A METHOD OF AND APPARATUS FOR USE IN MANUFACTURING A PACKAGING LAMINATE

Abstract: In an extrusion laminating system for production of a laminate for packaging liquid food, a thermoplastics product-contact layer (19) is extrusion coated onto an oxygen barrier substrate (10) from a low-temperature extruder (20), without the thermoplastics reaching its breakpoint temperature prior to its entry into the extruder (20) or at any point between its entry into the extruder (20) and arrival at a chill roller (16) immediately downstream of a die slot (21) of the extruder (20). Thereby, transmittal of off-flavour-producing compounds from the laminate to the liquid food can be avoided. Adhesion between the product-contact layer (19) and the substrate (10) is promoted in one or more of a number of ways, for example by co-extrusion of the product-contact layer (19) with a tie layer (18), to provide good adherence of the product-contact layer (19) in spite of its low-temperature extrusion.

Description: A METHOD OF AND APPARATUS FOR USE IN MANUFACTURING A PACKAGING LAMINATE This invention relates to a method of and apparatus for use in manufacturing a packaging laminate. The desire of US-A-4, 657, 614 is to build up a packaging laminate which has a carrier layer of paper, cardboard or for example polystyrene foam, whereby packages manufactured from the laminate can be formed by folding to a lasting shape and a good mechanical protection is imparted to the contents. Moreover, the desired laminate should be liquid-tight so as not to absorb moisture or liquid which may come into contact with the outside of the package, and the package should have an inside which is liquid-tight and which can be heat-sealed by bringing together plastics layers which can be fused together with the help of heat and pressure to a mechanically strong union. Furthermore, the inner plastics layer, which is in direct contact with the contents, should have a low characteristic taste level in cases where the contents are constituted of foodstuffs. The inner plastics layer should desirably be sufficiently strong and tough to withstand the stresses on the material occasioned by the fold-forming without causing the plastics to split. Furthermore, the desired laminate should include a gas barrier layer, preferably of aluminium (Al) foil .Thus, a number of different material layers are joined to one another and this is done with the help of different binder layers. US-A-4, 657, 614 adds that, conventionally, when manufacturing such a packaging laminate, broadly speaking, two differing methods are used. The first method is a dry laminating method, in which, after applying adhesive to laminated material (sub-substrate) such as a film made from plastics and drying the same, such a sub-substrate is pasted to the surface of the substrate. The second method is an extrusion laminating method, in which a thin layer of plastics melted at a temperature as high as 300°C, is extruded from an extrusion die and coating of the plastics is formed on the surface of the Al foil. Packaging comprised of such laminate manufactured by either conventional method can have an off-flavour which may be transferred to the contents. Such an improper change is more frequently found whenever the contents are packaged for longer preservation or at higher temperature. That is because, for packaging laminate manufactured by the dry laminating method, for example, there exists a layer of adhesive between the Al foil and the plastics layer located at the inner surface of the packaging container and the adhesive is eluted. On the other hand, the extrusion laminating method causes oxidative product (carboxyl group) in the plastics, and the oxidative product gives an abnormal smell and deterioration of the flavour. US-A-4, 657, 614 further discloses that a packaging container to be filled with a drink is not only required to have sufficient strength that the container may not be deformed, but also it is necessary that the inner plastics layer be firmly adhered to the Al foil so that leakage of the drink be prevented. The Patent states that, to adhere the plastics firmly to the Al foil, in the case of polyethylene (PE) , for example, the PE needs to be extruded at a high temperature over 300 * C. The plastics which is extruded at the high temperature sticks well to the surface of the aluminium foil, but, owing to its contact with the air for a relatively long time before forming a thin layer on the foil, produces a high proportion of oxidative product. Conversely, if the plastics is maintained at a lower temperature so as to prevent the formation of the oxidative product, the adherence with the foil becomes weaker and the workability of the container becomes worse. The oxidative product existing in the layer of plastics is gradually eluted into the packaged contents particularly in the situation of higher temperatures, wherein the degree of elution is increased. US-A-4, 657, 614 concluded that, for those reasons, a package using the laminate manufactured by a conventional method could not be readily heated so as to preserve food sensitive to taste and flavour for a long time. The solution proposed by US-A-4657614 to those conventional difficulties was to extrude a film of melted plastics between a substrate web and an Al foil web while a web of blown synthetic resin film was positioned at the opposite side of the Al foil, and then to pass these materials between a pair of rollers, whereby the melted plastics bonds the foil web to the substrate web and the blown film is sealed to a surface of the foil web by the heat conducted from the plastics through the foil web; and it is said that this method reduces the outbreak of oxidative product to an extreme extent. The Patent pointed out that the plastics in the film blowing process can be pressed out of the annular die at a temperature of 140 * -180"C whereas in the corresponding slot-extrusion process the temperature is approximately 250"-325°C. It said that one of the very considerable advantages is that the blown film gives off appreciably less taste which is of particular importance in the packaging of foodstuffs. In one preferred embodiment in US-A-4, 657, 614 a carrier layer of paper or cardboard, coated beforehand with an outer polythene layer is passed over a cylinder and down into a nip between co-operating pressure and cooling cylinders. An Al foil web is also guided in between the cylinders. Also introduced between the foil web and the carrier layer is a slot-extruded plastics layer. The warm plastics layer, not yet solidified, is then compressed, by the cylinders, between the carrier layer and one surface of the foil, a mechanical bond ensuing between the plastics and the carrier layer and between the plastics and the foil, at the same time as the plastics cools down and becomes stabilised. A coextruded film manufactured beforehand and comprising two plastics layers, namely a plastics layer of the ethylene acrylic acid (EAA)type and a plastics layer consisting of blown polythene is also introduced between the cylinders. Because those two plastics layers have been coextruded they have good adhesion to each other. The bulk of the heat content in the extruded plastics layer is given off to the foil since Al foil is a much better heat-conductor than the paper material of the carrier layer. At the same time as the extruded plastics layer cools down and is stabilized the foil layer is thus heated and the heat is transferred to the EAA layer which has a sealing temperature of approximately 93°C. Since the extruded plastics layer has to be heated to at least 300 * C, it has a relatively large heat content which has to be dissipated. If the temperature and the thickness of the extruded plastics layer are chosen correctly, sufficient heat for the achievement of a seal will be transferred to the EAA layer, which is raised thereby to a temperature exceeding 93 * C and is made to melt therefore along its surface facing the foil layer and after cooling is bonded to the foil layer. It seems that, in another preferred embodiment of US-A- 4,657,614, the EAA layer is omitted and the blown film is melted by direct conduction of the heat from the Al foil but, because of the extremely short period of time between the blown film being subjected to the heat and its being cooled by the cooling cylinder, the heating of the film does not give rise to any great amount of oxidative product. The above solution has the disadvantage that it is limited to the production of laminates incorporating a layer of material, in practice metal foil, which is a good thermal conductor. US-A-4, 747, 902 discloses that, in the extrusion laminating method, adhesive bonding mechanisms are classified into two kinds, i.e. mechanical adhesive bonding and chemical adhesive bonding. Mechanical adhesive bonding is relevant in cases where porous substrate materials such as cloth and paper are used, in which the molten polymer, particularly PE, extruded at a high temperature infiltrates into fine pores among fibres and is then solidified by cooling, providing adhesive bonding between the PE and the substrate. Chemical adhesive bonding utilizes the chemical intermolecular force and it is employed for substrate materials having smooth surfaces such as regenerated cellulose film, Al foil and plastics films or those having no functional groups. In order to obtain satisfactory bonding force in the chemical adhesive bonding, it is necessary that the substrate material be subjected to primer treatment, and it is often treated by corona discharge. It also mentions the preference that the PE used for extrusion coating be activated and that, as the method for this activation, it is a general practice that PE is extruded at higher temperatures so as to increase oxygen- containing polar groups in molecules before it is brought into contact with a substrate material so as to provide sufficient adhesive strength. The Patent adds that, with mechanical adhesive bonding, when the temperature of molten PE is high, its viscosity is low so improving the infiltration into the substrate material and thus the adhesive strength. Therefore, the lamination is done at temperatures as high as possible, provided that the PE is not decomposed. Accordingly, low density polyethylene (LDPE) is heated to, at the lowest, 280° to 310 * C in the conventional lamination process. In this process, it is necessary that the polyethylene is not decomposed and does not give out any offensive odour. In industrial practice, however, partial decomposition is caused to occur, giving off an irritating smell from decomposed product, and much smoke is emitted during the processing. We consider that this may be because the temperature readings given by the temperature detecting equipment of conventional extruders do not necessarily represent the maximum temperatures actually attained in the extruders. In spite of their inaccuracy such readings are often used unquestioningly as accurate readings or as appropriate temperature settings in the present technical field. In the solution proposed by US-A-4, 747, 902, the substrate is a sheet material and a layer of polyolefin (PO) laminated thereto. In a first method, a PO film to be laminated is used by being heated to a molten web and then applied to the PO of the substrate. In a second method, extruded PO, as a molten web, is directly applied to the PO of the substrate material. In these methods, the temperature of the molten web is set as low as possible in order to avoid thermal deterioration of the polyolefin. That is, the temperature of the molten web is in the range between the melting point or softening point of the polymer to be laminated and a temperature higher than that temperature by 100 ° C . The laminate can be used for such things as fragrant books, bookmarks, pamphlets, tickets, name cards, postcards, telegram paper, fans, cores of rolled paper, labels, posters, decorated building materials such as wall and ceiling materials, wrapping material, such as wrapping materials for lavatory tissue, greeting cards and letter paper, sealing paper for carton boxes of lavatory paper and casings of foodstuffs and flowers, refuse bags, waste bags, antifungal bags, antiseptic packaging materials for foodstuffs, mosquito repellent labels, insecticide sheets, wrapping sheets for growing fruits and rust-inhibitive packaging materials for steel pipes and machinery. In an example, a substrate was prepared by conventional extrusion lamination at 300 * C, in which a 20 micron thick layer of LDPE was applied to the surface of quality paper of 49g/m 2 . After that, a 50 micron thick web of LDPE was extruded at 170 * C and was laminated under pressure to the surface of the LDPE of the substrate to obtain the laminate. The Solution of US-A-4, 747, 902 has the disadvantages that it requires the provision of an additional layer of plastics to cover the plastics layer already coated onto the sheet material of the substrate and that the additional layer, unless undesirably thick, does not actually prevent permeation of the oxidation products. According to one aspect of the present invention, there is provided a method of manufacturing a packaging laminate, comprising supplying a substrate providing a good barrier to transmission of oxygen, and applying to said substrate a thermoplastics, product-contact layer by extrusion through a die slot at an extrusion lamination station, adhesion between said product-contact layer and said substrate having been promoted, characterized in that the temperature of said product-contact layer is maintained below the breakpoint temperature of the thermoplastics material of said product- contact layer throughout said method. According to a second aspect of the present invention, there is provided apparatus for use in manufacturing a packaging laminate, comprising an extruder including a die slot for producing a layer of thermoplastics material by extrusion, chilling means immediately downstream of said die slot for chilling said layer, means for supplying a substrate providing a good barrier to transmission of oxygen, roll means for guiding said substrate to said layer, and adhesion- promoting means for promoting adhesion between said substrate and said layer, characterized in that said extruder is a low- temperature extruder. Owing to these aspects of the present invention, and particularly whereby the thermoplastics material temperature is maintained below its breakpoint temperature throughout the method, the degree of oxidation of the material which can deleteriously affect the packaged product, especially produce off-flavour in packaged food, can be avoided. Furthermore, the fact that the substrate is a good barrier to oxygen minimizes transmittal of off-flavour-producing compounds from any other layers in the laminate to the outside of the barrier provided by the substrate. Moreover, the promotion of adhesion between the product-contact layer and the substrate provides for good adherence between the product-contact layer and the substrate in spite of extrusion of the product- contact layer below its breakpoint temperature. The breakpoint temperature of a thermoplastics material is the temperature at which begins significant oxidation of the thermoplastics material in an air atmosphere. The breakpoint temperature for a thermoplastics varies depending upon the particular composition of the thermoplastics. The breakpoint temperatures for a selection of thermoplastics each usable as the product-contact layer is given below:- Material ( αrade ) Melt flow Breakpoint xa±e. temperature MFR CO LDPE (high pressure autoclave) 4 250-280 LLDPE("DOWLEX" 3010E) 5.5 260-280 Ionomer ("SURLYN" 1652) 5.5 260-280 EVA (16% Vinyl acetate) 3 220-240 The low-temperature extruder can be a substantially screwless rotary extruder with a gear pump; a planetary roller extruder with a gear pump; a twin-screw extruder; a screw extruder with a screw designed for low-temperature extrusion; or an extruder of the character conventionally used for cast film of the thermoplastics material. The use of a low-temperature extruder has the advantage that the present system is readily applicable to a conventional extrusion-coating line, simply with replacement of a conventional, high-temperature, final extruder extruding the product-contact layer, by a low-temperature extruder performing the same function. Adhesion between the product-contact layer and the substrate can be promoted by one or more of surface treatment (such as flame treatment, corona-discharge treatment, or plasma treatment) of the receiving surface of the substrate; applying an adhesive to that receiving surface, which application can be carried out either by a manufacturer of the substrate or in an extrusion-coating line producing the laminate; pre-heating of the substrate; V, ALDYNE"® treating (employing electrodes to deposit a layer of glass on a substrate, particularly on an aluminium foil barrier layer) ; and co-extruding, in the low-temperature extruder, a low- temperature tie layer, for example "SURLYN"® or "PRIMACOR"®, in addition to the product-contact layer. The substrate consists of a barrier layer constituting a good barrier to oxygen and thus to the passage towards the product-contact layer of oxidation or other compounds which would give an off-flavour to the product contact layer and/or the product. A material which is a good barrier to oxygen in the present context is a significantly better barrier to oxygen than is LDPE. To be a good barrier to oxygen, a 25- micron thick sheet of the material has an oxygen transmission rate (TR) lower than 1,000 millilitres/square metre/atmosphere pressure difference/day, and preferably has a TR lower than 100. Examples of such material are ethylene vinyl alcohol (EVOH), polyamide (PA), cellophane, greaseproof paper; and coatings of silicon oxide (SiO x) ) (i.e. glass), metal (e.g. Al) , and metal oxide (e.g. A10 x ) . Thus the barrier layer may consist of one or more of metal (e.g. Al) foil; a polymeric barrier layer, e.g. EVOH or PA; a polymeric material coated by a deposition process (for example with a metal, metal oxide, or glass coating) ; greaseproof paper; and cellophane. The product-contact layer may be one or more polymers, especially one or more of PO, e.g. linear LDPE (LLDPE), LDPE, very LDPE (VLDPE) , high density polyethylene (HDPE) or polypropylene (PP) ; "METALLOCENE" POs; bi-modal POs; polyolefinic acid co-polymers (e.g. EAA); ionomers; ethylene vinyl acetate (EVA) ; blends of such polymers; and filled such polymers. Advantageously, the product-contact layer is of PO, preferably PE, most preferably LDPE with good organoleptic properties. In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 shows diagrammatically an extrusion-coating line manufacturing a packaging laminate, and Figures 2 to 4 show diagrammatic cross-sections through varieties of the laminate which can be produced by variations of the extrusion-coating line. Referring to Figure 1, paperboard 1 is fed from a roll 3 to the nip between a pair of rollers 4 and 5 of a first extrusion lamination station of which the roller 5 is a chill roller. An alternate, ready roll 2 of paperboard is also shown. To the nip is supplied a molten film of PE from a die slot of a high-temperature extruder 7. The PE layer so supplied is to constitute the outside (with respect to a carton to be formed of the laminate) surface of the packaging laminate. The web 2 so formed is forwarded to a second extrusion lamination station at which it enters the nip between a pair of rollers 8 and 9 of which the roller 9 is a chill roller. Also fed into the nip is a web 10 of an oxygen barrier substrate, in this example aluminium foil, fed from a supply roll 11, whilst between the paperboard layer 1 and the aluminium foil web 10 is introduced a molten film 13 of a polymeric tie material from a die slot of a high- temperature extruder 12. The web 14 so formed then advances to a third and final extrusion lamination station at which it passes between a pair of rollers 15 and 16, of which the roller 16 is a chill roller. Here, there is co-extruded, to between the aluminium foil layer 10 and the chill roller 16, a molten film 17 consisting of a polymeric tie layer 18 and a PE layer 19, which latter directly contacts the chill roller 16. The molten film 17 is co-extruded from a low- temperature extruder 20 which is so positioned that its die slot 21 is at a horizontal level as close as practical to the horizontal level of the nip between the rollers 15 and 16, so as to minimise the potential for oxidation of the molten film 17, particularly of the product-contact PE layer 19, before it is cooled by the chill roller 16. The web 22 so formed is then wound onto a roll 23 for transport to, for example, a converting plant where the web is converted into blanks for forming liquid-packaging cartons in which the outside PE layer 6 is heat-and pressure-sealed to the inside, product- contact, PE layer 19. The manufacturing line illustrated in Figure 1 has the particular advantage that it can be formed by simply replacing the high-temperature, final extruder in a conventional extrusion-coating line with a low-temperature extruder 2) . The low-temperature extruder 20 could be an "INSTAMELT"® rotary extrusion system available from Extrusion Systems, Inc., 1549 S. Fairground, Midland, Texas, United States of America, or a planetary roller extrusion system available from Entex Rust & Mitschke GmbH of Heinrichsstrasse 67, D-44805 Bochum, Germany. An example of the packaging laminate produced on the line shown in Figure 1 is as follows:- Layer Reference Layer Material Preferred range of Hc layer "thickness" (g/m 2 ) 6 LDPE 5-20 1 PAPERBOARD 140-450 13 LDPE 5-20 10 ALUMINIUM FOIL 15-25 18 "SURLYN"® 3-10 19 LDPE 10-50. The LDPE supplied to the extruder 20 should not have been brought to a temperature at or above its breakpoint temperature .prior to its entry into the extruder 20 and should not be raised to or beyond its breakpoint temperature at any point between entry into the extruder 20 and the chill roller 16. This is preferably also true of the tie layer 18. An example of a packaging laminate produced on a variation of the line of Figure 1 is shown in Figure 2 and includes an oxygen barrier substrate 10 in the form of a film 27 of polyethylene terephthalate (PET) with a deposited coating 28 of Al or SiO x . The substrate 10 is attached to the paperboard layer 1 and to the product-contact layer 19 by way of respective tie layers 25 and 29. The layer 29 may be co- extruded with the layer 19 and in those circumstances is such that its breakpoint temperature is above the co-extrusion temperature of the extruder 20. The coating 28 is to the inside of the PET film 27 to form a barrier against transmission of off-flavour-producing compounds from the PET film 27 towards the product. An example of the packaging laminate of Figure 2 is as follows:- Layer Reference Layer Material Preferred range of No. layer "thickness" (σ/m 2 ) 6 LDPE 5-20 1 PAPERBOARD 140-450 25 LDPE 5-20 10 Al-or SiO x -coated PET 8-20 29 EMA 5-20 19 LDPE 10-50. [EMA is ethylene meta-acrylic co-polymerl . Again, the LDPE supplied to the extruder 20 to constitute the LDPE layer 19 should not have been brought to a temperature at or above its breakpoint temperature prior to its entry into the extruder 20 and should not be raised to or beyond its breakpoint temperature at any point between entry into the extruder 20 and the chill roller 16. This is preferably also true of the tie layer 29. An example of a packaging laminate produced on another variation of the line of Figure 1 is shown in Figure 3 and includes an oxygen barrier substrate 10 in the form of a a layer 10 of EVOH extrusion-coated onto the inside of the to the paperboard layer 1. The layer 10 is attached to the product-contact layer 19 by way of a tie layer 32 of anhydride-modified PO, preferably anhydride-modified LLDPE co-extruded with the layer 19 and having its breakpoint temperature above the co-extrusion temperature. An example of the packaging laminate of Figure 3 is as fol lows : - Layer Reference Layer Material P re fe : rreU range of Ho_ layer "thickness" (g/m 2 ) 6 LDPE 5-20 1 PAPERBOARD 140-450 10 EVOH 3-25 32 LLDPE 3-10 19 LDPE 10-50 . The LDPE supplied to the extruder 20 to constitute the layer 19 should not have been brought to a temperature at or above its breakpoint temperature prior to its entry into the extruder 20 and should not be raised to or beyond its breakpoint temperature at any point between entry into the extruder 20 and the chill roller 16. This is preferably also true of the tie layer 32. An example of a packaging laminate produced on a further variation of the line of Figure 1 is shown in Figure 4 and includes an oxygen barrier substrate 10 in the form of a film 35 of EVOH carrying tie layers 36 and 37 at the outside and the inside thereof. The substrate 10 is extrusion-laminated to the paperboard layer 1 by the LDPE 13 and the product- contact layer 19 is extrusion-coated on the layer 37. The layers 36 and 37 are of anhydride-modified PO, preferably anhydride-modified LLDPE. An example of the packaging laminate of Figure 4 is as follows:- Layer Reference Layer Material Preferred range of NO. layer " hickness" (g/m 2 ) 6 LDPE 5-20 1 PAPERBOARD 140-450 13 LDPE 5-20 10 LDPE/EVOH/LDPE 8-20 19 HDPE 10-50. The HDPE supplied to the extruder 20 to constitute the LDPE layer 19 should not have been brought to a temperature at or above its breakpoint temperature prior to its entry into the extruder 20 and should not be raised to or beyond its breakpoint temperature at any point between entry into the extruder 20 and the chill roller 16.

Claims: CLAIMS 1. A method of manufacturing a packaging laminate, comprising supplying a substrate (10) providing a good barrier to transmission of oxygen, and applying to said substrate a thermoplastics, product-contact layer (19) by extrusion through a die slot (21) at an extrusion lamination station, adhesion between said product-contact layer (19) and said substrate (10) having been promoted, characterized in that the temperature of said product-contact layer (19) is maintained below the breakpoint temperature of the thermoplastics material of said product-contact layer (19) throughout said method. 2. A method according to claim 1, wherein the temperature of the thermoplastics of the product-contact layer (19) has been maintained below its breakpoint temperature prior to its supply to said extrusion lamination station. 3. A method according to claim 1 or 2, wherein said adhesion has been promoted by one or more of surface treatment of the receiving surface of the substrate (10) ; applying an adhesive to the receiving surface of the substrate (10) ; pre-heating of the substrate (10) ; employing electrodes to deposit a layer of glass on the substrate (10) ; and co-extruding, in a low-temperature extruder, a low-temperature tie layer (18) and the product-contact layer (19) . 4. A method according to claim 1, 2 or 3, wherein said barrier layer (10) consists of one or more of metal foil, a polymeric oxygen barrier layer, a polymeric material coated by a deposition process, greaseproof paper, and cellophane. 5. A method according to any preceding claim, wherein said product-contact layer comprises one or more of a polyolefin, a polyolefinic acid co-polymer, an ionomer, and ethylene vinyl acetate. 6. Apparatus for use in manufacturing a packaging laminate, comprising an extruder (20) including a die slot (21) for producing a layer (19) of thermoplastics material by extrusion, chilling means (16) immediately downstream of said die slot (21) for chilling said layer (19), means (11) for supplying a substrate (10) providing a good barrier to transmission of oxygen, roll means (15) for guiding said substrate (10) to said layer (19), and adhesion-promoting means (20,21) for promoting adhesion between said substrate (10) and said layer (19) , characterized in that said extruder (20) is a low-temperature extruder (20) . 7. Apparatus according to claim 6, wherein said low- temperature extruder is one or more of a substantially screwless rotary extruder with a gear pump; a planetary roller extruder with a gear pump; a twin-screw extruder; a screw extruder with a screw designed for low-temperature extrusion; and an extruder of the character conventionally used for cast film of said thermoplastics material.

Inventors: JOHANSEN GEIR MORTEN, JOHANSEN, GEIR, MORTEN
Applicants: ELOPAK SYSTEMS, JOHANSEN GEIR MORTEN, ELOPAK SYSTEMS AG, JOHANSEN, GEIR, MORTEN
Application year: 1997
Main classification: B32B31
IPC codes: B32B7, B32B27, B32B37