WO2005120811A1

WO2005120811A1 - Product packaging

Info

Publication number: WO2005120811A1
Application number: PCT/GB2005/050082
Authority: WO
Inventors: Michael Alan John Feast; Brenton John Zebedee; Howard White; Terence Michael Robins; Carl John Sofield; Denis Konstantinovich Koltsov
Original assignee: Stanelco Rf Technologies Limited
Priority date: 2004-06-11
Filing date: 2005-06-07
Publication date: 2005-12-22
Also published as: EP1753600A1; JP2008501589A

Abstract

A product such as meat or a ready meal (12) is packaged in a tray (14) of a polymeric material, covered with a cover sheet (34). The tray does not incorporate a sealing layer (surface bonding layer). For example the tray may be of crystalline polyethylene terephthalate (CPET) without a surface bonding layer. The polymer (such as APET) of the cover sheet is bonded to the CPET of the tray by dielectric welding between opposed electrodes (18, 24) of electrically conducting material, so as to form a continuous weld around the perimeter of the tray. This avoids the need for a bonding layer on the tray and the cover sheet, and enables a reliable hermetic seal to be obtained despite contamination of the surface of the tray.

Description

Product Packaging

This invention relates to a method and an apparatus for packaging a product, suitable for packaging food.

The packaging of foodstuffs in a plastic tray is known, and a suitable material for such a tray is the polyester, polyethylene terephthalate (PET) , which may be either crystalline (CPET) or amorphous (APET) . Other tray materials include polypropylene, or polyvinyl chloride (PVC) . The selection of material for the tray depends upon the nature of the contents, and so on whether the tray has to be heat resistant. For example polypropylene trays can be used for foodstuffs which are to be microwaved, whereas for foodstuffs to be heated in a conventional oven CPET trays are preferable. Such a tray may be covered and sealed by a transparent sheet of a plastic material that can be heat sealed to the rim of the tray. To enable heat sealing, the cover sheet would typically be of laminated form, incorporating a heat- sealable layer of for example polyethylene. It may also incorporate layers of other polymers, such as amorphous polyethylene terephthalate (APET) , polyvinylchloride (PVC) , polyamide (PA) , or polyvinylidene chloride (PVdC) . Similarly, to enable heat sealing, the tray is also typically of laminated form, with a heat-sealable layer on its upper surface; such a heat-sealable layer may be referred to as a surface bonding layer. The cost of the cover sheet for such packages would be less, and recycling easier, if the surface bonding layer could be omitted. These benefits would also arise from using a tray with no surface bonding layer.

According to the present invention there is provided a method for packaging a product, using a tray comprising a polymeric material, and a cover sheet, wherein neither the tray nor the cover sheet is provided with a surface bonding layer, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes, and the opposed electrodes forming a seal along a continuous line around the perimeter of the tray.

This provides the major advantage that a reliable hermetic seal can be formed despite the presence of contamination on the surfaces. This is not possible by heat sealing. Hence the method is particularly beneficial for packaging foodstuffs, more particularly foodstuffs with an oily or fatty surface, or an oily or fatty liquid or sauce. For example it is suitable for ready meals that include a sauce; or fresh cuts of meat (which may be bloody) , or slices of uncooked bacon (which are greasy) . Another advantage is that it is not necessary for there to be any surface bonding layer, or any adhesive.

Although the two items are referred to as a tray and a cover sheet, it should be understood that they may be formed of sheets of substantially the same thickness and flexibility. But preferably the tray is stiff whereas the cover sheet is thinner and so more flexible. In this case the tray should be sufficiently thick to be stiff, and is typically of thickness between 300 μm and 750 μm; in contrast the cover sheet is much thinner, typically less than 100 μm, more preferably between 10 μm and 50 μm thick. In the preferred embodiment the tray is comparatively rigid while the cover sheet is much more flexible . In some cases the trays may be formed from a continuous sheet of material, for example by thermoforming, immediately prior to being filled with the product and covered with the cover sheet; this might for example be the case where bacon is packed on a horizontal form fill seal machine which may be described as a thermoformer . In other cases the trays may be formed in a separate moulding operation, and supplied as separate trays to the packaging plant . Preferably the tray and the cover sheet are of chemically similar polymers. For example they may both be types of polyester (PET) . It will be understood that they may differ in their molecular weights, in the presence of additives such as plasticisers, or in their degree of crystallisation. The process of the invention is particularly suited to bonding polymers that melt at a temperature of above 150°C, for example at about 200°C. Each electrode may be coated with a layer of dielectric material. The electrically insulating material is preferably one that is not dielectrically heated, for example PFA (perfluoro alkoxyalkane) , or polytetra- fluoroethylene (PTFE) . As another example, the electrodes may be of aluminium, and be coated with a layer of alumina (by anodising) which is impregnated with PTFE. Another suitable material for the dielectric coatings is parylene, which can be deposited by a vapour- phase deposition and polymerisation process of para- xylylene (or its substituted derivatives) , and which has a low dielectric dissipation factor and a high dielectric strength. Alternatively or additionally a sheet of insulating material such as silicone rubber, providing good thermal and electrical insulation, may be interposed between the electrodes and the material to be bonded. The layer of electrically insulating material is preferably no more than 2 mm thick, and may be between 20 and 100 μm. A problem when dielectric welding a thin sheet to a thick tray is that the heating primarily occurs near the middle of the combined thickness of polymer, rather than at the interface. Coating the electrodes has a particular advantage in this situation, as the layers of dielectric material on the electrodes can redistribute the heating effect to nearer the interface. In this situation it may be desirable for the layer of dielectric material to be one that is dielectrically heated, at least to some extent. The layer of dielectric material is an electrical insulator, acting as a dielectric barrier, and also suppresses heat loss from the material being welded.

The radio frequency supply may in principle be at a frequency between 1 MHz and 200 MHz, usually between 10 MHz and 100 MHz, but stringent limits are imposed on any emitted radio waves. In practice therefore the choice of frequency may be more limited. For example the supply frequency may be 27.12 MHz, or 40.68 MHz.

Preferably the radio-frequency signal generator is a solid-state device, and the signals are supplied via a matching network. The matching network preferably is an active matching network, incorporating an inductor and at least one variable capacitor controlled by a servo motor; it monitors the radio frequency current and voltage, and automatically adjusts the value of the or each variable capacitor in accordance with variations in the load.

This may for example be such that the impedance presented to the generator remains at a constant value such as 50 Ω. Polyethylene terephthalate (polyester or PET) is a well-known material for such trays, and has a very much higher melting point than polyethylene. Polyethylene (PE) typically melts at about 120°C, whereas APET melts in a range of about 150 - 200°C and CPET at about 250°C, so that heat sealing of PET trays has been perceived as impractical (at least without very high pressures and temperatures, and long dwell times) . Bonding a PET film to a PET tray by heat sealing would have required heater elements at an even higher temperature, and so this has been perceived as impractical. In contrast, the present invention generates the requisite heat within the material, so avoiding the problems involved with high- temperature heater elements. The invention also provides equipment to seal a cover sheet to trays by this method; this may be achieved by modifying conventional equipment.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a welding apparatus for packaging a food product ;

Figure 2 shows a circuit diagram of the matching network of the apparatus of figure 1; and

Figure 3 shows a plan view of a packaging plant . Referring to figure 1, a welding apparatus 10 is shown, partly diagrammatically, for packaging a food product 12 (such as a ready meal) in a stiff, generally rectangular tray 14 of crystalline polyethylene terephthalate (CPET) with no surface bonding layer, that has rounded corners and a peripheral rim 16. The tray 14 is of material of thickness about 450 μm to ensure it is stiff and an adequate oxygen barrier; and the rim 16 is of width 4 mm. The apparatus 10 includes a lower aluminium die 18 which defines a generally rectangular aperture 20 into which the tray 14 locates, and the upper surface of the die 18 is coated with a 50 μm thick layer 22 of PTFE, so that when the tray 14 is located in the aperture 20 its rim 16 is supported by the upper surface of the layer 22 on the die 18. An upper aluminium die 24 has a 10 mm deep recess 25 of the same shape as the aperture 20, surrounded by a ridge 26 with a flat lower surface which is also coated with a 50 μm thick layer 28 of PTFE. The upper surface of the die 18 and the lower surface of the ridge 26 are each of width 3 mm, so the rim 16 projects just beyond their edge. The lower die 18 is earthed, while the upper die 24 is connected via a conductor 36 through an active matching network 30 to a solid-state RF generator 32. The matching network 30 incorporates variable capacitors controlled by servomotors which are operated so that the impedance presented to the generator 32 remains at a constant value such as 50 Ω. The earth to which the lower die 18 is connected is the earth of the matching network 30.

In use of the apparatus 10, a tray 14 containing a food product 12 is located into the aperture 20. A film 34 of amorphous polyethylene terephthalate (APET) is placed on top of the tray 14, and the upper die 24 is lowered so that the film 34 and the rim 16 are sandwiched between the PTFE layers 22 and 28 on the dies 18 and 24. The generator 32 is then activated (for example for 1.5 seconds) , such that the APET film 34 is welded to the rim 16 of the CPET tray 14. The upper die 24 is then lifted up, and the sealed tray 14 bonded to the film 34 is removed .

Although CPET has hitherto not been perceived as suitable for dielectric welding, this sealing process is surprisingly effective. Since no surface bonding layer (of say polyethylene) is required on the sealing film 34, the cost of this sealing film may be reduced. It should nevertheless be appreciated that the sealing film 34 may be a laminate, with APET as the bottom layer, and one or more other polymers such as ethylene/vinyl alcohol copolymer (EVOH) laminated to it to provide or enhance particular properties such as oxygen impermeability. For example the film 34 might comprise an upper layer 15 μm thick of APET; a 3 μm thick oxygen barrier layer of EVOH; and a lower layer 15 μm thick of APET; these layers may be bonded together by thin layers of adhesive. The proportion of EVOH is sufficiently small that waste film from the manufacturing process can be recycled to form new APET film. The sealing process is remarkably effective, and can provide a good seal despite the presence of contamination on the rim 16, such as traces of blood or fat from the product 12.

This welding process may be contrasted with that conventionally used, in which the tray itself may incorporate a 50 μm thick PE upper surface layer, and the film would typically incorporate a 50 μm thick PE lower layer, an EVOH oxygen barrier layer, and an APET upper surface layer. The two PE layers are surface bonding layers and enable the film to be heat-sealed to the tray, but they increase the costs of both the film and the tray, and they make recycling impractical.

Referring now to figure 2, this shows the matching network 30 in more detail. The signal from the generator 32 passes through a monitoring circuit 40 (shown diagrammatically) , a variable capacitor 41, an inductor 42, and a variable capacitor 43, and so to the conductor

36 leading to the upper tray 20. The monitoring circuit

40 monitors the radio frequency current and voltage, and adjusts the values of the variable capacitors 41 and 43 using servo-motors 44 and 45 so that the impedance presented to the generator 32 remains at a constant value such as 50 Ω. A capacitor 46 connects the junction between the capacitor 41 and the inductor 42 to earth potential, and by switching in other capacitors 47 and 48 the effective capacitance between this junction and earth can be adjusted. This has the effect of finely adjusting the radio frequency voltage applied between the live electrode (the upper die 24) and the opposed, earthed electrode (the lower die 18) , and the RF current supplied. It thus adjusts the efficiency with which power is supplied between the electrodes 18 and 24.

It will be appreciated that a welding apparatus may incorporate a different active matching network to that described above; for example the capacitors 47 and 48 may in some cases be omitted. In another modification, the upper die 24 may be connected to earth, and the lower die 18 be connected to the RF generator 32 via the conductor 36 and the network 30. In another modification, the dielectric coatings 22 and 28 may instead be of parylene, and may be of a different thickness, for example 70 μm. And in another alternative, in some situations it may be appropriate to omit at least one of the dielectric coatings 22 and 28.

Furthermore the welding apparatus may be used to seal trays and cover films of polymers other than those mentioned above, for example both the cover film and the tray might be of APET; the apparatus of the invention provides a reliable hermetic seal in this case too. Alternatively, both the cover film and the tray might be of CPET, which would enable the filled and sealed tray to be put into an oven to heat the contents without the need to remove the film. Alternatively the cover film may be of CPET and the tray of APET . Furthermore the cover films may be laminated to provide an oxygen barrier as mentioned above, or may be coated with such a barrier, or indeed no such barrier may be required.

Furthermore the tray and the cover film may be of other polymers, for example of PVC, or of starch-based thermoplastic polymeric materials. The cover film alternatively may be of a chemically-different polymer to that of the tray. For example a PET tray may be sealed with a PVC cover film.

The width of the opposed parts of the dies 18 and 24 between which the flange 16 is sandwiched determines the width of the weld, which as described above may be 3 mm. This may be even narrower, reliable welds having been made at widths down to about 1 mm. Consequently the rim 16 may be even narrower than mentioned above, for example only 2 mm wide .

The capacitance between the opposed parts of the dies 18 and 24 (referred to as the welding capacitance) is proportional to their area (in plan) and inversely proportional to their separation; it is also proportional to the dielectric constant of the medium occupying the space between them. In parallel with the welding capacitance is a parasitic capacitance, which is that between the upper, recessed part of the die 24 and the lower parts of the die 18 (for which the dielectric is mainly air, but also includes the food product 12) . The parasitic capacitance is associated with a much larger separation and lower dielectric constant, but since the effective surface area is much larger, the parasitic capacitance may be comparable to the welding capacitance. This issue becomes more significant if the dies 18 and 24 are designed to produce a narrow weld line, as the welding capacitance is correspondingly less. This leads a reduction in the power supplied to the weld region, and to an increase in energy wastage. The dies 18 and 24 should therefore be designed to ensure that the parasitic capacitance is significantly less than the welding capacitance, preferably less than a fifth of the welding capacitance. This can be achieved by ensuring that the width of the opposed parts of the dies 18 and 24 is less than that of the rim 16; and by arranging that the non- welding parts of the dies 18 and 24 are completely cut away, or are recessed well away from the welding plane.

The apparatus 10 of Figure 1 is shown as having a single aperture 20 so that a single tray 14 can be sealed in one operation. In a modification a lower die might provide a plurality of apertures so that a plurality of trays 14 can be supported, the upper die providing a corresponding plurality of projecting flat ridges so that the rims 16 of the trays 14 are sandwiched between the dies. The dies 18 and 24 are described as having 50 μm thick coatings 22 and 28 of PTFE on the opposed welding faces, but it will be appreciated that these coatings may be of a different thickness, though typically between 20 μm and 150 μm, and that the coatings may be of a different dielectric material . Furthermore an additional dielectric barrier, such as a layer of silicone rubber 0.5 mm thick, may be provided on one or more of the welding faces; preferably such a layer of silicone rubber is attached with an adhesive on top of the coating 22 on the upper die 24, so that it remains attached to the die 24 throughout a multiplicity of welding operations, but can be removed and replaced when it becomes worn or damaged . For example, referring now to figure 3, there is shown a plan view of a conventional tray sealing plant modified so that the sealing can be performed in accordance with the present invention. The plant 50 comprises a continuous belt made up of rectangular aluminium plates 52 each defining four apertures 54 to locate trays 14 (as shown in figure 1) , the plates 52 being linked together by chains 56 on each side of the belt. Trays are placed in each aperture 54, and are filled with the desired food product; the plates 52 carrying the filled trays move (in the direction of the arrow A) step-wise, passing under a covering module 58 in which the trays are covered by a cover film, and the cover film is sealed to the rim of each tray. The covering module 58 seals the cover film to the four trays in one plate 52, this sealing operation taking a few seconds, and then the next plate 52 is moved into position under the covering module 58.

Conventionally such a tray sealing plant would utilise heated plates and pressure to bring about the sealing, and would require use of trays and cover films incorporating appropriate sealing layers (surface bonding layers) , typically of polyethylene (PE) . In contrast the present invention enables trays to be used without any surface bonding layer, for example an APET tray can be sealed with an APET cover film, such as the APET/EVOH/APET cover film described above. Each plate 52 is supported by an insulating frame 60 of acetal (e.g. Delrin (TM) ) strips, those at the end being 25 mm thick, so it is electrically isolated. Each plate 52 is machined so as to define a 3 mm wide flat-topped rim 62 around each aperture 54 which projects 12 mm above the remainder of the plate 54, and this rim 62 is covered with a 50 μm thick layer of insulation such as parylene (as described above) . The covering module 58 is also electrically isolated from the remainder of the plant 50 by insulating acetal blocks. The covering module 58 is connected to the earth of the matching network 30. A copper spring strip (not shown) contacts the underside of the plate 52 carrying the trays undergoing sealing, the copper spring strip being connected via a conductor 36 and a matching network 30 to a signal generator 32 (as described in relation to figure 1) .

Thus as each plate 52 moves into position under the covering module 58, the covering module 58 presses the cover film on to each tray, and the rims of the trays and the cover film are sandwiched and compressed between the rims 62 and the corresponding ridges in the upper plate (not shown) which forms part of the covering module 58. The signal generator 32 is activated for 2 seconds so that an RF signal is applied between the earthed upper plate forming part of the covering module 58, and the plate 52 which is live. This seals the film to the rim of each tray. The total dwell time may be 5 seconds, so that the welded seal is held compressed for another 3 seconds after application of the RF energy. The pressure is then released and the next plate 52 in the belt moved into position. When packaging food products this plant 50 reduces the energy wastage associated with the use of conventional heated elements. In addition it enables a reliable hermetic seal to be formed despite the presence of water, fat or other contaminants on the rim of the tray or on the cover film. It does not require high pressures compressing the film on to the rim, and for example a pneumatic air supply at 80 psi (530 kPa) is ample; this may supply air to a 6 inch bore cylinder, providing a total load of about 1000 kg. The total sealing area for each plate 52 may for example be about 100 cm². The process of the invention has been found to provide better results in burst tests than conventional heat sealing, and the proportion of leaking seals is significantly less than with conventional heat sealing. If the strength of the seal is to be adjusted, for example to enable the film to be peeled off subsequently, this may be achieved by reducing the power supplied by the signal generator 32.

In another embodiment of the invention slices of bacon are packed into trays formed from a 450 μm thick sheet or web of PVC with no surface coating, the trays being formed by thermoforming within the same equipment . These are then sealed with a film comprising PVC laminated to a film of polyamide (which provides toughness) , this sealing process being carried out using RF welding between opposed electrodes as described above, so that the PVC layer of the film is bonded to the tray. The sealed trays can then be cut out from the remaining web. The use of RF welding in accordance with the present invention means that the trays can be formed of PVC without the laminated surface bonding layer of polyethylene as would be conventionally required. In a modification the film might comprise PVdC in place of PVC. In another alternative the polyamide film to provide toughness is replaced with PET. In another embodiment of the invention slices of bacon are packed into trays formed from a 450 μm thick sheet or web of APET with no surface coating, the trays being formed by thermoforming within the same equipment . These are then sealed with a film of APET incorporating an oxygen barrier (as described previously) , this sealing process being carried out using RF welding between opposed electrodes as described above.

Claims

1. A method for packaging a product, using a tray comprising a polymeric material, and a cover sheet, wherein neither the tray nor the cover sheet is provided with a surface bonding layer, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes, and the opposed electrodes forming a seal along a continuous line around the perimeter of the tray.

2. A method as claimed in claim 1 wherein one of the electrodes defines an aperture to locate the tray, and the electrode supports the tray along the said continuous line.

3. A method as claimed in claim 1 or claim 2 wherein the tray is stiff whereas the cover sheet is thin and flexible .

4. A method as claimed in claim 3 wherein the tray is of thickness between 300 μm and 750 μm, whereas the cover sheet is between 10 μm and 75 μm thick.

5. A method as claimed in any one of the preceding claims wherein the tray and the cover sheet are of chemically similar polymers.

6. A method as claimed in any one of the preceding wherein the tray and the cover sheet are of polymers that melt at a temperature of above 150°C.

7. A method as claimed in any one of the preceding claims wherein at least one of the opposed electrodes surfaces is coated with at least one layer of dielectric material .

8. A method as claimed in claim 7 wherein the dielectric material of the said coated layer is one that is not dielectrically heated.

9. A method as claimed in claim 7 wherein the dielectric material of the said coated layer is of parylene, perfluoro alkoxyalkane, polytetra- fluoroethylene, or alumina impregnated with polytetrafluoroethylene .

10. A method as claimed in any one of claims 7 to 9 wherein the dielectric material of the said coated layer is of thickness between 20 and 100 μm.

11. A method as claimed in any one of the preceding claims wherein the opposed electrodes are shaped such that the parasitic capacitance between them is significantly less than the welding capacitance.

12. A method as claimed in any one of the preceding claims wherein the bonding is carried out by dielectric welding using signals from a radio-frequency signal generator that is a solid-state device, and the signals are supplied via a matching network.

13. A method as claimed in any one of the preceding claims wherein the tray or the cover sheet or both comprise crystalline polyethylene terephthalate.

14. A method as claimed in any one of the preceding claims wherein the strength of the seal is adjusted by adjusting the power supplied to the opposed electrodes during the dielectric welding step.

15. Equipment for packaging food products in trays wherein each tray is supported in an aperture in a plate, wherein each plate is electrically insulated from the remainder of the equipment, the equipment also incorporating a radio-frequency signal generator, means to connect to earth and means to connect the plate to the signal generator.