GB2115770A

GB2115770A - Lined, gas-impermeable containers

Info

Publication number: GB2115770A
Application number: GB08205359A
Authority: GB
Inventors: Andrew Nicholas Ferrar; Arthur Neville Jones; Fuller Anthony Thomas Baden
Original assignee: Transparent Paper PLC; Keyes Fibre Corp
Current assignee: Transparent Paper PLC; Keyes Fibre Corp
Priority date: 1982-02-23
Filing date: 1982-02-23
Publication date: 1983-09-14
Also published as: GB2115770B

Abstract

A rigid container for packaging a product in a controlled atmosphere comprises a moulded rigid porous tray 7, such as pulp fibre, having a composite thermoplastic film B coherently bonded to the inner surface of the tray, the film being impermeable to gases. A lid F comprising a composite thermoplastic film is secured around the rim of the container to provide a gas-tight space in which the product can be packaged. The composite thermoplastic films preferably comprise three layers of which at least one 3, 4, 6 is an ionomeric polymeric material and a central layer 2, 5 is a gas- impermeable polymeric material such as polyvinylidene chloride, polyvinyl alcohol, or EVA copolymer. The lid or closing membrane preferably has a hydrophilic antifog surface adjacent the gas-tight space to facilitate inspection of a packaged product. <IMAGE>

Description

SPECIFICATION Rigid containers for product packaging This invention relates to rigid containers and is particularly concerned with containers for use in the gas packaging of products in which a significant free gaseous head space exists about and around the product. This form of packaging is termed "controlled atmosphere packaging" (CAP) and the technology of this form of packaging has demanded a rigid base container of an inherently gas-impermeable nature having a lid or closing membrane hermetically sealed thereto.

By virtue of the properties which the resulting package is required to have, the range of materials which can be utilized to form the package is limited. In practice, only thick laminates of unplasticized polyvinyl chloride and polyethylene have been used to form the base component of the package. The base components are prepared by thermoforming to yield deep traylike structures which are subsequently closed by heat-sealing a membrane around the peripheral lip of the base component to contain with the package a product and an appropriate gas mixture. The thick polyvinyl chloride (PVC) ply of the laminate is employed to provide the package with gas impermeability and to impart physical rigidity to the finished closed package.

Because of the specialized nature of the package material and the bulk of the container, the controlled atmosphere packaging process is limited to large scale factory operations employing extremely expensive and complex packaging equipment using large rolls of laminate material. Accordingly, the process can only be made use of by a few of the largest packers or processing companies.

According to one aspect of the present invention there is provided a rigid container for use in packaging a product by a controlled atmosphere packaging technology, wherein the container comprises a moulded rigid porous tray having a composite thermoplastic film coherently bonded to the inner surface of the tray, such film being impermeable to gases, and wherein a lid or closing membrane also comprising a composite thermoplastic film is provided for said container and is intended to be secured around the rim of the container to provide a gas-tight space in which the product may be packaged.

If the product to be packaged is, for example, meat or fish, it is highly desirable that a prospective purchaser should be able to inspect the contents, it is therefore desirable for the lid or closing membrane to have an antifog inner surface adjacent the gas-tight space, so that the lid does not become obscured by condensation of water from the product upon changes in temperature occurring.

The composite thermoplastic film may comprise any desired number of layers required to give the film the necessary thermoplasticity and gas-impermeability. Preferably, the thermo plastic film bonded to the container is a multilayer film comprising at least three layers, namely a first layer capable of bonding strongly to the surface of the porous container to give a bond at least equal to the inherent mechanical strength thereof, a central layer of gas-impermeable polymeric material, and a third layer of polymeric material capable of bonding to the thermoplastic material of the lid or closing membrane.

Conveniently, the composite thermoplastic film comprises an ionomeric polymeric material as said first and third layers, and the gas-impermeable layer may be of polyvinylidene chloride, polyvinyl alcohol or an ethylene/vinyl acetate copolymer which may be partly hydrolysed. The lid or closing membrane may be made of a similar composite material but with an outer layer of a polymeric material having a higher melting or softening point that the inner ionomeric layer.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings which illustrate diagrammatically and by way of example some embodiments thereof, and in which: Figure 1 a is a cross-sectional side view through a sealed container in accordance with the present invention, Figure 1 b is a plan view of the container shown in Figure 1 a, Figure 2 is an enlarged diagrammatic crosssection through a container, and Figure 3 is a plan view of a container with various cutouts.

Referring now to Figures 1 and 1 a, there is shown a container for use in controlled atmosphere packaging, the container comprising a base component C in the form of a tray with a peripheral flange D and a lid or closing membrane F heat sealed to the peripheral flange D. The base C and lid F define between them a space S in which the product to be packaged is contained in a controlled atmosphere.

Referring now to Figure 2, the structure of the tray C and the closing lid F are shown in greater detail. The tray C is formed as a substantially gas impermeable rigid container able to maintain an internal space S in isolation from the external atmosphere without any sensible interchange of gaseous substances. It will be seen that the tray C is composed of two materials a and p and these materials are of widely differing physical properties. The material a is a rigid porous material of relatively considerable thickness, strength and resistance to flexure or crushing but is inherently porous and gas-permeable. The layer p is itself formed of three separate layers 4, 5 and 6 and is a flexible polymeric laminate composite formed from plies of plastics material which have been bonded together to give a united structure.

The outer layer 6, outer in the sense that it lies remote from the interior space S consists of a film of an ionomeric polymer. The layer 5 consists of an oxygen- and other gas-impermeable substance applied to the film 6 and the layer 4 which forms the inner side of the package may also be a film of an ionomeric polymer, similar to the layer 6, but must be compatible with a layer 3 of the lid F.

The lid F is also formed of three separate layers 1, 2 and 3, and is also a flexible polymeric laminate composite formed from plies of plastics material which have been bonded together to give a united structure. The inner layer 3 is of an ionomeric polymer similar to or the same as that used for forming the layer 4, the layers 3 and 4 being compatible with one another to the extent that they can be united as by a heat-sealing, welding or other bonding operation. The middle layer 2 is, iike the layer 5, impermeable to oxygen and other gases and the outer layer 1 is a supportive layer which is of substantially higher melting point than either of layers 2 and 3 so that it can withstand a heat-sealing or welding operation whereby layers 3 and 4 are united.

In order to produce the container just described, a layer 7 of the material a is produced in the required tray form shown. The material is, as indicated above, porous and rigid and may be made of any desired fibrous material, such as paper, cardboard, fibre board or moulded pulp fibre, The material may be made from bonded wood or fibre chips or from other suitable gas pervious membranes. Alternatively the material may be foamed, open-cell or suitable filled fibrous plastic structures. It is necessary that the material a of the layer 7 should be capable of being cohesively bonded to and wetted by the polymer of the layer 6 when the polymer is applied at elevated temperature to the surface of the layer 7.

As indicated above, the layers 4, 6 and 3 are made of an ionomeric polymer, that is to say a polymer of that class of polymers in which ionised carboxyl groups create ionic cross-links in the molecular structure, which links are reversible 'broken at melt temperatures. The layers 4, 6 and 3 may all conveniently be made of the same material, such as that sold under the Registered Trade Marks "Surlyn". In order to ensure adequate bonding of the ionomeric polymer to the moulded porous base structure a, the layer 6 may be thicker than the other two layers. The layer 5 of oxygen- or other gas-impermeable material is preferably a film of polyvinylidene chloride or a copolymer thereof. Alternatively, the gas impermeable layer 5 may be a polyvinyl alcohol or a copolymer of ethylene and vinyl acetate which may have undergone a degree of hydrolysis.

As indicated above, the layer 3 of the lid or closing membrane F is of an ionomeric polymer material similar to the material from which the layer 4 is made, and the layer 3 must be heat sealable or weldable to the layer 4. It is a preferred feature of the layer 3 that the surface of this layer in contact with the gaseous atmosphere in the space S shall be hydrophilic so that, when the resulting package is in use, a continuous transparent film of water can form on the surface of the layer 3 so as to maintain the visibility of the contents of the package. The lid or closing membrane of the final package thus has an antifog inner surface adjacent the space containing the contents of the package. The layer 3 can be made hydrophilic in conventional manner as by incorporation of a surfactant into or onto the layer.

If desired, the layers 3 and 4 can be pigmented white or other desired colour to give an enhanced appeal to the subsequent package.

The middle layer 2 of the lid or closing membrane F is impermeable to oxygen or other gases and, like the layer 5, may be a vinylidene chloride polymer or copolymer or a vinyl alcohol polymer or a copolymer of ethylene and vinyl acetate which may be partly hydrolysed.

The outmost layer 1 of the lid or closing membrane F is one which has a substantially higher melting point than either of the layers 2 and 3 so that heat-sealing of the layers 3 and 4 can be effected by heat transmitted through the layers 1 and 2. The layer 1 is conveniently a film of a polyester or polyamide.

Figure 3 of the drawing shows an alternative structure for the tray 7. The structure is formed with cut-outs separated by bars (top half of Figure 3) or by cruciform members (bottom half of Figure 3). The cut outs are bridged by the composite material p and it will be appreciated that this arrangement allows the contents of the package to be inspected from underneath. In this case it may be desirable to make the inner surface of the layer 4 hydrophilic also.

The package of the present invention may be manufactured using conventional techniques. The composite material p and the material forming the lid or closing membrane F are multiply structures which may be made by adhesive combination of several plies of single layers, adhesive combination of composite plies or by a coextrusion process, including the extrusion of one layer onto an already formed layer.

The composite material p is then united with the porous base structure a by, for example, a conventional drawing-down or a vacuum-forming technique. In effecting this technique, a web of the material p is clamped over the space S so as to occupy a position similar to that occupied by the layer F in Figure 2. The layer p is then heated, as by radiant heat, and is then drawn down onto the layer 7 by applying a vacuum to the exterior of the layer, or by applying excess gas pressure on the layer 4 or by a combination of both such measures. The heat applied to the layer P softens the ionomeric layer 6 so that the softened polymer is drawn through and into the interstices of the porous substrate material .

The resulting tray if then filled with the contents to be packaged and sealed with a lid or closing membrane F after the space S has been filled with the controlled gas atmosphere appropriate to the food or other product being packaged.

It will be appreciated that the trays comprising the joined layers a and P may be manufactured separately and then fed to a packaging line to have the product inserted therein and for subsequent sealing. Alternatively, the manufacture of the trays may form part of the packaging line, each tray being fed, as it is produced, to receive the product to be packed.

The invention will now be further illustrated by the following Example.

Example A moulded pulp fibre tray sold under the Trade Mark "Shopak" by Keyes FibreCo., has an adhesively bonded laminate vacuum-formed onto its inner surface. The laminate corresponds to the layerp in Figure 2 and comprises a layer 6 of an ionomeric polymer sold under the Trade Mark "Surlyn" and of a thickness of 75 microns. The layer 4 is of the same material but is only 40 microns thick. The layer 5 between the layers 4 and 6 is a coating of polyvinylidene chloride applied to the layer 6 at a rate of 59 per m2 and adhesively bonded to the layer 4.

A food product to be packaged is then placed in the space S in the container and the lid or closing membrane F is applied on top of the package and heat-sealed thereto after the atmosphere in the space S has been controlled to the desired composition. The layer F comprises a layer 3 of the same material as the layers 4 and 6 but of 40 micron thickness. The layer 2 comprises a coating or polyvinylidene chloride applied at a rate of 39 per m2 to the layer 3 and adhesively bonded to the layer 1 which comprises a layer of polyester 12.5 micron thick.

The packaging of the food products is suitably conducted in a conventional packaging machine in which the atmosphere inside the space S is controlled, the choice of gas composition depending upon the particular food product being packaged. Mixtures of carbon dioxide and oxygen are generally used for packaging red meat and mixtures of nitrogen and carbon dioxide are used for packaging fish products.

Claims

1. A rigid container for use in packaging a product by a controlled atmosphere packaging technology, wherein the container comprises a moulded rigid porous tray having a composite thermoplastic film coherently bonded to the inner surface of the tray, such film being impermeable to gases, and wherein a lid or closing membrane also comprising a composite thermoplastic film is provided for said container, and is intended to be secured around the rim of the container to provide a gas-tight space in which the product may be packaged.

2. A container as claimed in Claim 1, wherein the composite thermoplastic film bonded to the container comprises at least three layers, namely a first layer capable of bonding strongly to the surface of the porous container to give a bond at least equal to the inherent mechanical strength thereof, a central layer of gas-impermeable polymeric material, and a third layer of polymeric material capable of bonding to the thermoplastic material of the lid or closing membrane.

3. A container as claimed in Claim 2, wherein the composite thermoplastic film comprises an ionomeric polymeric material as said first and third layers, and wherein the gas-impermeable layer is of polyvinylidene chloride, polyvinyl alcohol or an ethylene/vinyl acetate copolymer which may be partly hydrolysed.

4. A container as claimed in any one of Claims 1 to 3, wherein the lid or closing membrane comprises at least three layers, namely a first layer capable of bonding to the composite thermoplastic film bonded to the tray, a central layer of gas-impermeable polymeric material, and a third layer of polymeric material having a melting or softening point higher than that of said first layer.

5. A container as claimed in Claim 4, wherein the lid or closing membrane comprises an ionomeric polymeric material as said first layer and a polyester or polyamide as said third layer, and wherein the gas-impermeable layer is of polyvinylidene chloride, polyvinyl alcohol or an ethylene/vinyl acetate copolymer which may be partly hydrolysed.

6. A container as claimed in any one of Claims 1 to 5, wherein the lid or closing membrane has an antifog surface adjacent said gas-tight space.

7. A container as claimed in any one of Claims 1 to 6, wherein the porous tray is made of paper, cardboard, fibre board, pulp fibre, wood or fibre chips or foamed plastics material.

8. A container as claimed in Claim 7, wherein the tray is made of moulded pulp fibre.

9. A container as claimed in any one of Claims 1 to 8 wherein the composite thermoplastic film is contacted with and coherently bonded to the inner surface of said tray by a vacuum-forming technique.

10. A container as claimed in any one of Claims 1 to 9, wherein at least one of the layers of the composite thermoplastic film secured to the porous tray is pigmented.

1 A container as claimed in any one of Claims 1 to 10, wherein the porous tray is formed with cut outs to permit inspection of the contents, the cut outs being bridged by said thermoplastic film.

12. A rigid container for use in packaging a product by a controlled atmosphere packaging technology substantially as hereinbefore described with reference to the accompanying drawings.