide laminate described above was coated on the Melinex 800 surface with a water based adhesive (Swift 24095 ex Swift Adhesives) and wet laminated to a reel of 60 gsm bleached Kraft paper. Adhesive coat weight (dry) was 1.7 grams/metre2. The barrier of the resultant metallised polyester/oxide coated polyester/ paper laminate was not significantly different from that of its metallised polyester/oxide coated polyester precursor. When converted to a pouch for sausage and placed in a microwave oven, the sausage was cooked and browned with no significant cracking or distortion of the metal layer or the laminate.

Example 2 Example 1 was repeated using the same reel of 15 micron metallised co-extruded polyester film as the primary film for the laminate. The secondary film was a 12 micron polyester film (commercially available as Melinex 800 from ICI Films) coated with a layer of aluminium oxide by the technique of reactive

evaporation of aluminium with oxygenLdescribed in our UK patent application number 8928706.4. The resultant aluminium oxide coated polyester film had an oxygen permeability of 3.6 cc/metre2/24 hours at 23 " C, 50 % RH and an MVTR of 2.6 grams/metre2/24 hours at 38 C, 90 % RH.

The metallised Melinex 850 was laminated to the aluminium oxide coated Melinex 800, and further laminated to 60 gsm paper as described in Example 1.

The resultant laminate had an oxygen permeability of 0.31 cc/metre2/24 hours at 23 C, C, 50 % RH and an MVTR of 0.30 gram/metre2/24 hours at 38 " C, 90 % RH. When converted to a pouch for sausage and placed in a microwave oven, the sausage was cooked and browned with no significant cracking or distortion of the metal layer or the laminate.

Example 3 Example 1 was repeated using the same reel of 15 micron metallised coextruded polyester film as the primary film for the laminate. The secondary film was a 365 gauge regenerated cellulose film coated with polyvinylidene chloride, commercially available as Cello MXXT/W from British Cellophane Ltd, metallised on one side with aluminium to an optical density of 0.59, and then anodised as previously described.

The resultant aluminium oxide coated Cello MXXT/W film had an oxygen permeability of 3.9 cc/metre2/24 hours at 23 " C, 50 % RH and an MVTR of 5.0 grams/metre2/24 hours at 38 C, 90 % RH.

The metallised Melinex 850 was laminated to the aluminium oxide coated Cello MXXT/W as described in example 1. The resultant laminate had an oxygen permeability of 0.4 cc/ metre2/24 hours at 23 e C, 50 % RH and an MVTR of 0.5 gram/metre2/24 hours at 38 C, 90 % RH. When converted to a pouch for sausage and placed in a microwave oven, the sausage was cooked and browned with no significant cracking or distortion of the metal layer or the laminate.

Example 4 Example 2 was repeated using the same reel of 15 micron metallised co-extruded polyester film as the primary film for the laminate. The secondary film was a 365 gauge regenerated cellulose film coated with Cello MXXT/W from British Cellophane Ltd, coated with a layer of aluminium oxide by the technique of reactive evaporation of aluminium with oxygen as previously described. The resultant aluminium oxide coated Cello XXXT/W film had an oxygen permeability of 3.8 cc/metre2/24 hours at 23 ' C, 50 % RH and an MVTR of 4.8 grams/metre2/24 hours at 38 9 C, 90 % RH.

The metallised Melinex 850 was laminated to the aluminium oxide coated Cello MXXT/W as described in Example 2. The resultant laminate had an oxygen permeability of 0.36 cc/metre2/24 hours at 23 C, 50 RH and an MVTR of 0.55 gram/metre2/24 hours at 38 C, 90 % RH. When converted to a pouch for sausage and placed in a microwave oven, the sausage was cooked and browned with no significant cracking or distortion of the metal layer or the laminate.

Claims (8)

1) A laminate comprising: i) a thermoplastic or regenerated cellulose film vacuum deposited with a thin layer of a microwave absorbing metal, laminated to ii) a thermoplastic or regenerated cellulose film coated with a microwave transparent, barrier, metal or non-metal oxide, characterised in that the said laminate has an oxygen permeability of less than 5 cc/metre2/24 hours at 23 e C, 50 % RH and/or a moisture vapour transmission rate (MVTR) of less than 5 grams/metre2/24 hours at 38 " C, 90 t RH, which oxygen permeability and MVTR are significantly less than the sum of the respecive individual barriers of the two films.

2) A laminate according to claim 1, wherein film (i) is metallised polyester, polyamide, polysulphone, polyolefin or regenerated cellulose and film (ii) is oxide coated polyester, polyamide, polysulphone or polyolefin.

3) A laminate according to claim 1, wherein film (i) is metallised polyester, polyamide, polysulphone, polyolefin or regenerated cellulose and film (ii) is oxide coated regenerated cellulose.

4) A laminate according to any preceding claim wherein film (i) is poly (ethylene glycol) terephthalate metallised with aluminium to an optical density of from 0.2 to 0.3, and film (ii) is poly (ethylene glycol) terephthalate or regenerated cellulose coated with aluminium oxide.

5) A laminate according to any preceding claim wherein said laminate has an oxygen permeability of less than 1 cc/metre2/24 hours at 23 v C, 50 % RH and/or a moisture vapour transmission rate (MVTR) of less than 1 gram/metre2/24 hours.

6) A laminate according to any preceding claim wherein the regenerated cellulose film (ii) has a lacquer coating.

7) A laminate according to any preceding claim which is further laminated to a stock material (eg paper or board) and optionally includes an outer heat sealable surface on the non-metallised side of film (i).

8) A barrier container or barrier flexible packaging material for food stuffs, capable of browning the foodstuff when cooked in a microwave oven, produced from laminate described in any of the preceding claims.