The present invention relates to a paper cover, which is oxygen-impermeable, heat-sealable cnto a container with and peelably openable along a heat-sealed seam, for a food product package; said cover being at its inside surface facing the container coated with polymer coat layers, of which the innermost layer forms a hermetically sealed and readily heat-sealable seam between said container and the cover.

A heat-sealable cover of a package is used in unit packaging of food products, in particular those of the liquid-containing kind such as yoghurt, sourmilk, milk, juices, desserts and the like. Here, the cover must provide sufficient hermeticity and hygiene. The commonest cover material has been aluminium foil whose heat-sealing onto the rims of the container has been implemented by means of lacquer.

With the increasing demands for environmental compati¬ bility set on packaging, aluminium foil covers have become handicapped by the fact that they are nondegrad- able in waste processing. The aluminium foil cover is readily torn, yet not easy to open peelably along its seam line. Furthermore, aluminium covers are produced in an energy-consuming process which is deleterious to the environment and leads to high production costs. Known in the art are food product package covers made of paper coated by plastic layers in order to attain impermea¬ bility and heat-sealability, but the shelf life of products packaged using these covers has not matched that available with the use of an aluminium foil cover.

It is an object of the present invention to provide a paper cover for a food product container, said paper

cover being manufacturable by a simple process and offering a degree of hermeticity sufficient for assuring a long shelf life even for sensitive and easily decaying ood products. The paper cover according to the invention for a food product package is characterized by having its inside coat layers formed by a polymer bonding layer, a polymer layer of oxygen-impermeable material, another polymer bonding layer and a heat-sealable polymer layer.

The essential characteristic of the paper cover according to the invention for a food product package is its capability of preventing the penetration of oxygen from ambient atmosphere through the cover into the container. In general, oxygen causes a gradual degradation of product quality by oxidizing i.a. fats, flavour substances and vitamins contained in the product. An additional benefit of the polymer layer combination used to prevent oxygen from entering the package is its capability of preventing aroma substances from escaping the package. By virtue of the heat-sealable polymer layer, the cover is readily and nontearingly openable (peelable) along the heat-sealed seam. Thence, the paper cover according to the invention for a food product package renders the product such a shelf life and ease- of-use that facilitates the use of a paper cover that can replace aluminium foil covers in the packaging of easily decaying or degrading food products.

A polymer suitable for use as an oxygen barrier in the paper cover according to the invention for a food product package is ethylvinyl alcohol polymer (EVOH) .

Exemplifying the bonding materials, which are compatible with said polymer, can be mentioned, e.g., appropriately modified polyethene that can be laminated in either one or in both bonding layers with the EVOH layer. Thickness of the EVOH layer can be varied in the range 2...15 g/m ² , preferably 4...10 g/m ² .

As alternative polymers suitable for use as an oxygen barrier can be noted polyethene terephtalate (PET) , whereby grafted polyethene can be employed as a compatible bonding material, and polyamide (PA) , whereby a copolymer of ethene and unsaturated carboxylic acid can be employed, as a compatible bonding material. Thickness of the PET layer can be in the range 5...30 g/m ² , preferably 10...20 g/m ² . Thickness of the PA layer can be in the range 3...20 g/m ² , preferably 5...10 g/m ² .

Thickness of each bonding material layer can be in the range 2...15 g/m ² , preferably 4...10 g/m ² .

The outermost heat-sealable polymer layer of the polymer multilayer system of the cover contains a polymer, such as methacrylic acid and ethylvinyl acetate modified polyethene acting as the adhering component, and a polymer such as a copolymer of ethene methacrylic acid acting as the releasing component. Thickness of the heat- sealable polymer layer can be in the range 5...40 g/m ² , preferably 10...30 g/m ² .

Paper material of the cover can be, e.g., sulfate pulp based, bleached, filler-containing and coated paper grade with a basis weight in the range 30...200 g/m ² , preferably 50...120 g/m ² .

The cover paper can be either coated or left uncoated on its other side, which is not coated by said polymer layers, and thus forms the outer side of the package.

The invention also concerns a method for producing the paper cover described above for a food product package. In the method the paper base material is simultaneously coated with several polymer coat layers of which the uppermost is of a heat-sealable polymer or blended polymer material and is characterized in that the coat

layers, which comprise a polymer bonding layer, an oxyge -barrier forming pol mer layer, a second polymer bonding layer and a heat-sealable polymer layer, are applied in molten state against each other and at the same time against a moving paper web, and in that the paper coated thereby is used for fabricating said cover by pressing the paper against the rim of a ready-filled food product container and then adhering the cover by heat-sealing.

The invention is next examined in greater detail with the help of an exemplifying embodiment illustrated in the attached drawings, in which

Figure 1 shows a food product package sealed by a paper cover according to the invention,

Figure 2 shows a cross-sectioned detail of the seam between said package and the sealing paper cover,

Figure 3 shows the cross-sectioned detail of said cover in a greater scale, and

Figure 4 shows schematically the manufacturing process of the paper cover material.

Shown in Fig. 1 is a disposable unit package 1 of a liquid-containing food product such as yoghur . The package 1 comprises a container 2 and an easily detachable (peelable) paper cover 3 according to the present invention, said cover being heat-sealed onto the rim of said container.

The container 2 can be of any plastic conventionally used in food packaging such as polypropene or polystyrene, or alternatively, cardboard coated with LD polyethene. The material of the container is irrelevant to the invention;

for practical reasons, however, its degree of impermeability can be assumed to be at least as high as that of the cover.

According to Fig. 2, the paper cover 3 which is attached by heat-sealing to the package comprises a paper web layer 4 acting as a carrier for a polymer multilayer system 5 facing the inside of the package, said multilayer system being comprised of four superimposed polymer layers of which the innermost forms a heat- sealable seam 6 between the rim of the container 2 and said cover 3. In addition to its heat-sealability, the polymer coat 5 renders the cover an easy detachment and good hermeticity, which prevents the packaged product 7 from decaying.

Fig. 3 illustrates the polymer layers forming the inside coat 5 of the cover paper 4. Closest to the paper 4 is the polymer bonding layer 8, next is a layer of oxygen- barrier forming polymer layer 9, next to it is another polymer bonding layer 10, and finally outermost from the paper, the polymer layer 11 which provides the heat- sealable seam 6. The latter is advantageously composed of a blended polymer material containing an adhering component that facilitates heat-sealing and a releasing component that facilitates the detachment of the cover along the heat-sealed seam without tearing the cover.

For the purpose of the invention, an essential impetus is given by the oxygen-barrier forming polymer layer 9 that gives the cover the impermeability required for a satisfactory shelf life of the packaged product. The polymer material of the layer 9 can be, e.g., ethylvinyl alcohol copolymer, whereby modified polyethene can be selected as the material of the polymer bonding layers 8, 10. However, other material combinations than those mentioned above are also possible.

The heat-sealable polymer layer 11 can consist of methacrylic acid and ethylvinyl acetate modified polyethene as an adhering compound and a copolymer of ethenemethacrylic acid as a releasing compound.

According to Fig. 4, the fabrication of the paper cover according to the invention takes place as a continuous process by routing a continuous paper web 4 to the nip formed between rolls 14, 15, whereby the film webs of the polymer layers 8...11 to be superimposed on the cover base material are simultaneously routed from nozzles 16 to the same nip. The nozzles 16 communicate with containers 17...19, from which the polymer bonding material (container 17) , oxygen-barrier forming polymer (container 18) .nd heat-sealab_e polymer (container 19) are fed in molten state to said nozzles. As is evident from Fig. 4, the molten polymer layers 8...11 are first aimed against each other, after which they are applied in a single run to the paper 4 in the nip formed between said rolls 14, 15. As a result a coated paper web is obtained suitable for the stamping of the covers according to the invention either prior to the fill/heat- sealing stage of the package, or alternatively, in conjunction with this stage.

For those versed in the art it is evident that the invention is not limited by the exemplifying embodiment described above, but instead, its applicability can be varied within the scope of the enclosed claims.