TECHNICAL FIELD

The present invention relates to a packaging laminate for a packaging container for a food product, which packaging laminate comprises a bulk layer, outer heat sealable layers, and an electrically conductive layer. The invention also relates to a packaging container formed from the packaging laminate.

PRIOR ART AND PROBLEMS In the food packaging industry, there is a general strive to replace the aluminium foil that is commonly used today in many packaging laminates and packaging containers, especially for products for long shelf life and/or products stored at ambient conditions. In many countries, use of aluminium is considered undesired from environmental and recycling points of view. As used today, the aluminium foil has a dual function in such packaging laminates. Firstly, it acts as a gas barrier layer, and secondly it acts as an electrically conductive layer that enables heat sealing of outer heat sealable layers, by inductive heating. Extensive development is ongoing to find other types of materials that may function as gas barriers, and many alternative materials exist today, such as materials based on ethylene vinyl alcohol copolymers, polyvinyl alcohol and others. The second aspect, i.e. the enabling of inductive heat sealing, is however tricky to achieve and instead other sealing methods are used to seal packaging laminates that do not contain aluminium foil. Such other methods however have drawbacks and therefore it would be highly desired to be able to present an aluminium-free packaging laminate, preferably but optionally having an alternative gas barrier layer, which packaging laminate could still be sealed by inductive heat sealing.

For an alternative material enabling inductive heat sealing of a commercially produced packaging laminate, and for a packaging container formed there from, a number of requirements must be fulfilled. For example, the material must be stable, processable, producible in large amounts, and suitable for use in connection with food products.

WO 03/063548 relates to an agent for heating materials comprising (a) at least one plurality of electrically non-conductive susceptors and (b) at least one plurality of electrically conductive susceptors, in a polymeric or ceramic matrix. According to one aspect, the electrically conductive susceptors may be comprised of particles of an intrinsically conductive polymer.

Intrinsically conductive polymers (ICP's) are a type of polymeric material that has been under development for some time, but that in recent years has been improved to enable electrical conductivities that in some cases can compete with the electrical conductivities of metals. In year 2000, the Nobel Prize in chemistry was awarded the discovery and development of such conductive polymers. Intrinsically conductive polymers are used today in for example anti-static photographic films, batteries, transistors, displays and EMI and RFI shielding of aircrafts, but are to the knowledge of the present inventors not used today in packaging laminates and packaging containers for food products. ICP's are sometimes also referred to as inherently conducting polymers, organic metals, synthetic metals, etc.

BRIEF SUMMARY OF THE INVENTION

The present invention has for its object to provide a packaging laminate and a packaging container, by which the above mentioned problems are alleviated or eliminated.

Specifically, the present invention aims at providing an aluminium-free packaging laminate that all the same enables for inductive heat sealing of the laminate, as well as providing a packaging container formed from such a laminate.

These and other objectives are achieved by a packaging laminate according to the introduction, in which the electrically conductive layer is comprised of an intrinsically conductive polymer.

In connection with the development of the present invention, it has surprisingly been shown that intrinsically conductive polymers can be used to achieve inductive heat sealing in an aluminium-free packaging laminate comprising outer heat sealable layers. Successful tests have been made in which strong and durable seals have been produced in such a laminate.

The invention also relates to a packaging container for a food product, preferably a liquid or flowing food product, which packaging container is formed and sealed from a packaging laminate according to the invention. In the high-speed, continuous packaging processes well known for the paperboard packages of Tetra Brik® -type, a web of the packaging laminate is continuously formed into a tube, filled with contents and sealed off to pillow-shaped packaging containers by a simultaneous heat sealing and cutting operation. The pillow-shaped packaging container is then normally foldformed into a parallellepipedic packaging container. Any other type of packaging containers for food products, formed from a packaging laminate that is susceptible to heat sealing by inductive heating, is however also conceivable, such as packaging containers having a container body of such a laminate and a plastic top, e.g.

DETAILED DESCRIPTION OF THE INVENTION According to one aspect of the invention, the electrically conductive layer is an essentially homogeneous layer formed from the intrinsically conductive polymer. However, minor amounts of additives, such as adhesion promoting additives, may be allowed in the electrically conductive layer. Typically, the electrically conductive layer of an intrinsically conductive polymer may comprise up to 10 % by weight at the most, preferably 5 % by weight at the most and even more preferred 3 % by weight at the most, of one or more such additives, which additive(s) is/are preferably comprised of non-conductive material(s).

According to one embodiment of the invention, the electrically conductive layer is a continuous layer that extends throughout the entire laminate. However, according to another preferred embodiment of the invention, the electrically conductive layer is arranged only in sealing areas of the packaging laminate. This may be achieved since it is technically possible to apply the electrically conductive layer only at positions of the laminate that are to be sealed to each other when the laminate is to be formed into a packaging container, i.e. in the sealing areas. This is also beneficial since it reduces the amount of intrinsically conductive polymer needed, which intrinsically conductive polymer does not

anyhow add barrier functions to the laminate (as opposed to the aluminium foil layer commonly used today).

According to another aspect of the invention, the intrinsically conductive polymer is chosen from the group that consists of polyaniline, polypyrrole, polythiophene, polyethylenedioxythiophene, poly(p-phenylene vinylene), polyacetylene, polydiacetylene, polyphenylene sulphide, polythienylene vinylene, and polyvinylchloride. Preferably the intrinsically conductive polymer is doped to enhance its conductivity. However, the examples of intrinsically conductive polymers just given are only to be seen as examples feasible today. As development within the field of intrinsically conductive polymers progresses, new types or improved versions of existing types of intrinsically conductive polymers will surely be found, and such polymers will then also be part of the present invention.

According to yet another aspect of the invention, the intrinsically conductive polymer used can be defined by its electric conductivity. Accordingly, it should have an electric conductivity of at least 1 S/cm, preferably at least 10 ² S/cm, even more preferred at least 10 ⁴ S/cm and most preferred at least 10 ⁶ S/cm. It is difficult to define the thickness needed of the electrically conductive layer of the intrinsically conductive polymer, since the required thickness may vary dependent of type of polymer and the application. However, a thickness of 10 μm at the most, preferably 5 μm at the most, even more preferred 2 μm at the most and most preferred 1 μτa at the most, is generally expected to be enough.

The electrically conductive layer of the intrinsically conductive polymer may have any suitable position within the packaging laminate, as seen over the cross-section of the packaging laminate. According to one aspect, it is bonded to the bulk layer by means of a laminating layer. Such a laminating layer may be a known type of (a usually polymeric) barrier layer (other than aluminium), preferably a gas barrier layer, or a combination of barrier layers. Alternatively, tie layers are arranged between the bulk layer, any barrier layer(s) and the electrically conductive layer of the intrinsically conductive polymer. Suitably, the electrically conductive layer of the intrinsically conductive polymer is arranged in juxtaposition with the inner sealing layer(s) of the laminate, but it is also conceivable that one or more barrier layers and/or tie layers is/are arranged inbetween.

According to another aspect of the invention, it is preferred that the bulk layer is a paper or paperboard bulk layer. It is however also conceivable that it might be a polymeric layer. In case of a polymeric bulk layer, it is even possible to achieve a transparent, optionally coloured, packaging laminate that is sealable by inductive heat sealing.

The inner and outer heat sealable layers are suitably comprised of a thermoplastics in the group that consists of polyethylene, preferably metallocene polyethylene, and polypropylene. Use of polypropylene based heat sealable layers is for example plausible in the case or retortable packaging containers. Use of metallocene polyethylene is primarily of interest in the innermost heat sealable layer, i.e. the layer that will face the food contents of the packaging container formed from the packaging laminate.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The present invention will now be described in greater detail herein below, with reference to one preferred embodiment and with reference to the accompanying

Drawings. In the accompanying Drawings:

Fig. 1 is showing a cross-section of the structure of a preferred basic packaging laminate according to the invention; Fig. 2 is showing a cross-section of an alternative embodiment of a packaging laminate according to the invention.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 is schematically showing a preferred basic concept of a packaging laminate according to the invention. The packaging laminate 10 has a rigid, but foldable bulk or core layer 11 of paper or paperboard, as well as an outer, liquid-tight and heat sealable coating 13 and an inner, liquid-tight and heat sealable coating 12 on the respective sides of the bulk layer 11. The inner coating 12 will form the inside of the packaging container that is formed from the packaging laminate and is preferably composed of metallocene polyethylene. The packaging laminate 10 further displays an electrically conductive layer 14 of an intrinsically conductive polymer, disposed between the bulk layer 11 and the inner, liquid-tight coating 12. Suitably, the

electrically conductive layer 14 has been laminated to the bulk layer 11 by means of an intermediate laminating layer 15 of a thermoplastic material such as a polyethylene or polypropylene based material e.g. Optionally, a surface activation treatment such as corona, flame or plasma treatment and/or a tie layer is used to promote adhesion of the electrically conductive layer 14 of an intrinsically conductive polymer to the bulk layer 11 and/or to the inner heat sealable layer 12. In the embodiment of the packaging laminate 20 shown in Fig. 2, it has been schematically indicated that a dedicated barrier layer 16, preferably a gas barrier layer, may be present between the bulk layer 11 and the electrically conductive layer 14 of an intrinsically conductive polymer. It is also conceivable however (but not shown) that the barrier layer 16 is arranged between the electrically conductive layer 14 of an intrinsically conductive polymer and the inner heat sealable layer 12. The gas barrier layer 16 may be formed from any known gas barrier material except aluminium foil, such as ethylene vinyl alcohol copolymers, polyvinyl alcohol or others.

Tie layers or laminating layers (not shown) may be included in the packaging laminate as required.

Many other variants of the packaging laminate are conceivable, all having in common though that they have a polymer or paperboard bulk layer, an electrically conductive layer of an intrinsically conductive polymer, and outwardly facing surfaces of a heat sealable polymer.

The electrically conductive layer 14 of an intrinsically conductive polymer is suitably applied by dispersion coating, by means of any dispersion coating technique known per se. Optionally, it may be coated onto a carrier layer that is thereafter laminated into the packaging laminate. It is also conceivable, and preferable, that the electrically conductive layer 14 of an intrinsically conductive polymer is arranged only in the sealing areas of the packaging laminate.

EXAMPLE Two packaging laminates of the basic concept shown in Fig. 1 were heat sealed by inductive heating. A reference packaging laminate having an aluminium foil layer instead of the electrically conductive layer of an intrinsically conductive polymer

was also tested. Table 1 indicates the frequency tested for the induction heating equipment, as well as sealing times required to achieve a strong and durable seal. As can be seen, the known use of aluminium foil gave the shortest sealing times, but primarily the test showed that it was possible at reasonable times to achieve acceptable seals also when using an electrically conductive layer of an intrinsically conductive polymer instead of the aluminium foil layer. With further optimising, sealing times and other sealing conditions will be improved for the electrically conductive layer of an intrinsically conductive polymer.

Table 1

1) Ba)TtTOn P = poly(3,4-ethylenedioxythiophene) - (poly(styrene sulfonic acid))

2) PANI = polyaniline-dodecylbenzene sulfonic acid

By way of conclusion it should be observed that the present invention which has been described above with particular reference to the accompanying drawings, is not restricted to these embodiments described and shown exclusively by way of example, and that modifications and alterations obvious to a person skilled in the art are possible without departing from the inventive concept as disclosed in the appended claims.