MATERIAL FOR TRAYS OR PACKAGINGS TECHNICAL FIELD The present invention relates to a material for trays or packagings, which material is built up from a number of layers, these layers comprising, seen from the inside of the food tray, a grease-and water-repellent layer, a first greaseproof paper, a layer of adhesive and a second greaseproof paper. The material is conceived chiefly for trays for food, but can also be utilized for packagings or trays for other purposes, e. g. for the storage of drugs or even for toys etc.

PRIOR ART In today's society, there is great demand for packagings for e. g. foodstuffs to be adapted to the environment and recyclable. The present invention relates to material for such a packaging, or more specifically to material for a disposable tray for ready-cooked food or semi-prepared, refrigerated foodstuffs, especially of the pastry, pizza, gratin or similar type. For this type of material, at least the following requirements or wishes are posed.

1. It should be adapted to the environment, in which case manufacture should cause a minimal negative effect on the environment and the material should be able to be composted, recycled or incinerated.

2. The material should not be negatively influenced by fermentation in a climatic cabinet, baking in an oven or in a microwave oven, (rapid) freezing, defreezing, heating in an oven or in a microwave oven. Negative influence refers in particular to discoloration and a change in shape or stability.

3. The material should tolerate foodstuffs containing grease and liquid without being negatively affecte, in particular without permitting the penetration of grease or liquid or acquiring stains.

4. Trays manufactured from the material should not be affected negatively by a moist, possibly warm or hot atmosphere, in particular they must retain their shape.

5. Trays manufactured from the material should not stick to one another on stacking and shall part with the food easily without it falling out of the tray on this account.

6. The food which is in the tray should be capable of being examined with a metal detector, for which reason metal material should be avoided.

7. The material should not contain plastic, since there is a risk that plastic, at least in certain countries, will be taxed in the future.

8. The material should be approved for foodstuffs.

The conventional material for this type of trays for foodstuffs is aluminium. However, aluminium is far from satisfying all the above requirements/desires. In particular, it is not adapted to the environment and does not permit examination of the food with a metal detector.

Another type of known material is plastic-coated cardboard, which naturally has the disadvantage that it contains plastic. The cardboard usually has a basis weight of around 300-400 g/m2. The relatively environmentally friendly plastic polyethylene cannot be used in this type of material if the tray is to withstand high temperatures on e. g. baking, since polyethylene melts at these temperatures. Other plastics, e. g. polyesters, have the disadvantage that they have a tendency to release from the cardboard on heating, with bubbles forming.

A further type of known material is so-called greaseproof paper. A material is known from WO 96/11576 which from inside outwards is built up of a water-repellent layer, a greaseproof paper, a grease-repellent layer and a greaseproof paper. It is also indicated that the water-repellent layer can be a layer of lacquer and that the grease-repellent layer can be a layer of adhesive, which also has the function of holding the two greaseproof papers together. A decorative layer can lie outside the outermost greaseproof paper.

This type of material fulfils most of the above requirements/desires but not requirement 4. Moisture can enter the material from outside, e. g. in a warm oven with a moisture- saturated atmosphere, or on defreezing, in which case a tray manufactured from the material collapses and loses its dimensional stability.

SOLUTION AND ADVANTAGES By means of the present invention, a material is presented for trays, in particular for foodstuffs, which material satisfies all the above requirements/desires at the same time as

being relatively cheap. The material also results in a reduced weight of the trays manufactured from the material, in comparison with the heavier material plastic-coated cardboard according to the prior art. This is an important advantage from the environmental point of view.

The material according to the invention is defined in claim 1 and is a further development of the material presented in WO 96/11576, an outermost layer, i. e. the layer which is furthest away from the food in a tray manufactured from the material, being a water- repellent layer. This outermost, water-repellent layer is made preferably from or chiefly comprises preferably an acrylic polymer, in particular a modified one which also contains wax. A modified acrylic polymer which is specially preferred is made from an anionic aqueous dispersion of acrylate copolymer which contains a certain amount of stearate.

Other types of water-repellent material, e. g. silicone, polyacrylate copolymer dispersion, styrene acrylate, acrylic acid copolymer or a combination of said materials can however also be used for this outermost water-repellent material, but preferably not plastic material.

According to an aspect of the invention, a tray manufactured from a material according to the invention has a surface which can be unbleached, bleached, coloured or with printing and which in addition can be different on the inside and outside of the tray respectively. Furthermore, the material retains at least 60%, preferably at least 70% of its tear strength after being exposed to a baking temperature in an oven at around 200- 220°C for 20 minutes.

According to another aspect of the invention, the material can consist of a laminate with three or more layers of greaseproof paper. The material suitably has a total basis weight of 65-200 g/m2, preferably 80-200 g/m2, when two layers of greaseproof paper are used.

The upper limit of the ranges could however be as much 250 g/m2 if three layers of greaseproof paper are used in the laminate.

DESCRIPTION OF THE DRAWINGS The invention will be described below with reference to Figure 1, which shows the layers in a preferred material according to the invention diagrammatically, the items in the description below being numbered in the same way as the layers in the drawing. The

drawing is not according to scale. In reality, layers 2 and 5 are completely dominant in thickness.

1. Layer 1 in Figure 1 represents the layer which, in a tray manufactured from a material according to the invention, faces the food, i. e. is on the inside of the tray.

Layer 1 is made from a grease-and water-repellent layer, which preferably comprises chiefly silicone and has a basis weight of 0.1-2 g/m2, preferably 0.4-0.8 g/m2. The silicone is applied on manufacture of the laminate material to the next layer, i. e. by means of a so-called blade coater or in a so-called size press. Other types of grease-and water-repellent layer can also be used, for example chromium stearate, which however has the disadvantage that it is less environmentally friendly.

Silicone has the advantage that the surface releases the food well when this is taken out of the tray and that the trays do not stick fast to one another when stacked, which is an advantage in automated setting out of the trays when these are to be used. For applications other than foodstuffs, e. g. in trays or packagings for toys or drugs or similar, this inner layer 1 is not always necessary.

2. Layer 2 is made from a first greaseproof paper, which suitably has a basis weight of 30-100 g/m2, preferably 40-90 g/m2 and more preferredly 60-75 g/m2. This paper is preferably manufactured from paper pulp produced according to the sulphite method and which preferably has a degree of beating greater than 70 °SR Schopper-Riegler.

Such a highly beaten, greaseproof paper has a high density, at least 800 kg/m3 or around 900 kg/m3 at a basis weight of 75 g/m2 and has a grease resistance measured as so-called air permeance according to SCAN P26-78 of less than 0.7 nm/ (Pa-s), a low value being a measure of a high grease resistance. For the greaseproof paper to appear as white as possible, a substance is added during manufacture which produces this effect, preferably titanium dioxide in a quantity of around 1-10%, preferably 2.5-5% measured on dry matter, or alternatively e. g. China clay or calcium carbonate. Titanium oxide has the advantage here that it provides grease opacity, no unattractive dark grease stains being formed on the surface of the greaseproof paper. To give the greaseproof paper wet-strength so that a food tray manufactured from the material according to the invention retains its appearance and its stability on freezing, a wet strength agent, preferably of the type polyamide- epichlorhydrin-resin dissolved in water, which is known in Sweden for example under the trade name Kymene SLX, is added during manufacture of the greaseproof paper.

The fact that the greaseproof paper is preferably to be manufactured from sulphite pulp is due partly to the fact that sulphite pulp offers good properties in the form of grease resistance, but also to the fact that sulphite pulp gives better dimensional stability than the more common sulphate pulp. Pulp produced according to the sulphate method gives fibres which are stiffer than the fibres in sulphite pulp, which is why a food tray made of sulphate pulp loses its stability and shape. However, according to the invention 1-50% of sulphate pulp can be mixed into the sulphite pulp without the dimensional stability being dramatically impaired.

The first greaseproof paper in layer 2 is suitably coated 2a, 2b with starch adhesive, latex, polyvinyl alcohol or similar, preferably with starch adhesive and preferably on both sides. This coating is suitably created in connection with the manufacture of the greaseproof paper in a so-called size press. Each coating layer has a basis weight of 0.1-1 g/m2, preferably around 0.5 g/m2 and penetrates to a certain extent into the greaseproof paper. The task of the coating layers is to create barriers for the adjacent layers. If coating layer 2a did not exist, for example, the quantity of silicone required in layer 1 would increase considerably for the right surface properties to be obtained. The starch adhesive is preferred in this regard, since it provides a good barrier against the penetration of silicone and improves the grease resistance (air permeance) of the paper.

3. Layer 3 is a protective layer, the purpose of which is to give the material good sealing properties and good adherence to the adjacent layer. This protective layer has a basis weight of 0.1-1 g/m2, preferably around 0.5 g/m2 and preferably comprises mainly latex, or alternatively polyvinyl alcohol. However, in certain cases just a water coating will suffice to avoid curls in the paper. The coating is suitably carried out in a blade coater but can also be executed in a size press. This depends on the demands to be made on the final product.

4. Layer 4 is a layer of adhesive, the purpose of which is to join the two greaseproof papers to one another. Layer 4 has a basis weight of 1-10 g/m2, preferably 3-6 g/m2, depending on the paper's surface structure, and chiefly comprises a hardening adhesive, suitably polyurethane adhesive which is preferably solvent-free. Compared with a layer of plastic, this type of layer has the advantage that it tolerates high temperatures.

5. Layer 5 is formed from a second greaseproof paper which fully corresponds to the attributes indicated above for the first greaseproof paper 2.

The second greaseproof paper 5 is also coated 5a, 5b in the same way as the first greaseproof paper 2, with the sole difference that the coating layer 5a, 5b preferably comprises mainly a cellulose derivate, preferably sodium carboxymethyl cellulose (CMC). The primary purpose of the coating layer 5a, 5b is to form a barrier against the adjacent layer and to improve the grease resistance (air permeance) of the paper.

Alternatively, the coating layer 5a, 5b can consist of starch adhesive, polyvinyl alcohol or similar.

6. Layer 6 is formed according to the invention from a water-repellent layer. The function of this is to prevent moisture from penetrating the greaseproof paper, at which the dimensional stability of a food tray manufactured from the material can otherwise be lost, especially when this is exposed to a warm and damp atmosphere in the oven on baking of the contents of the tray or following defreezing. On defreezing, the moisture content of the air condenses on the cold surface, at which the tray risks collapsing if there is no outermost water-repellent layer 6. Layer 6 has a basis weight of 0.1-1 g/m2, preferably 05.-1 g/m2 and is formed preferably from or comprises principally preferably an acrylic polymer, particularly a modified one which also contains wax. A modified acrylic polymer which is particularly preferred is formed from an anionic aqueous dispersion of acrylate copolymer, which contains a certain amount of stearate. Other types of water-repellent material, e. g. silicone, polyacrylate copolymer dispersion, styrene acrylate, acrylic acid copolymer or a combination of said materials can however also be used for this outermost water- repellent layer, but preferably not plastic material.

When manufacturing the material according to the invention, one starts out from the two greaseproof papers, which have already been coated in conjunction with their manufacture with the layers 1,2a, 2b and 3 or 5a, 5b and 6. The greaseproof papers 2 and 5, with their coatings, are then joined to layer 4 by roller coating in a laminating machine.

EXAMPLE Four different laminates were manufactured, of which laminate A was formed according to the invention and the layers from inside outwards consisted of silicone, starch

adhesive, a first greaseproof paper, starch adhesive, latex, solvent-free hardening adhesive, CMC, a second greaseproof paper, CMC and finally a modified acrylic polymer with wax. Laminate B was also formed according to the invention in the same way as laminate A with the difference that the latex layer was excluded. Laminate C was also formed according to the invention in the same way as laminate A but with opaque greaseproof paper, with the first greaseproof paper coated with latex instead of starch adhesive and with water instead of the latex layer which was present in laminate A. A fourth laminate D was produced as a reference and consisted of an innermost layer of polyester film, CMC, a first greaseproof paper, CMC, a modified acrylic polymer with wax, solvent-free hardening adhesive, CMC, a second greaseproof paper, CMC and finally a modified acrylic polymer with wax. Some characteristic attributes were tested for the four different laminates, and these are shown in Table 1 below. It is to be taken into account in this regard that these results are derived from isolated measurements with an attendant uncertainty in the values.

Table 1 Test method A B C D Basis weight glm2 ISO536-1976 (E) 159 159 157 178 Density kg/m3 IS0534-1988 (E) 964 964 989 1053 Thickness 165 165 159 169 Brightness IS02470-1977 (E) 67 66 79 66 Opacity IS02471-1977 (E) 65 64 85 66 Air permeance nm/(Pa s) SCAN P26-78 <0 004 <0.004 <0.004 <0.004 Grease resistance s TAPP1454 of 89 > 1800 > 1800 > 1800 > 1800 Wet strength % IS03781-1983 27 25 25 45 Water absorbency ghmz IS0535-1991 (E) 12 12 12 0 Cob60inside Water absorbency g/m2 IS0535-1991 (E) 18 17 19 19 Cobb60outside Water vapour g/m2 24h 32 49 125 10 transmission 25°C50% RH Water vapour g/m2 24h 139 117 525 45 transmission 38°C90% RH Slip angle o Special method 11 12 13 18 Inside Slip Angle o Special method 13 13 15 14 Outside Heatresistance'% Special method 72 70 67 86 Baking test² -/% Special method 5/10-30 5/10-30 5/10-30 4/95

1) Remaining tear strength after 20 minutes at 200°C 2) Release factor 1-5, where 5 is best/proportion of surface which is covered with pastry (sponge cake) Table 1 shows that all four laminates have very good grease resistance and air permeance. Reference laminate D has a rather better result with regard to wet strength, water absorbency on the inside and heat resistance. Laminate D has a lower water vapour transmission than laminate A, B and C, which may be positive for certain

products. However, laminates A, B and C have better (lower) slip angles on the inside, which is positive since this is a measure of how easy it is to separate the trays from one another when they are stacked. Finally, the baking test shows that the surface on the inside of laminate D is covered with 95% remaining pastry when the pastry is removed from the tray, while laminate A, B and C are only covered to an extent of 10-30%.

Laminate C also has a high brightness and opacity, which has been achieved by the addition of titanium dioxide to the paper pulp. The titanium dioxide also gives the laminate a good grease opacity, which eliminates the risk of grease stains. In summary, it can be said that the reference laminate D certainly has a large share of positive attributes, but that laminate A, B and C are nevertheless to be preferred since certain important attributes are better for these laminates and the remaining attributes are still good, at the same time as these laminates do not have the disadvantages described previously for plastic-coated laminates of the type D. Laminate D is also more expensive than laminates A, B or C.

The invention is not restricted by the embodiments indicated above and the example can on the contrary be varied within the scope of the following claims.