Use of a packaging laminate

Abstract: A packing laminate for the manufacture of packing con­ tainers for edible oil must have an oil-resistant inner layer which is not affected by the oil, even during prolonged storage periods. A packing laminate meeting this condition has an ex­ truded inner layer of two types of plastics mixed with one an­ other, namely LLDPE and LDPE.

Description: The present invention relates to a particular use of a packaging laminate as mentioned in claim 1. Such packaging materials to be used for the packing of light-sensitive materials such as photographic roll-films, medicines, chocolate, fermentation products and developing solutions are described in JP―A―58 132 555. According to this prior art a carrier layer of paper or cellophane is covered on one side by an adhesive layer bonding an aluminum foil, a further adhesive layer bonding a layer comprising a mixture of a L-LDPE/LDPE/carbon black making the complete laminate light-shielding. It is an object of the present invention to provide a packaging laminate of such kind which is suitable for the manufacture of packing containers comprising a good oil resistance and without substantial stress-cracking during or after the contact of the laminate with oil. Indeed, the packing laminate should also comprise sufficient stiffness. Up to now, edible oil or products containing such oil are usually packed in lacquered sheet metal cans, glass bottles or blow moulded plastic bottles. However, such package containers are relatively expensive. The invention is represented by the use of the laminate of claim 1 and further embodiments thereof are described in the dependent claims. It has been found that a particular sequence of the respective layers solve the present problem very well. The packaging laminate comprises the desired strength and impermeability characteristics especially with regard to edible oil so that it can be well utilized as packaging material for edible oil or edible oil containing products. The present invention will now be described in more detail with special reference to the schematic drawing which in strong enlargement shows a cross-section through a part of a preferred embodiment of the packaging laminate in accordance with the invention. The packaging laminate 1 comprises a number of material layers laminated to each other which together form a packaging laminate of the desired strength and impermeability characteristics. The packaging laminate 1 has an external surface 2 and an internal surface 3 which after the packaging laminate has been converted to packing containers form the outside and inside respeclively of the packing container. It is evident from the figure that the greater part of the thickness of the laminate is taken up by a carrier layer 4 which consists of a material imparting stiffness, e.g. paper or foamed plastics. The carrier layer 4 has an appreciably greater thickness than the remaining layers of the laminate, but the exact thickness may vary depending upon the type and size of the packing container into which the packaging laminate is to be converted. In the case of the packing container sizes generally used of 0,282,0 litres, the paper material chosen for the carrier layer 4 may have a weight per unit area of 120―440 g/m 2 . A typical value in the manufacture of packaging laminate for 1 litre containers is 255 g/m 2 . The carrier layer 4 may be impregnated wholly or partly with an oil-repelling agent, which, however, belongs to prior art and is not, therefore, described in detail in this patent application. On both sides of the carrier layer 4 comparatively thin layers 5,6 of a liquid-tight, thermoplastics material, e.g. polyethylene, are present. The polyethylene layer 5 on the outside 2 of the packaging laminate imparts a smooth and liquid-tight exterior to the packaging laminate which, moreover, makes possible a sealing of the laminate with the help of heat and pressure, a so-called thermosealing. This external layer of thermoplastics consists of LDPE which has a weight of coating of 12―30 g/m 2 . When the packaging laminate is to be converted to packing containers with a volume of 1 litre, generally a weight of coating of 20 g/m 2 is used. The thermoplastic LDPE layer 6 applies to the opposite side of the carrier layer 4 comprises a similar weight of coating. Like the layer 5, the layer 6 is applied to the carrier layer 4 by means of extrusion which means that the layers 5, 6 are applied to the carrier layer 4 in liquid state and penetrate into the surface layer of the latter so that the seal between the layers 5, and 6, respectively, and the carrier layer 4 is very good. The main task of the layer 6 is to make possible the application of a metal foil layer 7 which in most cases consists of alumium foil of a thickness of 5―18 µm preferably 7 µm (corresponding to a weight of coating of 19 g/m 2 ). The layer 7 serves as a barrier layer which prevents light as well as oxygen from reaching the contents, which is advantageous since the contents are thus given better keeping quality. It is not desirable. however, for the contents to come into direct contact with the metal foil layer 7 and this is coated, therefore, in general with a further layer which in the packaging of milk, fruit juice and other beverages usually consists of polyethylene. However, polyethylene is not oil-resistant and the packaging laminate in accordance with the invention, therefore, comprises an inner layer 8 of two types of plastics mixed with one another, namely LLDPE and LDPE. An inner layer 8 of this type however, has comparatively poor adhesive properties and so as to improve these, an intermediate adhesion layer 9 is made use of which joins the inner layer 8 to the metal foil 7 and preferably consists of partially neutralized EAA. The inner layer 8 of mixed LLDPE and LDPE has a weight of coating of preferably 30 g/m 2 . In the manufacture of packing containers of smaller volume (0.2 litres) the weight of coating may be reduced a little further, but approx. 20 g/m 2 has proved to be a lower limit so as to avoid crack formation or other types of leaks. The intermediate adhesion layer 9 has a coating weight of preferably 15 g/m 2 , but very low coating weights may be used and even a weight as low as 1 g/m 2 has proved to give satisfactory results. It is important, though, that the combined thickness of the inner layer 8 and the adhesion layer 9 amounts to at least 30 g/m 2 , since otherwise, in unfavourable circumstances (mechanical damages etc.) the edible oil may come into contact with the metal foil layer 7. The manufacture of the packing laminate in accordance with the invention takes place substantially in a conventional manner, in that the carrier layer 4 by means of extrusion is coated successively with the different surface layers. The aluminium foil 7 is applied from a roll, the polyethylene layer 6 serving as a binder. The adhesion layer 9 and the inner layer 8 can be applied each separately directly after one another or through coextrusion. The conversion of the laminate to individual packing containers may also be done in a conventional manner, e.g. by supplying the packaging laminate in form of a roll to a packaging machine, wherein it is converted to a tube which is filled with contents and sealed off by means of repeated transverse seals so that cushion-shaped, filled packing containers are produced. The cushion-shaped packing containers are then form-processed to parallelepipedic shape in that their corner lugs are flattened, folded down and sealed to the outside of the packing container. By making use of a combination of LLDPE and LDPE for the inner layer 8 an inner layer is obtained which resists the effect of the oil even during prolonged contact, and any appreciable reduction in the stretching ability of the inner layer could not be observed in practical experiments. The mixing ratio between LLDPE and LDPE can be varied, but experiments and tests have shown that an admixture of LLDPE of approx. 85% by weight of the total weight (LLDPE+LDPE) of the layer provides a material layer of substantially the same elongation properties in transverse as well as longitudinal direction which is of decisive importance in order to prevent crack formation when the finished laminate is folded during its conversion to packing containers. If the admixture of LLDPE is reduced to between 50 and 85% by weight improved extrusion properties are obtained, that is to say the extrusion can be carried out at a faster rate without the thickness of layer or the uniformity being negatively affected. This has to be paid for though in part by poorer elongation properties of the finished layer, and if the admixture is reduced to below approx. 50% the transverse elongation properties of the layer are worsened to such an extent that it becomes unsuitable for use in the packaging of oil or oil-containing products. In corresponding manner there is an upper limit to the admixture of LLDPE at approx. 90% by weight. Above this limit on contact with oil the longitudinal properties are impaired and the material no longer can be used with such contents. An important factor regarding the possibilities of extruding the material is also the so-called "melt-index" of the material which can be varied to a certain extent depending upon the mixing ratio between LLDPE and LDPE together with the melt index of these two types of material making up the mixture. Thus preferably an LDPE of a melt index in the range of 5,5―8 g/10 min (dg/min) is chosen which together with an LLDPE of a melt index of approx. 3,7 g/10 min gives the mixture a melt index of at least 3,5 g/10 min. The melt index of the mixture ought to be between 3,5 and 7,0 g/10 min, which represents an easy-flowing mixture of good extrudability. A typical laminate in accordance with the invention which is used for the manufacture of packing containers of a volume of 1 litre (TETRA BRIK®) comprises the following layers (starting from the outside of the laminate): LDPE 12 g/m 2 Paper 225 g/m 2 LDPE 25 g/m 2 Al foil 7 µm EAA 8 g/m 2 LLDPE+LDPE 27 g/m 2 This material in practice proved to have excellent properties for the packaging of oil, and the oil-resistance of the inner layer permits storage over a long period without any stress-cracking or other defects arising.

Claims: Use of a packaging laminate including a carrier layer, an extruded plastics layer of a mixture of linear low density polyethylene (LLDPE) and of low density polyethylene (LDPE), a barrier layer of gas impervious material such as aluminium and an adhesive layer, for a package for packaging edible oil or food containing edible oil, characterized by the following sequence of layers a liquid-tight material layer (5) a carrier layer (4) a LDPE-layer (6) a barrier layer (7) an adhesive layer (9) a mixed LLDPE/LDPE-layer (8) comprising about 85% by weight LLDPE, that the mixture of LLDPE/LDPE has a melt index in the range of between 3,5 and 7 g/10 min and that the mixed LLDPE/LDPE-layer (8) is the inner layer of said package. Use of a laminate as claimed in claim 1, characterized in that the liquid-tight material layer (5) is formed by LDPE. Use of a laminate as claimed in claim 1 or 2, characterized in that the adhesive layer (9) is formed by a copolymer of ethylene and partially neutralized acrylic acid (EAA). Use of a laminate as claimed in one of the preceding claims, characterized in that the mixed LLDPE/LDPE-layer (8) has a coating weight of at least 20 g/m 2 . Use of a laminate as claimed in one of the preceding claims, characterized in that the adhesive layer (9) has a coating weight of between 1 and 15 g/m 2 . Use of a laminate as claimed in claim 4 or 5, characterized in that the combined coating weight of the mixed LLDPE/LDPE-layer (8) and of the adhesive layer (9) is at least 30 g/m 2 .

Inventors: ANDERSSON THORBJOERN, ANDERSSON, THORBJOERN, Andersson, Thorbjörn 
Applicants: TETRA PAK AB, AB TETRA PAK
Application year: 1984
Main classification: B32B27
IPC codes: B65D65, B32B27, B32B21, B32B15, B32B7