IMPROVED LAMINATED PACKAGING MATERIAL AND RELATIVE MANUFACTURING METHOD

Abstract: Laminated packaging material, in the form of a sheet, strip or web, from which to produce a form-stable, fluid-tight package by fold forming, thermoforming, or other mechanical process for shaping of the material. The material has a first and a second extruded layer; the first layer includes a first thermoplastic material selected from the group consisting of propylene-based polymers, and preferably a propylene homopolymer or a copolymer obtained from propylene and ethylene and includes 5% to 85% by weight of an inorganic mineral filler, preferably dolomite and talc. The second layer includes a second thermoplastic material selected from the group consisting of ethylene-based polymers, and preferably low-density polyethylene (LDPE). According to the present invention, the first layer and the second layer are laminated directly onto each other and no adhesive is needed in between.

Description: IMPROVED LAMINATED PACKAGING MATERIAL AND RELATIVE MANUFACTURING METHOD TECHNICAL FIELD The' present invention relates to a flexible packaging material in the form of a sheet, strip or'web, from which to produce form-stable, fluid-tight packages by fold forming and sealing, thermoforming or other mechanical material shaping processes. BACKGROUND ART As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature processed) or milk, wine, tomato sauce, etc., are sold in packages made of packaging material. A typical example of such a package is the parallelepipedal package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure obtained by laminating various layers of material one on top of the other, and comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, normally polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of oxygen-barrier material, defined for example by an aluminum film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer or layers of heat- seal plastic material forming the inner face of the package eventually contacting the food product. As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the strip- fed packaging material; the strip of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube. The tube is then filled with the sterilized or sterile-processed food product, and is sealed and cut at equally spaced cross sections into pillow packs, which are subsequently folded mechanically to form finished, e.g. substantially parallelepipedal, packages. Alternatively, the packaging material may be cut into blanks, which are formed on forming spindles into packages which are then filled with the food product and sealed. One example of such a package is the so-called "gable-top" package known as Tetra Rex (registered trademark). The properties of the fibrous material, such as paper or cardboard, forming the skeletal layer of the laminated packaging material are highly dependent on humidity, excessive absorption of which impairs the strength and rigidity of the material to the extent of rendering the packages unacceptable in terms of performance. In certain market applications in which the packages are exposed to an exceptional degree of humidity or to direct contact with water (on the outside of the package) - as, for example, when the packages are distributed in icewater buckets - packages comprising layers of fibrous material offer no guarantee in terms of strength or working life. A need is therefore felt for an alternative material, which may be used in place of fibrous material such as paper or cardboard, and which at the same time provides for a good degree of water -resistance. Various alternatives have been proposed by way of a solution to the problem, such as a layer of polypropylene film as a substitute for cardboard. As is known, however, a polypropylene or polypropene (abbreviated as PP) layer cannot be laminated directly onto polyethylene or polyethene (abbreviated as PE) layer defining the outer layer of heat-seal material of the packages described previously. In particular, serious problems are encountered when laminating polypropylene and low density polyethylene together. Joining the two layers requires the use of adhesive, which calls for altering the material manufacturing process: in particular, performing an additional step in which to add the adhesive, thus increasing cost. DISCLOSURE OF INVENTION It is an object of the present invention to provide a packaging material designed to eliminate the aforementioned drawbacks, and which provides for joining the aforementioned materials with no adhesive required. According to the present invention, there is provided a laminated packaging material, in the form of a sheet, strip or web, from which to produce a form-stable, fluid-tight package by fold forming and sealing, thermoforming, or other mechanical processing for shaping of the material; said material comprising at least a first layer and a second layer; said first layer comprising a first thermoplastic material selected from the group constituted by propylene-based polymers; and said second layer comprising a second thermoplastic material selected from the group constituted by polyethene resins; characterized in that said first layer and said second layer are laminated directly onto each other, and said first layer comprises 5% to 85% by weight of an inorganic mineral filler. BRIEF DESCRIPTION OF THE DRAWING The invention will be described below and further explained with particular reference to the enclosed drawing in which: - figure 1 shows a strongly enlarged cross-section of the material according to a preferred embodiment of the invention. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates as a whole the packaging material according to the invention. Material 1 , which may be in the form of a strip, a sheet or a web, comprises a first layer 2 forming the outer layer of the finished package; and a stiffening skeletal layer 3. In the embodiment shown, the laminated packaging material also comprises the following optional layers: a second layer 4; a barrier layer 5; a third layer 6; and a fourth layer 7 forming the inner layer of the finished package. According to the present invention, the skeletal layer 3 comprises a mixture of a polypropylene-based resin, an inorganic mineral filler, and additives, e.g. a compatibilizer. The polypropylene-based resin may be, for example, a polypropene homopolymer, a polypropene random copolymer made from a mixture of propylene and e.g., ethylene, a polypropene heterophasic copolymer made from propene and e.g., ethene in consecutive polymerization steps, or a mixture of these. Preferably, the polypropene-based resin is a polypropene homopolymer with an ASTM melt index of under 10 (2.16 kg; 230°C), or an ethene/propene copolymer with an ASTM melt index of 0.5 to 5 (2.16 kg; 230°C). The skeletal layer 3 preferably contains a quantity of inorganic mineral filler ranging approximately between about 5% and about 85% - and preferably of over about 50% - of the total weight of the skeletal layer, and preferably contains about 95% to about 15% - more preferably less than about 50% - of polypropylene-based polymer. The filler may be any known inorganic mineral filler, but is preferably dolomite, chalk, lime, talc or mica, each on its own or in any desired combination. More preferably, the filler is a mixture of dolomite and talc. The dolomite preferably defines over about 50% of the total weight of the filler, more preferably between about 80% and 90%, and even more preferably is about 85% by weight; the talc is preferably less than about 50%, and more preferably about 15%, by weight. Polypropene-based resins are normally incompatible with minerals. To obtain a skeletal layer 3 with acceptable technical properties, a compatibilizer, e.g. calcium stearate containing both polar and nonpolar segments, is added. The calcium stearate is preferably less than about 1 %, and more preferably less than 0.5%, by weight. Compounding is typically effected in a continuous mixer (e.g. double-screw extruder) or in a batch (e.g. Banbury) mixer, followed by pelletizing for convenient material handling, and consecutive extrusion operations. Typical melt temperatures in compounding and pelletizing are 180-280°C, and preferably 200-250°C. The thickness of the various layers may vary widely. By way of example, in packaging material used for manufacturing small volume portion packages (e.g. 250 ml packages), the thickness of skeletal layer 3 preferably ranges between approximately 90 and 150 μm, and more preferably between approximately 100 and 120 μm. Skeletal layer 3 may be a blown or cast film, and the typical melt temperatures in film manufacturing are 180-260°C, and preferably 200- 230°C. According to the present invention, the outer first layer 2 comprises polyethene resins such as high-density polyethylene (HDPE), medium- density polyethylene (MDPE), low-density polyethylene (LDPE), linear-low- density polyethylene (LLDPE), very-low-density polyethylene (VLDPE) and ultra-low-density polyethylene (ULDPE), and more preferably low-density polyethylene (LDPE). First layer 2 is also possibly used to cover print applied to skeletal layer 3. The thickness of the first layer 2 of polyethylene homopolymers preferably ranges between about 5 and about 50 μm, and more preferably between about 10 and about 25 μm. According to the present invention, the outer first layer 2 is laminated directly to the skeletal layer 3, whereas, according to the prior art, an adhesive is needed to laminate a layer containing polypropylene-based polymers to a layer containing polyethylene homopolymers. In the embodiment shown, the laminated packaging material also comprises a second layer 4 of polyethylene homopolymers on the other side of, and directly laminated to, the skeletal layer 3. Second layer 4 is preferably of the same chemical nature as the first or outer layer 2, and is therefore preferably a low-density polyethylene, and is preferably of a thickness ranging between about 5 and about 50 μm, and more preferably between about 10 and about 25 μm. A barrier layer 5 is applied on the other side of second layer 4, and is preferably a thin aluminum foil, preferably of a thickness ranging between about 5 and about 50 μm, and more preferably between about 5 and about 10 μm. A third layer 6 is extruded on the other side of the barrier layer 5. By way of example, in packages for milk, the third layer is preferably of the same chemical nature as the first or outer layer 2, and is therefore preferably low-density polyethylene. Third layer 6 is preferably oxidized for good adhesion to barrier layer 5. The third layer may comprise any suitable ethene-acid copolymer, e.g., ethene acrylic acid or ethene metacrylic acid. This ensures good adhesion to the barrier layer over a long period of time. This is particularly important when packaging products such as fruit juices, tomato products, wine. A fourth layer 7 is extruded on the other side of the third layer 6 and is preferably of the same chemical nature as the first or outer layer 2, and is therefore preferably polyethene. Fourth layer 7 (the inner layer of the finished package) is preferably non-oxidized to permit heat sealing of the packages manufactured from the packaging material. The thickness of third layer 6 and of fourth layer 7 is preferably between about 5 and about 50 μm, and more preferably between about 10 and about 25 μm. A laminated sheet packaging material 1 according to the preferred embodiment described above may be produced using the following preferred method, which comprises the steps of: extruding a skeletal layer 3 containing a mineral-filled propylene-based polymer; extruding a first or outer layer 2 of polyethylene homopolymer, using a conventional laminator equipped with extrusion dies, directly on the mineral-filled polypropylene skeletal layer 3; extruding a second layer 4 of polyethene resin, using a conventional laminator to bind the skeletal layer 3; to the layer of aluminum foil 5 ; forming a third layer 6 of polyethene resin or an adhesive resin, preferably an ethene acid copolymer using a conventional laminator equipped with extrusion dies; and forming a fourth layer 7 of polyethene resin. The present invention therefore provides, in a straightforward manner, using existing technology and with no additional steps in the processing method, for producing a laminated packaging material in the form of a sheet, web or strip, which exhibits good bonding strength of all the layers of the material, and v/hich may also be sealed with good bonding strength by so-called heat sealing when forming the material into packages. The invention will now be described with reference to an example, however, it should be observed that several modifications obvious for the specialist in the field, may be made without departing from the scope of the invention. For example, in another preferred embodiment of the present invention, the laminated packaging material may comprise only the skeletal layer, and an outer and inner layer of polyethylene homopolymers laminated directly onto the skeletal layer. EXAMPLE 1 A skeletal layer containing 48% by weight of polypropene resin, 51 % of a filler composition, and 1 % of a compatibiliser, is formed in an extrusion process into a 1 10 μm foil. Processing temperature is 225 °C. The filler composition is 85% by weight of dolomite and 15% by weight of talc. The compatibiliser is Ca-stearate. The skeletal layer is printed in a conventional stack press. A 12 g/m 2 layer of polyethene resin is extrusion coated onto the printed surface of the skeletal layer. The extrusion process is made using a conventional laminator equipped with extrusion dies. The polythene resin melt temperature at the exit of the die is 325 °C. Ozone, at a concentration of 30 g/m 3 air, is blown onto the surface of the melted polyethene resin. An 6.35 μm Al-foil is fixed to the non-printed side of the skeletal layer, by means of a lamination process where skeletal layer and barrier layer are entering the lamination station from different directions, and are combined with a 23 g/m 2 extruded polyethene resin layer which in melted form is contacting both the skeletal layer and the barrier layer. The extrusion process is made using a conventional laminator equipped with extrusion dies. The polythene resin melt temperature at the exit of the die is 325 ° C. Ozone, at a concentration of 30 g/m 3 air, is blown onto the surface of the melted polyethene resin. A first inner layer of 16 g/m 2 polyethene resin is extrusion coated onto the 6.35 μm Al-foil, which is already fixed to the skeletal layer. The extrusion process is made using a conventional laminator equipped with extrusion dies. The polythene resin melt temperature at the exit of the die is 321 ° C. Ozone, at a concentration of 30 g/m 3 air, is blown onto the surface of the melted polyethene resin, which shall contact the Al-foil. A 19 g/m 2 second inner layer of polyethene resin is combined with the first inner layer in a so called single slot co-extrusion die, and the two layers are thus simultaneously extrusion coated to the Al-foil. The second inner layer of polyethene resin is extruded at 275 ° C. A laminated packaging material manufactured as described above has, in any interface between layers, an adhesion not less than 240 N/m.

Claims: CLAIMS 1 .- Laminated packaging material, in the form of a sheet, strip or web, from which to produce a form-stable, fluid-tight package by fold forming, and sealing, thermoforming, or other mechanical processing for shaping of the material; said material comprising at least a first layer (3) and a second layer (4); said first layer (3) comprising a first thermoplastic material selected from the group consisting of propene-based resins; and said second layer (4) comprising a second thermoplastic material selected from the group consisting of polyethene resins; characterized in that said first layer (3) and said second layer (4) are laminated directly onto each other, and said first layer (3) comprises 5% to 85% by weight of an inorganic mineral filler. 2.- Laminated packaging material as claimed in Claim 1 , characterized in that said first layer (3) comprises 50% to 85% by weight of inorganic mineral filler. 3.- Laminated packaging material as claimed in Claim 1 or 2, characterized in that said inorganic mineral filler comprises at least one substance selected from the group consisting of dolomite, talc, chalk, mica, calcium carbonate. 4.- Laminated packaging material as claimed in Claim 3, characterized in that said inorganic mineral filler comprises a mixture of dolomite and talc. 5.- Laminated packaging material as claimed in Claim 4, characterized in that the total quantity of dolomite in the inorganic mineral filler ranges between 75% and 90% by weight; and in that the quantity of talc ranges between 10% and 25% by weight. 6.- Laminated packaging material as claimed in Claims 1 to 5, characterized in that said first layer further comprises a quantity of less than 1 % by weight of compatibilizer. 7.- Laminated packaging material as claimed in Claim 6, characterized in that said second thermoplastic material is low-density polyethylene. 8.- Laminated packaging material as claimed in any one of the foregoing Claims, characterized in that said first thermoplastic material is a propyethene resin. 9.- Laminated packaging material as claimed in Claims 1 to 7, characterized in that said first thermoplastic material is a copolymer of propylene and ethylene. 10.- Laminated packaging material as claimed in any one of the foregoing Claims, characterized in that said first layer (3) has a thickness of between 90 and 150 μm. 11.- Laminated packaging material as claimed in any one of the foregoing Claims, characterized in that said second layer (4) has a thickness of between 10 and 25 μm. 12.- Laminated packaging material as claimed in any one of the foregoing Claims, characterized in that it further comprises a barrier layer (5). 13.- Laminated packaging material as claimed in Claim 12, characterized in that said barrier layer (5) is an aluminum foil. 14.- Laminated packaging material as claimed in Claim 12 or 13, characterized in that said barrier layer (5) has a thickness of between 5 and 10 μm. 15.- Laminated packaging material as claimed in Claims 11 to 14, characterized by the fact that it further comprises a third layer (6). 16.- Laminated packaging material as claimed in Claim 15 characterized by the fact that said third layer (6) comprises a polyethylene homopolymer. 17.- Laminated packaging material as claimed in Claims 15 or 16, characterized by the fact that it further comprises a fourth layer (7). 18.- Laminated packaging material as claimed in Claim 17, characterized by the fact that said fourth layer (7) comprises a material selected from the group consisting of a polyethene resin and an adhesive polymer. 19.- Laminated packaging material as claimed in any of the foregoing Claims, characterized by the fact that it further comprises an outer layer (2) laminated directly onto said first layer (3) and comprising a polyethylene homopolymer. 20.- Method of manufacturing a laminated packaging material, in the form of a sheet, strip or web, from which to produce a form-stable, fluid- tight package by fold forming, thermoforming, or other mechanical processing for shaping of the material; said method comprising the steps of: - forming a first layer of a first thermoplastic material selected from the group consisting of propylene-based polymers; - forming a second layer of a second thermoplastic material selected from the group consisting of ethylene homopolymers; characterized by the fact that said first layer and said second layer are laminated directly onto each other; and said first layer comprises 5% to 85% by weight of an inorganic mineral filler. 21.- Laminated material according to claim 18, wherein said adhesive polymer is ethene-acid copolymer.

Inventors: BERGHOLTZ LARS, BERGHOLTZ, LARS
Applicants: BERGHOLTZ LARS, TETRA LAVAL HOLDINGS & FINANCE, BERGHOLTZ, LARS, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 2000
Main classification: B32B19
IPC codes: B32B27