The present invention relates to a lid for welding on and closing a package, preferably a disposable package of a polymer material, said lid comprising a metallized layer. More particularly, the invention relates to a weldable lid which tolerates pasteurization with temperatures up to 95 °C.

Pasteurization of the contents of disposable packages poses the problem that a welded lid on the package will frequently work loose during heating because of the increasing vapour pressure. Thus, ready-punched lids (wafers) are normally not used for pasteurization, but rather top web films which have sufficient adhesion, but owing to problems of curl (rolling of the material) these cannot be punched for loose lids. Thereby, it is difficult - in particular for small cups - to make the production profitable and to make it look acceptable, since it is difficult to produce a neat punching on some cups.

The lids comprise a metallized layer which is necessary to give a sufficient oxygen barrier in the product, so that any microorganisms, which have survived the pasteurization, do not receive oxygen to a problematic extent.

It has been attempted to use COEX coating, a laminated sheet, which is produced by co-extrusion of at least two polymers, at least one of which is formed by a polyolefin. However, so far, it has not been possible to achieve sufficient adhesion to the metallization layer for it to withstand the vapour pressure.

The previous solution has been to use a welding lacquer. However, the welding lacquer used has difficulty in withstanding the heat at the pressurization, as it becomes soft and thus cannot withstand the vapour pressure which occurs in the package or the cup. European Patent Application EP 2 100 729 A1 describes a multilayer sheet which lends itself for use in packages to be heat treated, e.g. pasteurized. The layers are co-extruded to achieve a directionally oriented stretched sheet which exhibits low shrinkage when subjected to high temperatures. A layer of the sheet may be metallized or coated with a ceramic material.

The object of the invention is to provide an alternative lid for packages which are to be subjected to heat treatment.

Thus, the invention relates to a lid for welding on and closing a package, preferably a disposable package of a polymer material, said lid comprising a PET sheet with a thickness from 23 to 50 μ, on one side of which a metallized layer is applied, said metallized layer having applied thereon a COEX coating in an amount of from about 8 to about 40 g/m ² , preferably 10 to about 35 g/m ² , or preferably 10 to about 25 g/m ² , said COEX coating containing a polymer which is selected to as to be compatible with the polymer material in the package. It has surprisingly been found that with this combination which includes a special COEX coating - applied to a PET with a special thickness and a metallized layer, it may be observed that the new material may be used for weldable lids which can withstand being subjected to a pasteurization process (95 °C for at least 1 hour).

The invention provides a lid which may be secured to a package, and which will not work loose when the package with contents is subjected to a heat treatment. The polymer, which is compatible with the polymer material of the package, is preferably selected to as to correspond to the polymer material in the package. The COEX coating comprises at least two layers, where the one layer forms a welding layer, and the other layer is a layer capable of binding to the metallized layer on the PET sheet. In some embodiments, the COEX coating will comprise two layers, and in other embodiments three layers. Also, lids may be produced where the COEX coating comprises four or five layers or optionally more layers.

Additionally, the COEX coating comprises at least one acid polymer. The acid polymer is present in the layer which binds to the metallized layer. Thus, the layer may be formed by a resin which contains the acid polymer, i.e. the acid polymer fully or partly constitutes a layer in the COEX coating, said layer facing toward the metallized layer.

It has been found that the best adhesion to the metallized layer is achieved if the acid proportion in the acid polymer constitutes from about 3 to about 18 percent by weight, preferably 3 to 13 percent by weight, more preferably 3 to 9 percent by weight.

Although the acid polymer may be selected from several possible acid polymers, it has been found that acid polymers selected from EAA (ethylene acrylic acid), EMAA (ethylene methacrylic acid) or co-polymer (ethylene acrylate MAH) give a layer with a particularly good adhesion.

The welding layer in the COEX coating is made from a resin which consists of one or more of the polymers: PP (polypropylene), PET (polyester), PS (polystyrene), HDPE (high density polyethylene) and/or PVC (polyvinyl chloride). The mentioned polymers frequently constitute the material in typical disposable packages, and, therefore, it will typically be possible to select from the mentioned resins a resin which is compatible with the polymer material in the package. In addition to the welding layer and the layer binding to the metallization layer, the COEX coating may comprise one or more layers which are disposed between the two mentioned layers. The intermediate layer or layers may consist of resins which contain PE (polyethylene), EAA (ethylene acrylic acid), EMAA (ethylene methacrylic acid), PE-TER-polymer or a combination of these. The intermediate layer or layers may form a transition between the welding layer and the layer binding to the metallization layer, whereby the risk of delamination may be reduced additionally.

In connection with the selection of polymers for the COEX coating, the melt index (MFI) of the polymers may be relevant. Melt index for polymers is measured in accordance with the standards ISO 1133 and ASTM D1238. PE and PP are some of the most frequently used polymers, and for these measurements in accordance with the standards for PE are: 190 °C, 2.16 kg (ISO 1133 and ASTM D1238), and for PP are: 230 °C, 2.16 kg (ISO 1133). In an embodiment, it is preferred to use a PE in the COEX coating where the melt index (MFI) for PE is 3-15 g/10 min.

Likewise, in an embodiment, it is preferred to use PP in the COEX coating where the melt index (MFI) for PP is 10-30 g/10 min.

The difference in melt index between two layers in contact with each other in the COEX coating is important for the ability to coat in homogeneous layers with an even layer thickness, since the smaller the difference in melt index is, the easier it is to produce a coating with homogeneous layers with an even layer thickness. Therefore, the invention also provides embodiments where the difference in melt index (MFI) between two layers in contact with each other in the COEX coating does not exceed 8, preferably does not exceed 4-6.

The difference in acid content in percent by weight between layers which are in contact with each other, should not be too great either owing to the adhesion, and it is preferred to use polymers where the difference in acid content between two layers in contact with each other in the COEX coating is 7% at most, preferably 5% at most. The new lid provides the following advantages:

It is possible to use a thinner PET sheet. It is e.g. possible to use 36 μ PET, it having been found to be possible to use 23 -50 μ PET. 50 μ PET is used on the wafers known till know.

According to the invention, the welding layer used is a co-extruded polymer layer - a COEX coating - which consists of at least two layers, but which preferably consists of 3 layers. Between 10 and 25 g/m ² co-extruded polymer layer may be used as a welding layer, e.g. 22 g/m ² . Depending on how the equipment is configured, it is possible to reduce the consumption to e.g. 5-17 g/m ² , or right down to e.g. 10-15 g/m ² .

The COEX coating adheres so well to the metallization at the top, and the welding side so well to the package, e.g. a cup, that it does not come loose because of the temperature, and it has so far been found that the vapour pressure in the cup does not knock off the lid according to the invention either.

The metallization layer on the PET film is generally necessary to give a sufficient oxygen barrier in the product, so that any microorganisms, which have survived the pasteurization, do not receive oxygen to a problematic extent, which may be the case if a non-metallized film is used. The metallization layer may e.g. consist of aluminium which has been vapour- deposited on the PET film. It is possible to produce a weldable lid which can withstand pasteurization, which means a temperature of 95 °C for at least 1 hour.

It is possible to produce a lid with a low grammage - e.g. 36 μ corresponding to 50.4 g + 22 g coating = total weight of 72.4 g/m ² , but with an optimum weight of about 65-67 g/m ² . The previously known materials (which have difficulty in withstanding pasteurization) have a weight of PET 50 μ = 70 g and 3-6 g/m ² , i.e. a total weight of 73 to 76 g/m ² .

COEX coatings may be produced e.g. for welding against: PP (polypropylene), PET (polyester); PS (polystyrene), HDPE (high density polyethylene) as well as PVC (polyvinyl chloride).

The invention will now be explained more fully with reference to the figures, in which:

Figure 1 shows the structure of a traditional lid with welding lacquer;

Figure 2 shows the structure of the lid according to the invention; and

Figure 3 shows a lid according to the invention with a COEX coating with 3 layers.

Figure 1 shows the layers in a traditionally produced weldable lid 1 , which may be used for packages where the contents are to be pasteurized.

The lid comprises a layer of PET 2 with a thickness of 50 μ. The PET layer 2 has applied thereon a metallized layer 3, on which a layer 4 with welding lacquer in an amount of 3-6 g/m ² is applied in turn.

Figure 2 shows the layers of a lid 10 according to the invention. This lid also comprises a layer of PET 12, which may have a thickness of about 23-50 μ. The layer 12 has applied thereon a metallized layer 13, which may e.g. consist of aluminium. In addition, the lid has applied thereon a COEX coating 4, which is the layer which is to bind to the package so that the lid closes and seals the package, also when it is heat treated. The layer 14 consists of two layers 15 and 16, which are co-extruded on the metallized layer 13, said layer 15 being an acid polymer, and said layer 16 being a polymer selected so as to be capable of binding to the package for which the layer is to be used. The COEX coating may be applied with a thickness of about 10-25 g/m ² .

Figure 3 shows a structure of a lid 20 according to the invention, where the COEX coating 24 comprises 3 layers. The lid 20 comprises a PET layer 22 of about 36 μ, on which a metallized layer 23, preferably consisting of aluminium, is applied. The metallized layer 23 is in contact with the COEX coating 24, which comprises the three layers 25, 26, 27. The layer 25 is an acid polymer with a melt index of about 13, while the intermediate layer 26 has a melt index of about 8, and the lowermost layer 27 has a melt index of about 4. The lowermost layer 27 may consist of a PE, and will be the layer by which the lid 20 is welded to the package.

The layers 25 and 27 would have difficulty in being in direct contact with each other because of the great difference in melt index for the layer 25 and the layer 27, respectively. The difference in melt index will mean that it will be very difficult or impossible to achieve homogenous layers with an even layer thickness in the COEX coating. The intermediate layer 26 with a melt index which is between the melt index of the layers 25 and 27, makes it possible to apply a co-extruded coating with homogeneous layers with an even layer thickness.

For the lowermost layer in the lid according to the invention (corresponding to the layer 27 in figure 3), the polymer is selected so as to be compatible with the material in the package on which the lid is to be welded. Some examples of suitable polymers for the lowermost layer in the lid in connection with packages of specific materials are stated below. A skilled person will readily be able to teach other suitable materials for use in connection with specific package materials.

When the material in the package is PP (polypropylene), the polymer in the lowermost layer may e.g. be PP or PE (polyethylene). When the material in the package is PET (polyester), the polymer in the lowermost layer may e.g. be PET.

If the material in the package is PS (polystyrene) or PVC (polyvinyl chloride), the polymer in the lowermost layer may e.g. en LDPE (low density polyethylene).

When the material in the package is HDPE (high density polyethylene), the polymer in the lowermost layer may e.g. be LDPE or HDPE.