MICROWAVE HEAT RESPONSIVE CONTAINER

Abstract: A container for food to be cooked by microwave having means responsive to heat developed on exposure of the container to microwaves to cause the container to open when food is present in it and has been heated ready for consumption.

Description: MICROWAVE HEAT RESPONSIVE CONTAINER The present invention relates to food packaging, in particular for food that is to be heated in a microwave oven. It is frequently necessary that some means be provided for indicating when food has been properly heated, and this is particularly true when the cook cannot be relied on to use his own judgement. So-called fast foods, such as those to be heated in a microwave oven, are often heated from frozen without great care, and insufficient heating is quite common. Sophisticated controls or indicators such as temperature probes are unlikely to be used for fast, cheap, meals. As a result the cook is likely to rely on given heating times which might not take into account the power output of the oven, the number of items and their position in the oven, nor their original temperature. We have now devised a simple, inexpensive, way of packaging food and drink etc that can give good indication of temperature, and if necessary amount of heat. Thus, the present invention provides an assembly comprising a container and (optionally non-integral with or otherwise separate therefrom) a heat-recoverable article that can be dimensionally-recovered at a desired temperature (and preferably after the container has been closed, to the environment or otherwise) to cause an externally-visible change in the container and optionally to allow the container if closed to be opened. Reference to the recovery allowing the container to be opened excludes merely a situation where recovery of a dimensionally-recoverable article causes an environmental seal to be made, as is the case with conventional heat-shrink tubing or sleeves for sealing cable splices etc, see for example GB 1155470. In the present case, by contrast, the recovery opens the container or has substantially no effect on the sealing provided by the container. The container is of particular use for foodstuffs, by which term we include food, drink and medicines etc. The container need not be closed to the environment, although we prefer that it is. It can then constitute the outer packaging for storage and sale, thus saving on further materials. It may, however, merely comprise some means of mechanically retaining the foodstuff, or of supporting it. In this latter case "opening" of the container is to be construed as merely allowing removal of the foodstuff . In many cases the container will comprise a box, of square or rectangular or other shape in plan view, with a lid. The lid may be substantially rigid and attached to the rim of a base or other part at one edge by a hinge line about which it can pivot. The remaining edge(s) of the lid may be temporarily attached to the base by some means such as a hot-melt adhesive of low softening point. The lid may cover the whole of a food receiving space defined by a base of the box and may be opened or closed a whole by being caused to pivot about the hinge line on heat-induced contraction of the dimensionally-recoverable material. In an alternative structure the base may be formed with a multiplicity of individual food receiving spaces separated by internal partitions, as where different foods are to be kept separate during cooking or to be cooked under different conditions, e.g with and without exposure to air. In that case the heat receiving means may be arranged to open part of each space, or only some of the spaces. Alternatively, the lid may be of flexible material such that the container can be opened by convolution or other conformational change of the lid. The visible change is a change in the container and (in general) not (solely) that of a heat-recoverable article separate from the container. Thus, a separate article may dimensionally recover and cause a visible change to the container, such as opening of its lid; or a part of the container, such as the lid, may itself recover for example by convoluting and opening the container. The change in the container may be a conformational change, such as a lid becoming convoluted (which term includes for example coiling-up and becoming corrugated), and also changes in arrangements of parts that in themselves do not change shape, such as a substantially rigid lid pivoting open. These various types of change are encompassed by the generic phrase "configurational change" . In general a dimensionally-recoverable article is an article that undergoes a significant dimensional change when subjected to some treatment, such as heating. Such articles may be produced by heating an article, deforming it while hot, and cooling it whilst deformed. It then retains its deformed shape until heated again, when it returns to its original shape. The article may have been cross-linked at some stage, generally before the initial deformation. The deformation may occur simultaneously with formation of the article, or an article may be made from deformed material, in which case the article on heating will adopt a new shape. The deformation may be by stretching, in which case the article will be heat-shrinkable. The articles are commonly made from compound based on semi-crystalline polymers such as polyethylene. Polypropylene and polyvinylchloride may also be used. Various additives such as carbon black, and antioxidants etc may be added. Heat-shrinkable articles are well known in other arts, such as environmental sealing of cables, and no more need be said about the general principles of heat-shrink technology. For the present invention we prefer that the heat-recoverable article is in sheet form and is substantially uniaxially shrinkable in the plane of the sheet. When heated they can shrink in a linear dimension ratio of 2:1 to 3:1 or less usually in a ratio of up to 10:1. Such a sheet of heat-shrinkable material may extend across a hinge line of the container and be fixed to surfaces of the container, such as a top of a lid and a side panel of a base, joined by the hinge line. The hinge line may comprise a mechanical hinge with pivot pin etc, but we prefer that it comprise a fold or other live hinge, optionally with a line of weakness produced for example by a so-called kiss-cut or by a pre-creased hinge. This ensures that the hinging occurs at a precise location. The heat-shrinkable sheet preferably is substantially unidirectionally shrinkable, the shrinkage direction being substantially perpendicular to the hinge line. Lack of shrinkage parallel to the hinge line will in general avoid undesirable distortion of the container. Tension in the heat-shrinkable sheet that occurs above the shrink temperature as the sheet attempts to shrink, will cause the lid to pivot about the hinge line and, provided that the heat shrinkable sheet is applied to the outer face of the hinge, therefore to open. This may be aided by shaping of the base and lid at the hinge line. For example the base and lid may each be rounded or bevelled, or may have a recess between them. In this way a portion of the heat shrinkable sheet can lie oblique to the surfaces of the lid and base to which it is attached. In some circumstances this might enhance opening of the lid by ensuring that tension in the shrinkable sheet was not solely parallel to the surfaces of the lid and base. The recoverable article may be bonded, welded or otherwise fixed to the container. We prefer that it be bonded, and suitable adhesives include curable adhesives, or latex-based adhesives such as that known by the Trade Mark, Copydex. Hot-melt adhesives are not at present preferred due to the heating required for installation. They may, however, be used if they have a softening point greater than the recovery temperature and if the recoverable article is held-out during bonding or if application time is sufficiently short. Other arrangements may be used by means of which a recoverable article causes one surface of the container to move relative to another such as opening of the container if closed. For example, resilient means may be provided for opening the container, which resilient means is activated by recovery of the recoverable article. Such resilient means may comprise some spring separate from the container proper, or it may comprise resilience of the walls of the container. The container may have some form of latch or other snap-shut closure, which on release allows the lid to spring open. Recovery of the recoverable article may force the latch open. The recoverable article could comprise an elongate member such as a rod that expands in diameter (optionally with longitudinal shrinkage) on heating. Such a rod may be positioned between the lid and the base and therefore force them apart, overcoming the latch, when the desired temperature is reached. Additionally or alternatively, recovery of the recoverable article may overcome some sealing means such as an adhesive or sealant. The adhesive may comprise a hot-melt adhesive that softens, preferably at or below the recovery temperature. In addition or instead of the configurational changes described above, recovery of the recoverable article may cause concealment (either totally or partially) or exposure of a surface of the container. The effect of this may be to change a message on a visible surface of the container. For example a message such as "Ready" or "Eat Now" may initially be obscured from view, for example by an opaque covering that is removed e.g by sliding on recovery of the recoverable article. In another possibility, an initial message may read "Not Ready", the word "Not" becoming obscured when the desired temperature is reached. This may be achieved by a closure member that slides over the remainder of a lid of the container. In the same way a sliding closure member controlled by a heat shrinkable member can give access to a single food compartment of a multi-compartment food container. Any of various mechanisms may be provided by means of which recovery occurs at the correct moment. In general it is desirable that recovery occur after a time that reflects a certain temperature and time of heating of the foodstuff within the container. We prefer that the recoverable article be heated by heat from the foodstuff which reaches the recoverable article by thermal conduction through a wall or walls of the container, and more preferably substantially solely by that means. Thus, the recoverable article is preferably substantially unresponsive to microwaves (at least to those of the wavelength and intensity of a domestic microwave oven, generally about 2450 MHz and 400-700 watts), or is shielded from them. Then, its only source of heat will be by conduction of heat generated in the foodstuff, and possibly in the container or optionally in another heat generating member or material within the container positioned so that its heat passes to the recoverable article. The recovery temperature may be chosen to be equal to or greater than or less than the desired final temperature of the foodstuff, and the thermal path chosen to achieve a suitable thermal gradient. In this way recovery can be made to occur at just that moment when the foodstuff has been at its desired temperature for the desired length of time. It will be seen that this relationship between time of recovery and correct heating time will be substantially independent of initial temperature, microwave power and number of containers in the oven. That may not be true for very short heating times and very high thermal gradients, but in practice we have found satisfactory results . The recovery temperature and thermal path can be easily determined empirically for various different foodstuffs. Recovery of shape of the shape recoverable article may be accelerated or retarded by suitable means. ' Where recovery of shape is to be retarded, there may be provided an insulating layer or layers between the recoverable article and the container. For example, an extra layer of the container e.g an extra cardboard layer could be incorporated, or an additional layer of a polymer as such or in an expanded state could be incorporated. Conversely the rate of conduction from the foodstuff to the recoverable article may be increased by provision of a thermal conductor on or within the container adjacent to the recoverable article, for example, by fabricating a container at least in part of conductive material or by employing a conductive or heat generating member within it. Thus, there may be provided a heat generating member that absorbs microwaves and in thermal contact with the heat recoverable article via an intervening way of the container. Such thermal contact may be additional to or instead of a thermal connection between the foodstuff and the recoverable article. Although assessment of degree of heating of the foodstuff is more indirect when using this further member, greater flexibility in design is possible. Better thermal contact can be made since the further member may comprise a sheet attached to an inside of a lid or base face-to-face with an external recoverable article. Also, the response of the further member to microwaves, its heat capacity and its conductivity can be varied at will. The invention is further illustrated by the accompanying drawings, in which: Figure 1A shows a container in a nearly closed state and Figure IB shows it in an open state; Figures 2A is a cross-section through a lid and a base of another container, and Figure 2B is a diagramatic side view of the container in its open state; Figures 3A and 3B show a design where a lid becomes convoluted when the container is heated; and Figures 4A and 4B show a design where a surface becomes exposed or concealed by relative sliding movement. Figure 1A shows a container 1 in the form of a paperboard box comprising a base 2 having a bottom panel and upstanding wall panels defining a food receiving space and a and a lid 3 which is complementary to and fits over the base 2 to cover the food receiving space. The base 2 and the lid 3 may be formed integrally from e.g a single piece of paperboard about 100-500 micrometres thick as is well known in the box- or carton-making art, and its base dimension may be about 10cm x 15 or 20cm as is typical for a single portion food container. A heat-shrinkable sheet 4 of typical thickness 100-400 micrometres is fixed by adhesions to surfaces of the base 2 and lid 3, across a hinge line 5 at which the lid 3 hinges towards or away from the base 2. In this instance the hinge line 5 arises at an intermediate position, in this instance at approximately mid-height of the back or hinge containing wall of the box. Other edges 6 of the base and lid may be temporarily sealed together so that the container is closed to the environment. Thus, the contents of the container can remain sterile. When foodstuff within the container has reached a certain temperature due for example to absorption of microwaves, heat in the interior space of the container will be conducted through the material of the container to the sheet 4 which will shrink causing the lid to open. This will indicate to the cook that sufficient heating has occurred. The sheet 4 or other recoverable article can be provided during manufacture of the container, during packaging of the foodstuff, or just prior to heating. The sheet 4 may be applied in such a way that in the early stages of shrinkage an adhesive attaching it to the base 2 and lid 3 is in sheer. Later, when the lid is for example wide open the angle between the relevant surfaces of base and lid may be such that the adhesive is in peel. In this way, the shrinkage can exert an opening force initially, but adhesive failure will later occur to prevent excessive opening which might cause the container to take up too much room in the oven. It might, however, be preferable if the sheet does not fall free since that may allow the lid to fall closed. In a preferred embodiment therefore the sheet remains bonded at its opposing edges, but between those edges it is or becomes free to separate from lid and base. In this way thermal contact to the container is lost over a large part of the sheet and shrinkage stops (see Figure IB). In any case it will be appreciated that a visual change such as lid opening is unlikely to go unnoticed, and is preferable to a mere colour change. A wide variety of materials can be used, and for many applications we prefer that the container comprise cardboard (though it could also be of moulded plastics), and the shrinkable sheet comprise polyethylene. Preferably the materials have no smell and do not taint the foodstuff. The sequence of events that occurs on heating will generally be as follows: foodstuff, container or other conductive member absorbs microwaves and gets hot, adhesive joining edges 6 fails by melting or reduction in tack etc, shrinkable sheet 4 receives heat by conduction and begins to shrink, after a time lapse the lid 3 opens perhaps as far as vertically, a bond holding the shrinkable sheet to base and/or (preferably or) lid fails by peel or otherwise, foodstuff is ready to be eaten or drunk. The heat activation temperature of the sheet 4 may be 80-175°C and the adhesive joining edges 6 may be a hot melt adhesive that softens or becomes molten more than 10°C and typically about 20°C below the activation temperature of the sheet 4. Even after it has melted, the adhesive at edges 6 will tend to maintain the container closed until the force developed by sheet 4 has become sufficient to overcome the resistance of the adhesive and pull the edges apart. As mentioned above, various aspects of the design can determine when shrinkage starts and any time lapse before a chosen visible change has occurred. The following may be considered: for the container, its thickness, response to microwaves, its heat capacity, any coating, its colour, and its design; for the recoverable article, its shrinkage temperature and force and recovery ratio, its response to microwaves and its heat capacity, its colour, its surface chemistry which will affect adhesion etc and the thermal route between it and the foodstuff or other absorber of microwaves; for any such other absorber, its response to microwaves, its area and overlap with the recoverable article, and its surface characteristics. Figure 2A is a cross-section through a hinge-containing edge of a top edge hinged box of paperboard or other suitable material having a base 2 and lid 3 which meet at a hinge line 5. Here the lid, when closed as shown, is substantially perpendicular to the base. An adhesive 7 which does not melt under the expected service conditions of the box (or which melts or softens relatively slowly at the temperature reached by the box when the food is ready to eat) can be seen holding a heat-shrinkable sheet 4 to the base and to the lid. Various adhesives that cure other than by the action of heat are preferred for the adhesive 7. For example, it could be the adhesive sold under the tradename "COPYDEX", a latex curable adhesive, a UV curable adhesive, an acrylic resin, a cold curing epoxy resin, PVA or a solvent based rubber. The sheet 4 is preferably held only at opposing edge portions 4A and 4B thereof, in order that some slippage can occur between it and the container on shrinkage and consequent opening of the lid. The remaining area of the sheet 4 may be at least partially held to the container by a weaker adhesive etc 8 that fails as shrinkage begins. The function of the adhesive 8 is simply to help retain the sheet 4 prior to use, to improve appearance of the product. Adhesive 8 may comprise an adhesive of low melting point typically about 40-80°C. It may be, for example, a paraffin wax or a hot melt adhesive (40- 80°C) such as a butadiene rubber that melts or softens at the appropriate temperature and does not interfere with the action of the sheet 4. Double-sided adhesive tapes (preferably using pressure-sensitive or contact adhesives) may also be used, especially for the adhesive 7. A further member 9 is shown that has the function of absorbing microwaves, becoming hot, and transmitting heat by conduction to the sheet 4. It may be a sheet of metallised heat resistant material e.g aluminsed mica or heat resistant plastics the metallisation being very thin as in a vacuum deposited coating so that excessive heat is not absorbed and the oven is not damaged. It can be seen to be positioned face-to-face with at least part of the sheet 4. Such a member can be used to bring forward the moment when the lid opens. This is simply because heat does not have to pass the air gap within the container that has to be passed by heat from the foodstuff. Adjustment the other way, i.e. delaying lid opening, can be achieved for example by provision of an insulating layer e.g an additional paperboard sheet or a plastics sheet between sheet 4 and the base and lid of the container. Figure 2B shows the container with the lid open. The sheet 4 can be seen to have separated from lid and base, breaking thermal contact, and preventing further shrinkage. The embodiment shown in Figures 3A and 3B may be preferred for shallower or wider containers, such as trays . Here the lid 3, which in general will be highly flexible and thin, convolutes, especially by coiling. In Figure 3A it is shown before heating, closing the container, and in Figure 3B it is shown coiled at 10. The lid may be a laminate of layers of different shrinkage, for example a lower layer (as drawn) that is not shrinkable and an upper layer that is highly shrinkable. The differential shrinkage will result in some coiling. In Figure 4A, film 4 is bonded to a surface 11 of a container by an anchoring adhesive 12. Before heating a first message 13 (if any) is in view. After heating as shown in Figure 4B the film 4 has shrunk to move message 13 and/or to expose a new message 14. The same kind of translational movement of a movable panel across a fixed lid of the container may be used to expose e.g one but not both compartments of a two compartment container in which the compartments are in adjacent relationship along a base of the container and are separated by an internal wall.

Claims: CLAIMS 1. A container for food to be cooked by microwave having means responsive to heat developed on exposure of the container to microwaves to cause the container to open when food is present in it and has been heated ready for consumption. 2. A container according to claim 1 having within it a single food receiving space. 3. A container according to claim 1 having within it a multiplicity of food receiving spaces . 4. A container according to any preceding claim wherein the heat receiving means is arranged to open the whole of the food receiving space or spaces . 5. A container according to any preceding claim wherein the heat receiving means is arranged to open only part of said single food receiving space, only part of each of said multiplicity of food receiving spaces or less than all said multiplicity of said food receiving spaces . 6. A container according to any preceding claim wherein the heat responsive means is connected to two relatively movable parts of the container to bring about a relative rotation thereof when the food has been heated ready for consumption. 7. A container according to claim 6, which comprises a body defining a food receiving space and a lid which conforms to the body and covers the food receiving space, the lid being hingedly connected to the body and the temperature responsive means being arranged to pivot or allow to pivot the lid away from the body when the food is ready for consumption. 8. A container according to claim 6 or claim 7, wherein the heat responsive means is arranged so as to be in physical contact with the container when the container is closed so as to receive heat developed within the container and to move into spaced relationship with the container on relative rotation of the movable parts to open the container. 9. A container according to any of claims 1 to 5 wherein the heat responsive means is connected to two relatively movable parts of the container to bring about relative sliding movement. 10. A container according to any of claims 1 to 5, which is arranged to open by a conformational change in a lid thereof. 11. A container according to claim 10, wherein the conformational change comprises convolution. 12. A container according to claim 10 or claim 11, wherein the lid comprises a heat responsive means . 13. A container according to any preceding claim wherein the heat responsive means is a heat shrinkable member. 14. A container according to claim 13, wherein the heat shrinkable member is substantially unidirectionally shrinkable. 15. A container according to any preceding claim wherein the heat shrinkable member is a sheet. 16. A container according to any preceding claim wherein the heat responsive means comprises a heat shrinkable member fastened at spaced apart end regions thereof through different relatively movable portions of the container by means of an adhesive which does not soften or melt or does so only relatively slowly at the temperature at which the heat shrinkable member shrinks . 17. A container according to claim 16, wherein the heat shrinkable member is adhered to the container in a region between the spaced end regions by an adhesive that softens or melts at a temperature at or below that at which the heat shrinkable material shrinks . 18. A container according to any preceding claim, which is openable by pivoting motion of a lid thereof relative to a body part thereof at a hinge line, and the heat ' responsive means comprises a sheet of heat shrinkable material that extends across the hinge line and is fixed to surfaces of the container joined by the hinge line. 19. A container as claimed in claim 18 wherein the sheet is substantially unidirectionally shrinkable, the shrinkage direction being substantially perpendicular to the hinge line. 20. A container as claimed in claim 19 wherein the hinge line comprises a Line of weakness in material joining the body to the lid of the container. 21. A container as claimed in any of claims 1 to 5 having resilient means for opening the container, which resilient means is enabled to operate by the heat responsive means when the food is ready for consumption. 22. A container according to any preceding claim further comprising means for holding the container closed, which means is overcome by the heat responsive means when the food has been heated ready for consumption. 23. A container according to claim 22, wherein the means for holding the container closed is a hot melt adhesive. 24. A container according to any preceding claiir. in which the heat responsive means is substantially unresponsive to microwaves from a domestic microwave oven. 25. A container according to any preceding claim further comprising means for retarding the passage of heat from the container to the heat responsive means . 26. A container according to claim 25, wherein the heat retarding means comprises thermal insulating material within the container located adjacent to the heat responsive means. 27. A container according to any preceding claim further comprising means for accelerating the heating of the heat responsive means on exposure to microwaves . 28. A container according to claim 27, wherein the heating acceleration means comprises a body located within the container adjacent to the heat responsive means which itself becomes heated in response to microwaves. 29. A container according to claim 28, wherein the body is a sheet of heat resistant insulating material having a metallised surface. 30. A container according to claim 29, which contains a foodstuff. 31. A container according to claim 30 which does not contain a foodstuff. 32. An assembly comprising a container and a heat recoverable article that can be dimensionally recovered at a desired temperature to cause an externally visible change in the container and to allow the container if closed to be opened. 33. An assembly according to claim 32 in which the heat recoverable article is heat shrinkable. 34. An assembly according to claim 32 or claim 33 in which the recovery causes a configurational change in the container. 35. An assembly according to claim 32, 33 or 34 in which recovery causes concealment or exposure of a part of the container. 36. An assembly according to claim 34 in which the configurational change comprises movement of one part of the container relative to the other. 37. A container according to claim 36 in which the relative movement of the surfaces results in opening of a closed said container. 38. A method of preparing a foodstuff, which comprises: (a) placing the foodstuff in a container, having a thermal indicator that is substantially unresponsive to microwaves; (b) applying said microwaves to the foodstuff to cause the food or drink to become hotter and heat to be conducted from the foodstuff and/or container to the thermal indicator; (c) allowing a configurational change of the thermal indicator to cause a visible change to the container; and (d) stopping application of the microwaves to the container in response to the physical change. 39. A container to be cooked by microwaves having means responsive to heat developed on exposure of the container to microwaves to cause a major surface of the container to move relative to other portions of the container when the food is ready for consumption to give a visible indication of that fact.

Inventors: LIVESLEY DAVID, LIVESLEY, DAVID
Applicants: CAMBRIDGE CONSULTANTS, CAMBRIDGE CONSULTANTS LIMITED
Application year: 1991
Main classification: H05B6
IPC codes: B65D81