TECHNICAL FIELD

The present invention relates to a method of producing a laminate material, for example a packaging laminate, comprising layers of aluminium and plastic, according to which method molten plastic is extruded at and caused to harden on a web of aluminium, for the formation of a well- integrated laminate material.

BACKGROUND ART

In packaging technology, use is made of single-use disposable packages for packing and transporting foods, and a very large group of these so-called single use disposable packages is produced from a laminate material (packaging laminate) comprising a core layer of paper or paperboard and outer, liquid-tight layers of plastic, normally polyethylene, disposed on both sides of the core layer.

The composition of the packaging laminate is based on the concept of providing the best possible protection for the product which is to be packed in the finished package and can, therefore, vary from one product to another depending upon the shelf-life and storage sensitivity etc., of the product in question.

A packaging laminate consisting solely of paper or paperboard and plastic, for example polyethylene, imparts good mechanical strength and configurational stability to the package, as well as good tightness properties vis-a-vis liquids, but generally lacks tightness properties against gases, in particular oxygen gas, which may therefore readily penetrate into the package and come into contact with the packed product. In particular oxygen-gas sensitive products such as, for instance, juice, wine, cooking oil and the like would thus, already within a very short period of time from the date of packing, be seriously damaged or even completely destroyed as a viable food if they were packed in such a package.

In order to make the above-described prior art packaging laminate usable for packages for such oxygen gas sensitive products, the packaging laminate is normally provided with one or more additional layers of a material possessing the desired gas tightness properties, and the hitherto

most common material for this purpose has been aluminium. In this instance a thin foil (so-called Alifoil) is applied to the one side of the paper or paperboard layer between the paper or paperboard layer, respectively, of the packaging laminate and the one outer plastic layer. In recent times, barrier layers of other materials than aluminium have also been employed, for example polymer material (so-called barrier polymers) such as ethylene vinyl alcohol copolymer (EVOH).

An aluminium foil (Alifoil) possesses advantages as compared with such alternative barrier polymers in that it is as good as completely tight against gases, at the same time as it is an efficient light barrier and is, therefore, suitable in packages for products which are both gas- and light- sensitive.

A further advantage is that a packaging laminate including an Alifoil layer can be sealed using inductive heating technique, which is an efficient and rapid sealing technique, preferred to, for example, conventional conductive heat sealing, during conversion of the packaging laminate into finished packages.

Although an Alifoil is a relatively expensive material and is relatively sensitive to outer tensile stresses which occur during package production, it is still a material that, because of its remaining superior properties, is largely employed within the packaging industry for producing packages for perishable foods, in particular foods which are sensitive to oxygen gas and light.

Packaging laminates comprising mutually bonded layers of aluminium and plastic, for example polyethylene, as in the above-described conventional packaging laminate, are normally produced by an extrusion process of the type described by way of introduction, in which molten polyethylene is extruded onto a web of aluminium. The web is led through the nip between rotating cooling rollers which, under pressure, cause the extruded polyethylene layer to harden for the formation of a well-integrated laminate material.

One problem in the prior art production method is intimately related to the tendency of the aluminium rapidly to oxidise on contact with oxygen gas, forming an oxide layer on both sides of the web. The oxide layers impart to the web a smooth surface topography which impedes or prevents physical penetration of the extruded plastic into the web. Since the strength (adhesive

bonding strength) with which the web secures and retains the extruded plastic layer depends, on the one hand, on the physical penetration of the plastic mass into and its mechanical engagement or bonding with the web, and, on the other hand, on reactive (polar) chemical groups on the surface of the web and the surfaces of the extruded plastic layers, respectively, which together form continuous bonds (van der Waals', dipole-dipole and hydrogen bonds), it will be readily appreciated that the adhesive bonding strength decreases and becomes weaker the smoother the surfaces of the aluminium web are. It will just as readily be appreciated that the adhesive bonding strength increases and becomes greater the coarser the surface topography and thereby the greater potential bonding areas which the web displays.

The problem inherent in weakened adhesive bonding strength between the aluminium web and the applied plastic layer as a result of the oxidised surface layers of the aluminium web can, to some degree, be solved by carrying out the extrusion operation in an ozone atmosphere which creates reactive surface groups in the extruded plastic and thereby renders the plastic layer better inclined to develop chemical bonds with the aluminium web by way of compensation for reduced physical penetration and mechanical engagement or bonding as a result of the smooth aluminium oxide layer.

It is also known in the art to create such reactive surface groups in the plastic by so-called flame-treating, heat-treating, corona-treating etc., with the intention of increasing the adhesive bonding strength between the aluminium web and the plastic.

OBJECTS OF THE INVENTION

It has now proved that good adhesive strength between aluminium and plastic which is extruded or applied in the form of a prefabricated film against an aluminium web may be attained without the employment of either the one or the other of the above-outlined treatment methods, and one object of the present invention is, therefore, to propose a method of producing a laminate material, for example a packaging laminate, possessing superior adhesive bonding strength between individual layers of aluminium and plastic included in the laminate material, without consequential adhesion problems of the above-outlined type.

A further object of the present invention is to realise a method of producing a well-integrated laminate material, for example a packaging laminate, on a continuous industrial scale with good internal adhesive bonding strength between the incorporated layers of plastic and aluminium in the laminate material.

SOLUTION

These and other objects and advantages will be attained according to the present invention by a method of the type which is disclosed in appended Claim 1.

OUTLINE OF THE INVENTION

When aluminium, in this particular case the aluminium web, is brought into contact with a hot aqueous medium, the surface layer of the aluminium web is converted, by a chemical hy dration reaction, into a layer of oxide hydroxide (AIOOH) with a porous, gently crystalline physical structure (so-called false boehmite). The hydration reaction between aluminium and water takes place in accordance with the general chemical reaction formula: Al + 2H2θ=> AIOOH +3/2H2. In comparison with the smooth surface structure of the aluminium oxide, a layer of false boehmite has a much coarser surface topography Λvhich is easily wetted by the extruded plastic melt and considerably improves the possibility of the plastic melt to penetrate into and mechanically bond with the false boehmite layer for reliable mechanical retention of the hardened plastic layer on the aluminium web.

The improved adhesion between the aluminium web and the extruded plastic layer as a result of the hydration reaction may be even further improved by carrying out the extrusion operation in an atmosphere of, for example ozone which is aspirated against the extruded plastic in connection with the extrusion of the plastic. Reactive (polar) surface groups may also be provided for through other known methods, such as heat treating, plasma-treating, corona-treating, flame-treating etc.

As aqueous medium for converting the surface layer of the aluminium web into a surface layer of false boehmite, use may be made of an aqueous bath through which the aluminium web is led.

The water temperature in the bath should be between approximately 90 and 120°C, preferably approximately 100°C, since at temperatures below approximately 90°C duplex layers are formed consisting of a layer of false boehmite most proximal the aluminium and a layer of bayerite crystals in contact with the water. At temperatures above approximately 100°C, boehmite is the only oxide phase that has been demonstrated.

Instead of an aqueous bath, it is possible according to the present invention to employ hot water vapour as the aqueous medium for contact with the aluminium web. In such instance, the water vapour is sprayed against the one or both sides of the aluminium web before the aluminium web is coated with the extruded plastic melt.

Whether the aqueous medium be in the form of an aqueous bath or hot water vapour, the aluminium web must be dried after contact with the aqueous medium and before the molten plastic is extruded on the aluminium web. Drying of the aluminium web may be put into effect with the aid of, for example, a gas flame and/ or air knife, or using IH technique (inductive heating).

According to the present invention, it has also proved that the temperature at which the plastic is extruded has some degree of influence on the adhesive bonding strength between the aluminium web and the extruded plastic layer. Thus, it has proved that the extrusion temperature for, for example, polyethylene (in this case LDPE), should be above approximately 280°C, preferably above approximately 300°C, extraordinarily high adhesive bonding strengths being obtained when the extrusion temperature is approximately 320°C or higher, as measured in a 180°T-test using an Instron 1122 test apparatus.

A further factor that influences the adhesive bonding strength between the aluminium web and the extruded plastic layer in the method according to the present invention is the so-called melt index of the extruded plastic melt, and in this context it has proved that the higher the melt index the better will be the adhesion (higher adhesive bonding strength) that can be achieved. In particular, it has proved that there is a limit value above which adhesion is good and increases with increasing melt index, while the adhesion is slight and declines with decreasing melt index below this limit value. The absolute level of this limit value depends on the type of plastic that is employed and,

among other things, on the temperature at which the selected plastic is extruded, and may readily be determined by a person skilled in the art using simple experiments in each individual case.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

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

Fig. 1 schematically illustrates an apparatus for carrying out the method according to a first embodiment of the present invention; and

Fig. 2 schematically illustrates an apparatus for carrying out the method according to a second embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the Drawing, Fig. 1 schematically illustrates an apparatus for producing laminate material (packaging laminate) 1 employing the method according to a first embodiment of the present invention. As shown, the apparatus comprises three mutually subsequent stations: A, B and C as follows. A designates a chemical hydration station in which an aluminium web 2 is brought into contact with an aqueous medium 3 for hydration of the aluminium web for forming surface layers of false boehmite on both sides of the aluminium web.

B designates a drying station in which the hydrated aluminium web 2 is dried after contact with the aqueous medium 3 in station A.

Finally, C designates an extrusion station in which the dried aluminium web 2 with formed surface layer of false boehmite is coated with molten plastic 4 which is extruded on one or both sides of the aluminium web (in the illustrated example, on the one side of the aluminium web), and is caused to harden on the aluminium web by cooling under pressure with the aid of rotating cooling rollers 5, 5, through the nip between which the aluminium web 2 is led with the extruded molten plastic layer for forming the finished laminate material 1 in which the hardened extruded plastic layer 4 is bonded with good adhesion (high adhesive bonding strength) to the aluminium web 2, as is illustrated within the enlarged, encircled area at D in Fig. 1.

The chemical hydration station A comprises an open bath 6 containing the aqueous medium 3 which, in this example, consists of hot water. As was pointed out above, it is important that the temperature of the aqueous medium 3 is kept within a temperature range of between 90°C and 120°C and, in order to keep the water temperature within this range, there are disposed within the bath 6 four heater elements 7, 7 and 8, 8 arranged in pairs, and being, for example, in the form of inductive heater elements between which the aluminium web 2 is disposed to be led when it passes through the aqueous medium 3 in the bath 6. In order to ensure complete contact between the aluminium web 2 and the aqueous medium 3, a rotating roller 9 is immersed in the aqueous medium 3 centrally at the bottom of the bath 6 and around which the aluminium web 2 is led on its passage through the aqueous medium 3. As shown in Fig. 1, the one pair of heater elements 7, 7 is disposed in the region in which the aluminium web 2 is led down into the aqueous medium 3, while the second pair of heater elements 8, 8 is correspondingly disposed in the region in which the aluminium web 2 is led up out of the aqueous medium 3 after its passage around the immersed roller 9. By suitable regulation (not shown) of each respective heater element 7, 7; 8, 8, constant and uniform contact temperature will be ensured between the aluminium web 2 and the aqueous medium 3 during the passage of the aluminium web 2 through the chemical hydration station at A.

As shown in Fig. 1, the subsequent drying station B includes elongate heater elements 10, 10 disposed on both sides of the aluminium web 2 and preferably being inductive heater elements, in which event the aluminium web 2 is disposed to be led through the gap between the heater elements 10, 10 for complete evaporation of the accompanying aqueous medium 3 from the station A.

As was mentioned above, the extrusion station C after the drying station B comprises rotating cooling rollers 5, 5 which positively rotate with light pressure against one another in the nip region between the rollers 5, 5. Above the rollers 5, 5, there is provided an extruder 11 with the extruder nozzle 12 directed downwardly towards the nip between the rollers 5, 5 for the extrusion of molten plastic 4 onto one side of the dried aluminium web 2. In the region above the rollers 5, 5 between the extruded plastic film 4 and the dried aluminium web 2, there is further provided an ozone unit 13 which is disposed to aspirate ozone against the extruded plastic film 4 for oxidation

of the molten plastic prior to contact with the aluminium web 2 in the nip between the rollers 5, 5.

As was mentioned previously, the temperature at which the plastic is extruded has some degree of influence on the final adhesive bonding strength between the aluminium web 2 and the extruded plastic layer 4 and, in the illustrated embodiment, the extruder 11 is disposed to extrude the molten plastic at a temperature above 300°C, preferably approximately 320°C, which has proved to give extraordinarily good adhesion (high adhesive bonding strength) when the plastic type under consideration here consists of LDPE (Low Density Polyethylene).

Using the above-described apparatus according to Fig. 1, a well- integrated laminate material (packaging laminate) 1 was produced comprising layers of aluminium and plastic 2 and 4, respectively, in the following manner according to the present invention. An aluminium web 2 is unwound from a magazine reel 2' (in the left-hand section of Fig. 1), and is led via a bending roller 14 down into the bath 6 for contact with the aqueous medium 3 which, with the aid of the heater elements 7, 7 and 8, 8, is kept at a temperature suitable for chemical hydration of the aluminium web 2, i.e. within the range of between approximately 90°C and approximately 120°C. After the passage round the immersed roller 9, the aluminium web 2 is led up out of the aqueous medium and introduced, via a bending roller 15 disposed above the bath 6, into the station B where the web is led through the flat gap space between the elongate heater elements 10 disposed on both sides of the web, which heat the web to a sufficient level to completely evaporate accompanying aqueous medium from the station A.

From the drying station B, the dried aluminium web 2 is led further into the extrusion station C, where the extruder 11 extrudes molten plastic (LDPE) at approximately 300-320°C onto one side of the aluminium web 2, at the same time as the ozone unit 13 aspirates ozone against the extruded plastic melt. The aluminium web 2 with applied molten plastic layer 4 is led through the nip between the two rotating cooling rollers 5 which, under light pressure, cause the molten plastic to harden during passage of the web through the nip between the rollers 5, 5 for forming the finished laminate material (packaging laminate) 1 in which the layers of plastic and aluminium 4 and 2, respectively are bonded to one another with good adhesive bonding strength.

Fig. 2 schematically illustrates an apparatus for producing a well- integrated laminate material (packaging laminate) according to a second embodiment of the present invention. In order to facilitate a comparison between Fig. 1 and Fig. 2, corresponding or identical stations/ parts of the apparatus in Fig. 2 have been given the same reference numerals as in Fig. 1, but with the addition of a prima symbol (').

The apparatus according to Fig. 2 thus comprises three mutually subsequent stations A', B' and C as follows.

A' designates a chemical hydration station in which an aluminium web 2' is brought into contact with a hot aqueous medium 3' which, in this example, consists of hot water vapour which is sprayed onto the aluminium web 2' on both sides of the aluminium web 2' for hydration of the aluminium web 2' for forming a surface layer of false boehmite.

B' designates a drying station in which the hydrated aluminium web 2' is heated for complete evaporation of accompanying aqueous medium from the station A.

Finally, C designates an extrusion station in which the dried aluminium web 2' from the drying station B' is coated with molten plastic 4' which is extruded on one or both sides of the web (in the illustrated example, on the one side of the web), and, with the aid of rotating cooling rollers 5', 5', is caused to harden on the web for forming the finished well- integrated laminate material (the packaging laminate) 1' comprising layers of plastic and aluminium 4' and 2' respectively, bonded together with good adhesion (high adhesive bonding strength), as is illustrated within the enlarged, encircled area at D'.

The only major difference between the previously described apparatus according to Fig. 1 and the apparatus according to Fig.2 is that the hot aqueous bath in the chemical hydration station A in Fig. 1 has been replaced by treatment with hot water vapour 3' in the chemical hydration station A' in Fig. 2. In other comparable respects, the two compared apparatuses are identical. In order to avoid unnecessary repetition of parts relating to the stations B' and C in the apparatus according to Fig. 2, reference is therefore made to the earlier description of stations B and C, respectively, in the apparatus according to Fig. 1. Using the apparatus according to Fig. 2, there was thus produced a well-integrated laminate material (the packaging laminate) 1' in the

following manner according to the invention. The aluminium web 2' is unwound from a magazine reel 2" (in the left-hand section of Fig. 2), and is led through the chemical hydration station A' where the aluminium web is brought into contact with the hot (90-120°C) water vapour which is sprayed onto both sides of the web for chemical hydration of the aluminium web for forming surface layers of false boehmite. The hydrated aluminium web 2' is thereafter led through the drying station B' and the extrusion station C where the aluminium web is treated in the same manner as in the above- described stations B and C, respectively in connection with Fig. 1. Although the present invention has been described in the foregoing in connection with the production of a laminate material (packaging laminate) comprising layers exclusively of plastic and aluminium, it is naturally not restricted solely to such two layer laminates. According to the invention, it is possible to produce laminate materials comprising one or more additional layers above and beyond the two previously described, without departing from the inventive concept as defined in appended Claim 1. For example, the aluminium web may, by an interjacent layer of adhesive, binder or sealing plastic, be bonded to a web of paper or paperboard or to a web of plastic, and be coated by the method according to the present invention with a plastic layer on its other side using the method according to the invention. Such material combinations in which the laminate material includes additional layers of paper, paperboard or plastic thus fall within the scope of the inventive concept as herein disclosed and claimed, while not being specifically described or shown on the Drawing. Further, the present invention may be employed in connection with plastics other than LDPE as specifically described above. Examples of such other plastics may be polyester and polypropylene which, in practical trials, have given good results.