WO1998052751A1

WO1998052751A1 - A method of producing a well-integrated packaging material comprising layers of aluminium and plastic

Info

Publication number: WO1998052751A1
Application number: PCT/SE1998/000933
Authority: WO
Inventors: Knut Rosenberg
Original assignee: Tetra Laval Holdings & Finance S.A.
Priority date: 1997-05-22
Filing date: 1998-05-19
Publication date: 1998-11-26
Also published as: AU7680698A; SE9701909D0; SE9701909L

Abstract

The disclosure relates to a method of producing a packaging laminate comprising layers of plastic and aluminium, in which method an aluminium-clad web of paper is coated with a first plastic layer which is extruded on the aluminium-clad side of the paper web, whereafter the thus plastic-coated paper web is coated with a second plastic layer which is extruded on the first plastic layer applied on the paper web. Increased adhesion between the extruded first plastic layer and the foil cladding of the paper web is achieved by extending the contact time between the warm, extruded first plastic layer and the foil cladding of the paper web and extruding the second plastic layer while the first plastic layer is still warm.

Description

A METHOD OF PRODUCING A WELL-INTEGRATED PACKAGING MATERIAL COMPRISING LAYERS OF ALUMINIUM AND PLASTIC

TECHNICAL FIELD The present invention relates to a method of producing a laminated packaging material comprising layers of plastic and aluminium, in which method an aluminium-clad paper web is coated with a first plastic layer which is extruded on the aluminium- clad side of the paper web, and in which method the thus plastic-coated paper web is coated with a second plastic layer which is extruded on the first plastic layer of the paper web.

BACKGROUND ART

A laminated packaging material comprising layers of plastic and aluminium is previously known in the art and is often employed for aseptic packages. More precisely, the prior art aseptic packaging material includes a core layer of rigid, but foldable paper and outer, liquid-tight coatings of plastic. Between the core layer and the one of the two outer, liquid-tight plastic coatings, the packaging material has an aluminium foil which serves as oxygen gas barrier and which, by means of a suitable sealing layer, is bonded to the core layer on the one side of the core layer, i.e. that side which is intended to face inwards when the packaging material is reformed into aseptic packages.

The above-described prior art packaging material is produced by a lamination/ extrusion process in which a web of paper is clad with an aluminium foil which is permanently bonded to the one side of the web by means of a suitable sealing or adhesive layer which is extruded between the paper web and the aluminium foil. The aluminium-clad paper web is thereafter coated with a first plastic layer which is extruded on the aluminium- clad side of the paper web in connection with the paper web being led through a nip between a first, smaller press roller and a first, larger counter roller which is cooled internally by water at approx. 8-10°C for rapidly solidifying the first extruded plastic layer. The thus plastic-coated paper web is thereafter coated with a second plastic layer which is extruded on the first plastic layer of the paper web in connection with the plastic - coated paper web being led through a nip between a second, smaller press roller and second, larger counter roller which is likewise cooled internally by water at approx. 8-10°C for rapidly solidifying the second plastic layer.

Even though the prior art packaging material as disclosed above possesses good integrity (i.e. displays good adhesion between the individual material layers included in the packaging material), it has surprisingly proved that the adhesion between these material layers, in particular the adhesion between the aluminium layer of the packaging material and the first plastic layer, may be improved considerably using only simple means.

OBJECT OF THE INVENTION One object of the present invention is therefore to realise such a laminated packaging material possessing improved internal cohesion or integrity.

SOLUTION This object is attained according to the present invention in that the method disclosed by way of introduction has been given the characterizing feature as set forth in appended Claim 1.

Advantageous characterizing features of the method according to the present invention are further apparent from the appended subclaims.

OUTLINE OF THE INVENTION

In accordance with the present invention, it has been ascertained by practical experiments that the adhesion which is achieved between the aluminium foil and the first plastic layer of the packaging material is greatly dependent upon the time during which the extruded first plastic layer in the molten state is in contact with the aluminium foil. In particular, it has proved that this adhesion increases the longer the contact time between the extruded molten plastic layer and the aluminium foil.

The extended contact time between the aluminium foil and the first molten plastic layer which promotes adhesion will be achieved according to the invention by avoiding cooling the first larger counter roller at the application nip for the first plastic layer. While the outer peripheral surface of the counter roller was previously kept at a temperature of approx. 25°C by internal cooling with water at approx. 8-10°C, the outer peripheral surface of the counter roller should thus, according to the present invention, be kept at as high a temperature as possible for as long a contact time as possible in order to achieve maximum adhesion between the aluminium foil and the first plastic layer.

According to one preferred embodiment, the outer surface of the first counter roller is thus kept warm by means of heat which is transferred from the extruded first plastic layer. In certain cases, it may also be appropriate to heat the roller internally, for example by hot oil, in order to speed up the start procedure and achieve stable operational conditions. If the first plastic layer consists, for example, of polyethylene which has an extrusion temperature of about 300°C, this implies that the outer peripheral surface of the first counter roller is heated to and kept at a corresponding temperature of approx. 300°C.

Since the extruded plastic melt has a tendency to fasten to a surface which is not cooled (in this case the outer peripheral surface of the first counter roller), it is important according to the present invention that the outer counter roller surface is clad by a coating which reduces or eliminates the tendency of the extruded plastic layer to adhere to the surface. One preferred example according to the present invention of such a material is TEFLON®.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings, in which:

Fig. 1 is a schematic cross section through a conventional laminated packaging material; and

Fig. 2 schematically illustrates a method of producing the laminated packaging material of Fig. 1 according to the present invention.

In order to facilitate a comparison between the figures in the accompanying Drawings, the same reference numerals have been employed for identical or similar details in the two figures.

DESCRIPTION OF PREFERRED EMBODIMENT

Fig. 1 thus schematically illustrates the make-up of a conventional packaging material for so-called aseptic packages. The laminated packaging material carrying the generic reference numeral 10 comprises a core layer 11 of rigid, but foldable paper of conventional packaging quality for such dimensionally stable packages of the single use disposable type. The packaging material 10 has further two outer, liquid-tight coatings 12 and 13 of plastic, preferably polyethylene, on both sides of the core layer 11. Between the core layer 11 and the one outer plastic coating 12 of the packaging material, there is further a foil 14 of aluminium which serves as an oxygen gas barrier and which, with good adhesion, is bonded on the one hand to the core layer 11 by the intermediary of an interjacent layer 15 of suitable lamination or adhesive agent, and, on the other hand, to the above- mentioned one outer plastic coating 12 by the intermediary of an interjacent layer 16 of plastic, preferably polyethylene.

From a laminated packaging material 10, dimensionally stable aseptic packages are produced using modern packing and filling machines of the type which, from a web or from prefabricated sheet blanks of the packaging material, form, fill and seal finished packages under sterile (aseptic) conditions. From, for instance, a web, aseptic packages are produced in that the sterilized web is, under sterile or aseptic conditions, reformed into a tube by both of the longitudinal edges of the web being united to one another in an overlap joint seal. The tube is filled with sterilized contents, for example liquid foods, and is divided into closed packages by transverse thermoseals of the tube below the level of the contents in the tube. The packages are separated from one another by incisions or cuts in the transverse sealing zones and are given the desired geometric configuration by an additional forming and sealing operation.

An extremely well-known aseptic package of the type described above is TETRA BRIK® which is often employed for packing and transporting liquid, sterilized foods with extended shelf life.

Fig. 2 illustrates how the packaging material 10 in Fig. 1 may be produced using the method according to the invention.

A web 20 of foil-clad 14 paper 11 with an outer coating 13 of plastic, for example polyethylene, is unwound from a magazine reel (not shown) and is led through a nip at 21 between a first, smaller press roller 22 and a first, larger counter roller 23, at the same time as the foil-clad side 14 of the web 20 is coated with a first layer 16 of plastic, e.g. polyethylene, which is extruded on the foil-clad side of the web by means of an extruder 24 disposed in connection with the nip 21.

In order to achieve improved adhesion between the foil 14 and the extruded, first plastic layer 16, the first larger counter roller 23 is allowed to be heated by the extruded plastic film 16 so that the outer peripheral surface of the counter roller 23 in contact with the extruded first plastic film 16 is kept at a temperature corresponding to the extrusion temperature of the plastic film 16. If the extruded plastic film 16 consists, for instance, of polyethylene, this implies that the surface of the counter roller 23 is heated to and kept at approx. 300°C.

As has been mentioned above, there is a risk that the extruded, molten plastic film 16 may adhere (so-called sticking) to the surface of the counter roller 23 unless the counter roller is cooled, as in the prior art technique. However, a cooling of the roller entails that the molten plastic film 16 is instantaneously cooled and hardens on contact with the cooled roller surface, whereby the contact time between the aluminium and the molten plastic is reduced, with consequentially poorer adhesion between the two layers.

However, this problem, is solved in an extremely elegant and efficient manner according to the present invention in that the surface of the counter roller 23 is clad or coated with TEFLON® (polytetrafluoroethylene) which is a plastic material which is wholly repellent of or incompatible with polyethylene which cannot, therefore, adhere to it even if the contact time between the two materials is increased. The thus created increased contact time without the risk of so-called sticking results, according to the present invention, in increased adhesion between the aluminium foil 14 and the extruded first plastic film 16 as compared with the prior art technique employing a cooled roller surface as has been previously explained.

The plastic coated web is removed from the TEFLON®-clad first counter roller 23 and is led through a second nip at 25 between a second, smaller press roller 26 and a second, larger counter roller 27 at the same time as the still warm first plastic film 16 is coated with a second plastic film 12 which is extruded on the first plastic film 16 by means of an extruder 28 disposed in connection with the second nip 25. The pair of second press roller 26 and second counter roller 27 is substantially identical with the pair of the first press roller 22 and first counter roller 23, apart from the fact that the second counter roller 27 is cooled internally by water at 8-10°C, and, as a result, has a surface temperature of approx. 30°C instead of the approx. 300°C as in the first counter roller 23. It often happens that the applied plastic film 26 is extruded with greater width than the width of the web and, in order to provide support to the projecting plastic strips, it may therefore be appropriate to allow these to be transported on two Teflon® belts which run in an endless loop on both sides of the web, as is illustrated schematically with the endless Teflon® belt 29 in Fig. 2.

The finished web-shaped packaging material 10 is removed from the second counter roller 27 and may thereafter be wound up on a magazine reel for storage prior to its intended final use. Comparative experiments which have been carried out with a packaging material produced by the prior art method using two "cold" roller nips and a packaging material produced by the method according to the present invention with a first "warm" roller nip and a second "cold" roller nip have shown that the latter packaging material - the packaging material according to the present invention - has a dramatically improved adhesion (approx. 200 per cent) between the aluminium foil and the first plastic coating as compared with the conventional packaging material.

It will thus be apparent from the foregoing description that the present invention attains the previously set out objects and makes for the production of a packaging material comprising layers of aluminium and plastic with surprisingly good adhesion between these layers using but a very simple modification of the prior art technique. While the present invention has been described above with particular reference to polyethylene by way of example of plastic coating, it is not restricted exclusively to this example of usable plastics. The present invention may also be applied on other types of extrudable plastics which occur in packaging contexts, for example SURLYN®.

Claims

WHAT IS CLAIMED IS:

1. A method of producing a laminated packaging material (10) comprising layers of plastic (12, 16) and aluminium (14), in which method an aluminium-clad paper web (20) is coated with a first plastic layer (16) which is extruded on the aluminium-clad side (14) of the paper web (20), and in which method the thus plastic-coated paper web is coated with a second plastic layer (12) which is extruded on the first plastic layer (16) of the paper web, characterized in that the second plastic layer (12) is extruded on the paper web while the extruded first plastic layer (16) is still warm.

2. The method as claimed in Claim 1, characterized in that both the first (16) and the second plastic layer (12) consist of polyethylene.

3. The method as claimed in Claim 1 or 2, characterized in that the first plastic layer (16) is extruded on the aluminium-clad side (14) of the paper web in connection with the aluminium-clad paper web (20) being led through a nip (21) between a first, smaller press roller (22) and a first, larger counter roller (23) whose surface is heated by the extruded first plastic layer (16) and is kept at a temperature corresponding to the extrusion temperature of the first plastic layer (16).

4. The method as claimed in Claim 3, characterized in that the outer peripheral surface of the first, larger counter roller (23) is clad with Teflon®.

5. The method as claimed in any of the preceding Claims, characterized in that the second plastic layer (12) is extruded on the first plastic layer (16) of the paper web in connection with the plastic-coated paper web being led through a nip (25) between a second, smaller press roller (26) and a second, larger counter roller (27) whose outer peripheral surface is cooled.