GB2254126A - Material having a temperature-dependent passage therethrough - Google Patents

Abstract

A sheet 2 for use in packaging vegetable produce incorporates an element with a temperature-dependent variable- registry hole system consisting of superposed discs 5 retained by a grommet 4. The discs 5 have ventilation holes 8 which are in registry at one temperature but slide out of registry at another temperature on thermal change of the element. Alternatively the element may be used (i) in a roadside sign to display a backing colour or a message e.g. FROST, on a certain ambient temperature, (ii) as one fabric of a two-fabric attachment system wherein the other fabric has hairs which can be locked in holes 8 on a temperature change, (iii) in a medical dressing, a ventilation controller for other than foodstuffs, a radiation absorber or reflector, or a variable vapour barrier e.g. for clothes and shoes, or (iv) to control dosage release e.g. of deodorant. <IMAGE>

Description

MATERIAL HAVING A TEMPERATURE-DEPENDENT PASSAGE THERETHROUGH The present invention relates to a material having a passage therethrough. Such material has a wide variety of applications.

For illustration, the invention will be described with reference to one such application, namely packaging of edible produce, in particular to maintain it in a preservative atmosphere, bearing in mind that the requirements can vary with temperature.

All harvested vegetables and fruits and also meats continue to respire, absorbing oxygen and emitting carbon dioxide. A number of parallel biological paths are involved leading to loss of weight (although loss of water is usually the main cause of weight loss), some heat production, loss or change of flavour, change of texture, and discolouration of outer surfaces or cut surfaces or in the body of the product. The growth and excretions of micro-organisms present at harvesting can produce other changes. Virtually all these changes are deleterious to the acceptability of the product to the consumer.

Respiration increases with increasing temperature. The main method of increasing storage life is therefore to hold the product at lowered temperatures, often a little above OOC.

However some products (e.g. runner beans, cucumbers, green peppers, tomatoes, bananas) cannot be stored below 5-lloC.

Respiration rates show a wide range, broccoli being nearly 102 times as fast as onions or potatoes. Values also vary with variety within a given species, cropping and harvesting methods used, year of crop and degree of ripeness.

It might be thought that the respiration rate (in particular the carbon dioxide production rate) could be sharply reduced by lowering the concentration of oxygen. However below 2-6% (depending on product) anaerobic processes begin, due to both the product and its accompanying micro-organisms. These, usually, produce rapid and unacceptable tainting of flavour. Alcohol production by yeasts often predominates but is often not the main cause of unacceptability.

If a product is wrapped in a sealed impermeable packaging system then even a modest respirer such as tomatoes soon lowers the oxygen concentration to anaerobic conditions. Ripening of tomatoes is completely inhibited and does not resume when the package is opened and many of the fruits develop rots or suffer fungal attack. (An atmosphere of 6% oxygen retards ripening, which however recommences on opening to air, with no adverse effects on eating quality.) To overcome this rot problem, it is known to pass such a sealed impermeable pack (preferably after a short period of storage and temperature reduction, to allow the oxygen concentration to drop rapidly), under an adjustable mechanical 'pecking' head which punches in the necessary holes, e.g. typically three holes of 0.4mm2 per pack.Changing their diameter or number provides easy adjustment for product type, variety type, weight and expected downstream conditions before consumption etc - always provided the basic knowledge is available. However, by itself, this has been found to give inadequate variation with temperature.

An alternative approach to this problem has been to devise selectively permeable materials which can be used as panels in gas-impermeable wrapping materials for fresh fruit, vegetables or flowers. These panels can be of a microporous material having greater C02 permeability than 02 permeability, and it has been proposed to use that area of material as will transmit oxygen at the same rate as the packaged contents will consume it. However, this presupposes a certain temperature of storage, as oxygen consumption of produce increases much more sharply with temperature than does the oxygen transmissibility of the material. At accidentally high temperatures, therefore, the oxygen within such a wrapping will be consumed much faster than it can be replenished, leading to anaerobic conditions and their drawbacks as already described.

It is known that high carbon dioxide concentrations inhibit micro-organisms so that it could be advantageous to maintain such levels for packaged fruit and vegetables which tolerate such concentrations.

Regarding water vapour, the optimum requirement seems to be a relative humidity of just under 100%, but with no liquid water present, both from appearance of the pack and for the discouragement of fungal growths.

It would therefore be desirable to devise a material having a passage therethrough, which passage changes in effective size with temperature at a higher rate than the thermal coefficient of expansion of the material, over at least a certain temperature range, e.g. increasing as temperature rises from -50C or OOC to 200C.

According to the present invention, a temperature-dependent obturator comprises two apertured layers, wherein the apertures in adjacent layers are in disregistry or only partial registry at certain temperatures and are arranged on thermal size-change to move into more complete registry.

The aperture may be constant or adjustable ranging from one hole (which would typically be from 0.05 to l.0mm2 for most retail packs and products), to many holes of few microns diameter.

The gas permeability of the apertured laminate should increase at least threefold from 50C to 200C. This would make it especially suitable to be used in a package (e.g. comprising all or part of the wrapping) for edible produce.

A packaging film has to fulfil many demands, e.g. of strength, toughness, often clarity, sealability, printability, ability of the inner surface to disguise or spread water droplets, and finally price. To achieve this while offering a controlled and specific series of permeabilities with a prescribed temperature variation is to steer between Scylla and Charybdis while gazing on Medusa. Therefore, since an obturator according to the invention cannot be used as the packaging film itself, it would be fabricated to form a small area in a package or other container made otherwise from a conventional commercial packaging film. The small area needs no transparency etc, only a suitable permeability and strength.

The invention may be realised for packaging edible produce and other uses (exemplified later) by layers of plastics, polymers or other materials, and/or of stouter sheets such as of metal foil or sheet and/or of seml-rigid sheets of plastics or polymers, which themselves may be loaded with metal powder. The layers may be the same or different, e.g. one layer is polyester and/or the other is polyolefin (usually polyethylene).

Alternatively, polyamide, cellulosic or polyethylene terephthalate film can be paired with low density polyolefin (e.g. polyethylene) film, which may be metallised, or with ethyl vinyl acetate film.

The invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 shows in cross section and Figure 2 in plan a temperature dependent variable-registry hole.

A bag or a label portion of a bag, 2, has an opening with a variable-registry hole system mounted therein and sealed by a grommet or pair of retaining rings 4 having a coefficient of thermal expansion comparable with that of the bag. The grommet 4 retains two discs 5 having a high thermal coefficient of expansion. The discs can for ease of production be identical.

Alternatively, differential expansions of the discs can be exploited if they are of materials having widely differing coefficients of thermal expansion, e.g. polyester and polyethylene. The discs each have a wide circumferential slot 6 to accommodate thermal contraction so as to allow the main part of the disc to remain flat while constrained by the grommet. The discs 5 are mounted with slots 6 antiphase, in the grommet 4. In production, the system is taken to its maximum expected service temperature and an appropriate number of holes 8 0.05mm across or even down to O.Olmm across is laser-drilled through the overlap portion ("main part") of the pair of discs 5.

The system is then mounted in a bag 2 which is filled with produce. When the produce is stored at OOC, the discs 5 contract but remain flat as stretched in the grommet 4. The laser-drilled holes 8 thus slide out of registry as between the two discs, reducing or stopping the passage of gas into and out of the bag 2 of produce. As temperature rises towards the maximum expected, so the holes 8 slide back towards registry, increasing the gas flow rate as required.

It will be readily appreciated that, with no or minimal modification, these systems can be used in medical dressings (e.g. for burns), ventilation control other than for foodstuffs, (if metal-filled) temperature-dependent radiation absorbers or reflectors, dosage release (e.g. of deoderant), variable vapour barriers (e.g. vapour transmission control in shoes and clothes), thermal valves such as for appropriately permeable boil-in-bag sachets, and as part of one fabric of a two-fabric temperaturedependent attachment system, the other fabric being hairy and the hairs being locked into the holes 8.. On temperature change, the opening holes can reveal a backing colour or message appropriate to the temperature (e.g. "Frost", on a roadside sign exploiting negative temperature excursions from a OOC set point).

Claims (7)

1. A temperature-dependent obturator comprising two superposed apertured layers, wherein the apertures in adjacent layers are in disregistry or only partial registry at certain temperatures and are arranged on thermal change to move into more complete registry.

2. An obturator according to Claim 1, wherein a first of the layers is any of polyester, polyamide, cellulose, or polyethylene terephthal ate.

3. An obturator according to Claim 1 or 2, wherein a second of the layers is polyolefin, which may be metallised, or ethyl vinyl acetate.

4. A container comprising, an obturator according to any preceding claim.

5. A method of packaging edible produce, comprising using the obturator of any of Claims 1 to 3 as part of the wrapping.

6. Edible produce packed into a container according to Claim 4.

7. An obturator according to any of Claims 1 to 3, in the form of a medical dressing, ventilation controller, radiation absorber or reflector, dosage release controller, variable vapour barrier, thermal valve, temperature-dependent display or a female fabric of a two-fabric attachment system.