The invention relates to a multi-layered heat-sealable material for structures closed by sealing and capable of being opened from the sealing point by pulling free, such as for heat-sealable product packages, whose material comprises at least, in the order mentioned, a base layer, an inner polymeric support layer and an outer, polymer-based heat-sealing layer. In addition the invention relates to a heat-sealed product package in which the material in question is used.

In consumer packages, such as cup-, goblet- or. box-models of foodstuffs packages, lids fastened to the mouth of the package by heat-sealing are used, wherein the lids are to be pulled free when opening the package. In order to be workable the lid must come off essentially in one piece from the sealing point between the mouth of the package and the inner surface of the lid. Not only in package lids, such a material, which is heat-sealable and removable from the point of sealing, can also be used in packaging containers which are provided with a lid of the same or some other material, or in container packages formed by folding or bending the material, such as bag containers which are closed by heat-sealing the material against itself.

Package lids that are heat-sealable and removable in the manner described above by pulling free (peelable) are made especially of aluminium, but also in growing amounts of comparably more environmentally friendly polymer-coated papers and cardboards, and their main applications for use have been for consumer packages of liquid foodstuffs such as milk products, juices and water, edible fats and prepared food. The container packages in which these lids are used have typically been of polymer such as polyolefin, polyester or polystyrene, or polymer-coated cardboard.

From US patent publication 5 626 929 is known a polymer- or aluminium foil- based heated-sealable container lid, where in the heat-sealed layer are mixed ethylene homopolymer or copolymer, such as for example ethyl methyl acrylate (EMA), butene ethylene copolymer and an inorganic filler material, such as for example talcum. According to the publication the container is of plastic, such as polyethylene or the better-recommended polypropylene.

Examples of known heated-sealable and by pulling free removable lids made of polymer-coated papers or cardboards are clarified for example in Finnish patent publications 97285 and 104887, Finnish patent application 93 AAl β and publication WO 95/34489. In the multi-layered coating structures of these lids an attempt has

been made to achieve not only the demanded heat-sealability and pulling-free ability, but additionally the air- and aroma lock required for preservation of the product as well as prevention of the curving of the lid that hinders the packing procedure. In the lids binding agent layers may also have been needed to bind the polymer layers, which serve various purposes, to the base paper or cardboard as well as to one another.

In the development of lid materials an aim has also been to find polymer compositions for the innermost heat-sealing layer of the lid, wherein these compositions would seal as well as possible to the different surface materials used in container packages, such as different qualities of polymers, or uncoated cardboard, glass or even metal. The ability of know lid materials to seal to polyethylene, which is one of the most common heat-sealable coating polymers used in plastic and cardboard packages, has however had a problem with sticking too tightly, which makes opening the lid by pulling it free difficult and complicates removal of the lid in one piece.

The object of the invention is therefore to form a simply-structured layered aluminium foil-free packaging material, which can be sealed together tightly and securely to polyethylene over a wide range of temperatures such that the adhesive strength of the seal allows the material to easily be pulled free in one piece without producing the fibre-like hairs due to tearing in the heat-sealed layer, which are a known commonly-occurring disadvantage of heat-sealing compositions. The multi- layered structure of a material according to the invention comprises in a manner known per se a base layer, which basically gives the material its required stiffness, a heat-sealing layer, with which the required heat-sealing and pulling-free abilities of the material are achieved, and a polymeric support layer between these two that works as a bond between the heat-sealing layer and the base layer and with which the material gains its desired product-protecting steam-, gas- and aroma lock characteristics. A material according to the invention is characterized in that in the heat-sealing layer are mixed polystyrene, acrylate copolymer, and a fine inorganic substance.

The said inorganic substance produces a cohesive strength in the seal, which loosens the tightness of the seal in a controlled manner. In addition to polyethylene, a material according to the invention has been found to be capable of being heat- sealed in a peelable manner to other polymers as well, such as polypropylene and

polyethylene terephthalate, as well as to itself when heat-sealing layers are brought together against one another.

The base layer of a material according to the invention is most preferably of a fibre- based material, such as paper or cardboard.

As the material of the support layer between the base layer and the outer heat- sealing layer may be for example polyolefin, such as for example polypropylene, polyamide, polyester, such as for example polyethylene terephthalate (PET), or ethylene vinyl alcohol polymer (EVOH). Polyamide, PET or EVOH give the layer advantageous gas-lock characteristics. If desired, the material may comprise between the base layer and the heat-sealing layer a polymeric multi-layered structure implementing various functions.

In the heat-sealing layer a suitable acrylate copolymer is for example ethyl methyl acrylate (EMA), and the fine inorganic substance evenly-dispersed in this layer can be for example talcum. For the acrylate copolymer, possibilities can also include for example ethyl butyl acrylate (EBA) or ethyl ethyl acrylate (EEA). As alternatives to talcum, there may be mentioned for example mica or calcium carbonate. A composition that has been noticed to work well is one in which are mixed approximately 17.5 % polystyrene, approximately 52.5 % ethyl methyl acrylate, and approximately 30 % talcum.

The support layer and the heat-sealing layer are to be applied to the base layer, such as to packing paper or cardboard, simultaneously by coextrusion.

A heat-sealed product package according to the invention, comprised of the packaging material described above, may be formed from a container that is closed by a lid of the material in question that is heat-sealed to the mouth of the container and removable from there by pulling it free.

A package according to the invention is in this case a cup- or box-model foodstuffs package whose container portion is either entirely of polymer material or cardboard having an inner surface with a polymer coating. In cardboard the outer surface may also be polymer-coated. As a polymer for the container or its inner and outer surfaces over which the lid is adhered from its heat-sealing layer, polyethylene is particularly suited, but a lid according to the invention also works well when sealed to other polyolefins, polystyrene or polyester, such as polyethylene terephthalate.

Along with said lidded packages the invention comprises a heat-sealed product package which is formed of the layered packaging material described above by bending or folding and by heat-sealing the heat-sealing layers brought to be against the material to one another along the sealing line. Considered in particular are bag packages shut by heat-sealing and capable of being opened by a pulling directed to the sealing point.

In the following the invention is described in detail with the aid of examples with reference to the accompanying drawing, in which

Fig. 1 shows a box-model product package according to the invention, suitable for example for a package of edible fat or prepared food and provided with a lid that can be pulled free,

Fig. 2 is a section H-II of Fig. 1, showing a cross-section of the sealing point betweer* the mouth of the container and the lid,

Fig. 3 is a schematic cross-section IH-III of Fig. 2, showing the multi-layered structure of a material according to the invention which is used in the lid,

Fig. 4 shows a sheet of packaging material according to the invention, which can be folded and heat-sealed into a bag package, and

Fig. 5 shows a bag package obtained from the sheet of Fig. 4 in partially opened state.

A box-model package 1 as shown in Fig. 1 and 2, which typically forms for example consumer packages of edible fat 2, comprises a container portion 4 having an edge flange 3 as well as a lid 5 fastened to the edge flange by heat-sealing. Package 1 can be opened by pulling on the extending lip 6 on the package edge 3 that is a part of the Hd 5. When pulling free, the sealing point 7 gives way such that the lid 5 is removed in one piece from the mouth of the container 4.

The multi-layered lid 5 comprises in the embodiment shown a base layer 8, which is for example paper, against which is a polymeric support layer 9, to which, using the chosen polymer, an air- or aroma lock protecting the packaged product 2 can be achieved, and the polymer-based heat-sealing layer 10 forming the inner surface of the lid, from which the lid is sealed to the edge flange 3 of the container. According to the invention in the heat-sealing layer 10 is mixed polystyrene, aery late copolymer, and a fine inorganic substance.

The container portion 4 contained in the product 2 is in the exemplary embodiment packaging cardboard 11, whose inner surface has a layer of polymeric coating 12 that seals together with the heat-sealing layer 10 of the lid. As a suitable polymer for the coating 12 for example polyethylene can be mentioned.

Fig. 3 shows a schematic cross-section of a multi-layered packaging material according to the invention, such as is used in a product package lid 5 according to Fig. 1 and 2. The base layer 8 of the material is paper, whose weight is for example 40-120 g/m ² . Depending on the intended use, the base layer can also be cardboard having a weight for example 130-500 g/ m ² . Onto the fibre-based base layer 8 is applied by extrusion a two-layer polymer coating 9, 10, of which the inner support layer 9, located against the base layer 8, is for example polypropylene. Alternatively as the material of support layer 9 can be chosen for example some known air- sealant polymer, such as for example polyamide, PET or EVOH. The weight of the support layer 9 is for example 10-30 g/m ² . In the outer heat-sealing layer 10 is mixed 8-30 % polystyrene, 30-70 % acrylate copolymer, and 10-40 % a fine inorganic substance. A suitable mixture composition is for example 17.5 % polystyrene, 52.5 % ethyl methyl acrylate, and 30 % fine talcum. The weight of the heat-sealing layer 10 can be for example 5-20 g/m ² .

Depending on the intended use the layer composition of a material according to the invention may differ from that presented above. It is also possible to add to the structure polymeric barrier layers as well as possible layers of a binding substance between the layers as needed.

A bag package 13 that can be effected as in Fig. 4 and 5 can be made for example of a sheet 14 of packaging material having a multi-layered structure of Fig. 3. The sheet 14 is folded double along the fold line 15 such that the heat-sealing layers 10 of the material come to be across from one another in the folded sheet. The folded sheet is heat-sealed along the edges into a closed bag package 13. The bag 13 can be opened by taking hold of the sides of the bag 16 and pulling them away from each other, wherein the sealing line 17, in those areas where the pulling force is exerted, gives way. In Fig. 5 can be seen a bag 13 partially opened at its mouth 18.

Example 1

The peelable multi-layered structure was produced using coextrusion techniques by coating Lumiflex paper of basis weight 90 g/m ² . 15 g/m ² polypropylene WG341C was used for the support layer, the peelable layer being 10 g/m ² in strength. The

composition of the peelable layer was 17.5 % polystyrene, 52.5 % ethyl methyl acrylate, and 30 % talcum.

The web speed during coating was 100 m/s. The temperature profile of the polypropylene extruder was 240-290-315-314-315-315-285-285-285 ⁰ C when the melt temperature was 315 ⁰ C. The temperature profile of the extruder feeding the peelable mixture was 200-240-290-290-290-290-275-275-275 ⁰ C when the melt temperature was 290 ⁰ C and the temperature of the nozzles was 300 ⁰ C.

The resulting multi-layered coated paper was sealed against itself, wherein a bag- like flexible package was created. The peelability of the seal was tested using a T- peel test on a Hounsfield pulling apparatus. The seal tester was a Kopp Hot Tack -seal tester and the sealing time was 0.5 s. When the seal temperature was 9O ⁰ C, the strength needed for peeling was 150 N/m, at a temperature of 100 ⁰ C 180 N/m, at a temperature of 110 ⁰ C 250 N/m, at a temperature of 120 ⁰ C N/m and at 130 ⁰ C 220 N/m.

Example 2

The suitability of the peelable layered material as the lid material for different types of cardboard cups was tested by sealing a layered material produced as in Example 1 to coated cardboard cups.

From the material produced were cut lids which were then sealed using a cup sealer. The sealing pressure was 2 bar and the sealing time 0.6 s. The width of the sealing jaw was 3 mm. When sealing the material to polyethylene-coated cardboard cups, the lid peeled when the seal temperature was 145-180 ⁰ C. When sealing the material to polystyrene-coated cardboard cups, the lid peeled when the seal temperature was 145-185°C.

Example 3

The suitability of a peelable layered material produced as in Example 1 as the material for different types of peelable cardboard packages was tested by sealing it against coated cardboard.

The material was tested by sealing it to coated cardboard using a Kopp Hot Tack -sealing apparatus. The width of the jaw of the sealer was 20 mm, the sealing time 1.0 s and the sealing force 100 N (0.5 bar). The temperature area, in which a peelable seal was achieved, was when sealing to polyethylene-coated (PE-)

cardboard (Cupforma PE 210+15) 100-130°C, when sealing to polyethylene terephthalate-coated (PET-) cardboard (Trayforma WPET 350+40) 130-140 ⁰ C, and when sealing to polypropylene-coated (PP-) cardboard (Trayforma PP 420+20) 130-140°C.