WO1996016877A1

WO1996016877A1 - Laminates with coatings

Info

Publication number: WO1996016877A1
Application number: PCT/GB1995/002760
Authority: WO
Inventors: Jan Eggestad; Mohan Lal Sankuja; Paul Thottathil
Original assignee: Elopak Systems Ag
Priority date: 1994-11-26
Filing date: 1995-11-27
Publication date: 1996-06-06
Also published as: JPH10509123A; EP0793605A1; GB9423906D0; MX9703842A; AU3932295A

Abstract

A packaging container or a packaging web comprises a laminate comprised of layers of differing substances of which one is a stiffening layer or substrate and at least another is a layer which is bonded directly or indirectly onto the stiffening layer or substrate by chemical grafting.

Description

LAMINATES WITH COATINGS

This invention relates to laminates with coatings, and concerns in particular the provision of coatings, especially barrier coatings, of packaging board to provide laminate that is subsequently going to be converted into containers for foodstuffs, detergent chemicals and the like.

Conventional laminates used in packaging, such as gable-top and flat-top containers, are basically a layer of paperboard, giving stiffness and the necessary support to the container, coated with low density polyethylene (LDPE) on both sides to protect the paperboard against moisture penetration from either the contents or the surroundings, and to give sealability.

Often there is included a barrier layer, typically a layer of aluminium foil, to give additional protection to the contents against the penetration thereinto of oxygen and other, possibly harmful, substances from the environment. The barrier requirements vary in dependence upon the intended contents of the container.

A gable-top carton is a versatile container, suitable for many different products. Typical products that can be packed in such cartons are foodstuffs (such as milk and orange juice) and detergents and fabric softeners; such products need to be protected against leakages and against weight loss through moisture transmission. Moreover, many of these products - especially detergents and the like - also normally contain perfume, and in order to protect the environment from the smell (which could unpleasantly contaminate other products in the immediate neighbourhood) a barrier against gas permeation is needed. It is therefore commonly known that these products need a laminate with a barrier layer - a barrier laminate - and that the barrier should be of a kind appropriate to the task it is to perform.

The conventional laminate incorporating aluminium foil as its barrier layer is suitable for packaging many of these products, but the environmental impact of the aluminium foil

(by virtue of the high energy consumption used in its production) and the cost are disadvantages of this sort of laminate. Other laminates with good barrier properties are produced by applying to a basic laminate including paperboard an additional layer using methods including extrusion coating, co-extrusion coating or extrusion lamination of a barrier polymer such as ethylene vinyl alcohol (EVOH) and polyamide (PA). For example, WO92/01558 teaches that a packaging carton should consist of a laminate comprising inner and outer layers which serve to stiffen the laminate, and an intermediate layer by way of which the inner and outer layers are tied together. In one embodiment, the inner layer and the intermediate layer both serve as relatively good barriers to O₂ migration, the intermediate layer being EVOH or PA extruded or co-extruded onto the inner layer, which comprises M-PET (metallized polyethylene terephthalate), G-PET

(glass-coated PET), M-OPP (metallized, oriented polypropylene), G-OPP (glass-coated, OPP), or greaseproof paper, the coated surface of the M-PET, G-PET, M-OPP, or

G-OPP, facing outwards. In another embodiment, an aluminium-metallized EVOH film constitutes the inner stiffening layer and has been extrusion-laminated by an intermediate LDPE layer to the outer stiffening layer.

The aforementioned laminates have the disadvantages that they require costly equipment for their manufacturing, even though the polymer they use does make an efficient and satisfactory barrier layer.

US-A-4206249 teaches a process for producing a paper container having high impermeability to liquid comprising spray coating (by means of hot-melt airless spraying) a polymerizable solution containing at least a prepolymer and a photo-sensitizer and, if necessary, a reactive diluent onto a wall surface of a previously fabricated paper container, and causing the prepolymer (and reactive diluent) to polymerize by ultraviolet irradiation to thereby form a liquid impermeable coating on the wall surface of the container. Spraying and then UV irradiation of containers is a disadvantageous way of producing a barrier coating, being inefficient in time required and unreliable in consistency of coating thickness and curing, especially for the interiors of containers of a high height/width ratio.

Chemical Abstracts, volume 80, no.10, 11 March 1974, Abstract no. 48876u discloses that biaxially stretched poly (vinyl alcohol) film was coated with a vinyl polymer to give a packaging material having low permeability to moisture and gases. In an embodiment, a poly (vinyl alchohol) film was biaxially stretched to 225% of its original area to give a film having a reduced swelling rate when immersed in water. The film was coated with an emulsion containing 10 parts vinyl chloride - vinylidene chloride co-polymer to give a film having low permeability to moisture and oxygen compared with uncoated poly (vinyl alchohol) film.

The invention tackles the problems of the prior art by suggesting that a layer of some suitable material typically polyvinyl alcohol (known either as PVA or PVOH) or PA - could be formed on the basic paperboard (with or without the usual LDPE coatings thereon) in a quite different way, and specifically by a chemical grafting [graft polymerisation] process.

Accordingly, in one aspect the present invention provides a packaging container comprising a plurality of layers of differing substances at least one of which is a suitable stiffening layer and at least another of which is a suitable layer which is bonded directly or indirectly onto the stiffening layer by chemical grafting.

In a second aspect the invention provides a packaging material in the form of a laminate comprising a plurality of layers of differing substances at least one of which is a suitable substrate and at least another of which is a suitable layer which has been formed in situ directly or indirectly on the substrate by chemical grafting.

The laminate may take the form of film or board.

In a third aspect the invention provides a method for making a packaging material or a packaging container of the invention - that is, in the form of or made from a laminate comprising a plurality of layers of differing substances at least one of which is a suitable substrate or stiffening layer and at least another of which is a layer suitable for some other purpose - in which method the said other layer is bonded directly or indirectly onto the substrate or stiffening layer by chemical grafting.

The invention provides a packaging material and a packaging container - that is, it provides a material and a container each of which is intended for use in the packaging of products such as foodstuffs, e.g. milk, yoghurt and fruit juices, and such as basic household chemicals, e.g. washing powders, liquid detergents, and fabric softeners. The packages will normally be in the form of gable-top or flat-top cartons.

The packaging material/container is in the form of, or made from, a laminate comprising a plurality of layers of differing substances at least one of which is a stiffening material and suitable substrate. Typical such substrates are of fibrous and/or plastics substances, for example polymers and fibrous cellulose-based materials such as paperboard (of an appropriate weight), which may already have a coating thereon of a waxy substance such as LDPE, and fabrics woven out of fibrous synthetic polymers such as polyethylene (PE) itself.

At least another of the layers making up the laminate packaging material of the invention is a chemically-grafted layer. Because this layer is bonded on by chemical grafting - that is, it is chemically attached (by strong covalent bonds) to the molecules making up the underlying layer - the layers themselves may be more firmly interconnected, and thus less likely to delaminate, than if they relied upon mere physical bonding (as is the case with common extrusion and gluing). Moreover, the conventional use of tie layers can be avoided.

The chemically-grafted layer may have one or more of a number of functions. Thus, it may serve as a barrier to oxygen or water vapour, for instance - and/or it may act as an oxygen scavenger and/or it may have some microbiological function. In this latter connection it is possible to employ as part of the laminate a material that will effect some biological control on the contents of any container made from the laminate. For example, there might be included a substance having a high affinity either for biological cells or for some growth factor for the cells, or there might be incorporated a material having either some direct inhibiting effect on cell growth or an indirect effect (as by removing necessary oxygen). Such use of biological-control substances is disclosed in a number of our earlier Patent Specifications, including WO 89/05,762 and that of our co-pending British Application No: 94/21,123.2. Where a foodstuff is to be packaged, and if the grafted layer is likely to be exposed to that foodstuff, then the layer should be of a composition which does not transfer an off-taste to the foodstuff nor be otherwise detrimental thereto. Moreover, if the foodstuff is to be subjected to heat treatment - for instance, by microwaving - while still in the packaging, then naturally the layer material should stand up both to the microwaves themselves and to the high temperatures - say, 100°C - engendered.

If the packaging material is a board to be formed into cartons, then the grafted layer may be bonded either directly to a stiffening layer/substrate, such as paperboard, or indirectly thereto, such as via an intermediate layer of

PE, particularly LDPE. Moreover, the grafted layer may be either a surface layer of the board or it may itself be covered by a further layer, for example an LDPE one.

Perhaps the most common purpose for the grafted layer will be as a barrier layer. Typical barrier layers are much the same as those polymeric materials already used in the Art to provide good oxygen/moisture barriers save that they are grafted (and perhaps polymerised) into place rather than being merely physically affixed to the substrate. Such materials are various PAs and PVOH [although PVOH tends to be water soluble, and so, if it is likely to be exposed to water, may need a protective surface coating, typically of an acrylic polymer such as NEOCRYL A10512, from ICI]. Poly-vinylidene chloride (PVDC) also makes excellent barrier layers, although its use is restricted in view of possible toxic side effects of the initial raw materials.

According to a fourth aspect, the present invention provides a packaging container comprising a plurality of layers of differing substances at least one of which is a suitable stiffening layer and at least another of which is a layer comprised of PVOH which is bonded directly or indirectly onto the stiffening layer by chemical grafting. In a fifth aspect the invention provides a laminate comprising a plurality of layers of differing substances at least one of which is a suitable substrate and at least another of which is a layer comprised of PVOH which has been formed in situ directly or indirectly on the substrate by chemical grafting.

In a sixth aspect the invention provides a method for making a material or a packaging container of the invention - that is, in the form of or made from a laminate comprising a plurality of layers of differing substances at least one of which is a suitable substrate or stiffening layer and at least another of which is a PVOH layer - in which method the PVOH layer is bonded directly or indirectly onto the substrate or stiffening layer by chemical grafting.

Advantageously the PVOH is bonded to cellulose, for example paperboard, or to PE.

As already noted, the grafted layer is bonded either directly or indirectly to the substrate. Thus, for example, where the substrate is paperboard the layer formed thereon by chemical grafting may either be formed directly thereon, so that it is bonded directly to the underlying paperboard material, or, as is generally preferred, the paperboard substrate may have an intermediate coating (of LDPE, say) so that the layer is formed on the intermediate coating and thus indirectly on the paperboard.

The grafted layer is most conveniently formed in situ by graft polymerisation - that is to say, the substrate is exposed to monomer- or prepolymer-type chemicals which react not only with each other, to form the layer, but also with the substrate, so "grafting" themselves, and the layer, chemically to the substrate (as already noted, with the chemicals usually employed, the graft is by way of covalent bonds, and so a properly-formed grafted layer should adhere strongly to the substrate).

Graft polymerisation is in general a fairly standard technique. As applied to the present invention, the surface on which the graft polymer is to be formed is, if necessary, first treated with an initiator (to provide active sites at which grafting can take place), and then it is treated with the "monomer" to be grafted (this "monomer" can with advantage be a low molecular weight prepolymer), possibly in the presence of one or more co-monomer and/or cross-linking agents (to assist in the creating of a tight film).

A typical initiator is silver perchlorate (which can be regenerated in situ using hydrogen peroxide), which is a free-radical generator, and provides a multiplicity of free bonds/radicals on the substrate surface to which the polymer-forming monomer can then attach itself.

The monomer (or prepolymer) will naturally be one appropriate to the layer required. For example, a typical PVOH starting material is that polyvinyl alcohol prepolymer sold by Air Products under the name AIRVAL 523, while a typical prepolymer of PVDC is that available from Dow Chemicals under the name SARAN FC310. A possible acrylic/PVDC prepolymer is Prepolymer 76 Res M3 153, from Unical Chemicals.

The monomer may be used with a co-polymer or cross-linker. Examples of these are the silane monomer Silane A1100, available from Union Carbide, sodium vinyl sulphonate, available from Air Products, the acrylic Prepolymer 76 Res 5517, from Unical Chemicals, and hydroxy ethyl acrylate, available from Dow Chemicals. Another cross-linker is the polyaziridine CX-100, from ICI.

Examples of the use of these materials are given hereinafter. The application of these chemicals to the substrate can be combined so that there is used a single treatment composition that can simply be applied (by wiping or printing, say) to the substrate surface. The composition can even be printed onto the substrate using the same flexographic, offset-litho or rotogravure equipment already commonly in use for printing text and graphics onto packaging laminates, and this printing can conveniently be carried out actually during a conversion stage - that is, as the laminate is being converted from a web into individual container blanks - and can be performed in-line with the printing of whatever textual and pictorial matter is required on what will be the external surface of the container. This will naturally keep down the cost of the operation.

The grafted layer is normally placed on the inside surface zone of the laminate that defines, say, the carton blank to be produced, although it need not cover the whole of that surface zone. However, if it does extend over those inside surface portions which are to be heat- and pressure-sealed in forming the carton then the grafted layer should be, or include, a substance which enables this sealing.

Multilayer packaging laminates made with grafted layers can be relatively inexpensive, and have been shown to have good oxygen- and moisture-barrier characteristics, and are thus suitable for use with cartons and containers for all sorts of materials, including many foodstuffs and household chemicals.

Various embodiments of the invention are now described, though by way of illustration only, with reference to the following Examples and Test Results.

Examples and Test. Results

The Formulations identified in the Table below (in each case except Formulation No: 24 the initiator used was silver perchlorate, as 0.1 parts by weight of a 0.1% solution: in No: 24 it was 0.1 pbw of 0.1% silver nitrate solution) were prepared as described hereinafter and then used to form barrier coating layers on LDPE substrates. The substrates were provided by conventional milk-packaging laminate consisting of paperboard extrusion-coated on both faces with LDPE. Where the formed barrier coating layers were water soluble (shown by the asterisk in the Table) they were given a water-resistant topcoat as described hereinafter.

The resultant coated laminate substrate was then tested for oxygen and moisture permeability by the Tests described below: the Results of these Tests are also shown in the Table.

Formulation preparation

1) The coat or prime coat of Formulations Nos: 2, 10, 14 and 24 was prepared as follows:- The AIRVAL PVA prepolymer was added in small portions to the indicated amount of de-ionised water in a container, with stirring. Stirring was continued for 5-10 minutes, the mixture's temperature was raised to 180°-200°F (about

80°-95°C), and stirring was continued for 30 minutes. The mixture was then cooled to room temperature, and the

A1100 silane monomer added with stirring. The other ingredients were then added with continuous stirring cross-linker followed by co-monomer, then the regenerator and lastly the initiator.

The contents were stirred to a uniform solution, and the resultant Formulation was then ready for use.

Top coat

Where the prime coat was a water soluble polymerised vinyl alcohol it was given a protective top coat, as follows:-

A) For use with the prime coat provided by Formulation No:

10, 50pbw NEOCRYL A612 were mixed into a container of 16pbw de-ionised water. Separately, 1pbw of crosslinker polyaziridine CX-100 were mixed with 0.1pbw of 0.1% urea peroxide solution. The two were then stirred into a mixture of 11pbw de-ionised water and 6.5pbw butyl cellosolve. The thus-prepared Formulation was used to provide the required top coat.

B) For use with the prime coat provided by Formulation No: 24, 90pbw acrylic/polyvinylidene chloride prepolymer 76 Res M₃ 153 were mixed into a container of 10pbw isopropyl alcohol. 0.1pbw of hydrogen peroxide were then added, followed by 0.1pbw hydroxy ethyl acrylate. Finally, 0.1pbw of 0.1% silver perchlorate were added, and the thus-prepared Formulation was used to provide the required top coat.

2) Formulations Nos: 8, 17 and 22 were prepared as follows:-

The requisite amount of polyvinylidene chloride prepolymer was mixed in a container with the other ingredients - solvent, regenerator, co-monomer and finally initiator - to provide the required Formulation. Formation of a barrier layer

For the purposes of testing, samples of each Formulation were applied to the substrates using a conventional Meyer rod applicator. The layer was then cured in an oven at 100°C for 2-5 minutes (in an air-circulating oven this curing time can be reduced to merely seconds).

Where the prime coat needed a top coat, the latter was applied to the cured prime coat in exactly the same way that the prime coat had been applied to the substrate with a Meyer rod applicator, followed by curing at 100°C for 2-5 minutes.

The Tests

The several Formulations prepared were applied to packaging board of a number of different grades. Each such board included a paperboard stiffening layer of a 300g/m² weight carrying (both inside and outside) an LDPE layer

(26g/m² inside, 14g/m² outside), and the Formulation was used to make a graft polymer barrier layer on the inside LDPE layer. The coating was found to be between 0.1 and 0.2 mil (2.5 and 5 microns) thick.

The resultant barrier layered paperboard samples were tested for oxygen transmission by the standard ASTM D3985-81 Test (carried out at 23°C and at 50% relative humidity), and some for moisture transmission (Mocon Permatron-W Twin in accordance with ASTM F 372-73). The results are shown in the Table above.

Some of the samples were also tested for sealability - to see at what minimum temperature they could be heat-sealed (as would be necessary were the treated board to be used to make a carton). The results for the samples coated with Formulations Nos: 2, 8 and 10 are, respectively, 105°, 115° and 105°C, as shown in the Table above.

Claims

1. A packaging container comprising a plurality of layers of differing substances at least one of which is a stiffening layer and at least another of which is a layer which is bonded directly or indirectly onto the stiffening layer, characterised in that the other layer is bonded directly or indirectly onto the stiffening layer by chemical grafting.

2. A packaging material in the form of a laminate comprising a plurality of layers of differing substances at least one of which is a substrate and at least another of which is a layer which has been formed in situ directly or indirectly on the substrate, characterized in that the other layer has been formed in situ directly or indirectly on the substrate by chemical grafting.

3. A container or a material according to claim 1 or 2, wherein said other layer serves as a barrier layer.

4. A container or a material according to claim 3, wherein said other layer serves as a barrier to oxygen and/or water vapour.

5. A container or a material according to claim 3 or 4, wherein said other layer comprises any one of PA, PVOH and PVDC.

6. A container or a material according to claim 1 or 2, wherein said other layer serves as an oxygen scavenger.

7. A container or a material according to claim 1 or 2, wherein said other layer has a microbiological function.

8. A container according to claim 7, wherein said other layer has a micro-biological control on contents of the container.

9. A container or a material according to any preceding claim, wherein said other layer is bonded to said stiffening layer or said substrate by way of a layer of PE.

10. A container or a material according to any preceding claim, wherein said other layer is a surface layer of said container or said material.

11. A container according to claim 10, wherein said other layer is an internal surface layer of said container.

12. A container or a material according to any one of claims 1 to 9, wherein said other layer is covered by a further layer.

13. A container or a material according to any preceding claim, wherein said stiffening layer or said substrate comprises fibrous and/or plastics substance.

14. A container according to claim 1 or any one of claims 3 to 13, and containing a foodstuff or a household chemical.

15. A container according to claim 1 or any one of claims 3 to 14 and in the form of a gable-top or flat-top carton.

16. A method for making a packaging material or a packaging container, comprising bonding directly or indirectly onto a substrate or stiffening layer at least another layer, characterised by bonding the other layer onto the substrate or stiffening layer by chemical grafting.

17. A method according to claim 16, wherein said other layer is bonded indirectly to said substrate or stiffening layer by way of a layer of PE.

18. A method according to claim 16, wherein said other layer is bonded directly to said substrate or stiffening layer, which comprises fibrous substance.

19. A method according to claim 16, 17 or 18, wherein said chemical grafting comprises graft polymerisation utilizing monomer - and/or prepolymer-character chemicals which react not only with each other but also with said substrate or stiffening layer.

20. A method according to claim 19, wherein said chemicals are applied as a single treatment composition to said substrate or stiffening layer.

21. A method according to claim 20, wherein said chemicals are applied to said substrate or stiffening layer by wiping or printing.

22. A method according to any one of claims 16 to 21, wherein said other layer is applied to said substrate or stiffening layer in a system in which said packaging material in web form is converted into container blanks.

23. A method according to any one of claims 16 to 22, wherein said other layer is applied to said substrate or stiffening layer outside zones of said substrate or stiffening layer which are to be subjected to heat and pressure in a subsequent sealing step.

24. A packaging container comprising a plurality of layers of differing substances at least one of which is a stiffening layer and at least another of which is a layer comprised of PVOH which is bonded onto the stiffening layer, characterised in that said PVOH is bonded onto said stiffening layer by chemical grafting.

25. A laminate comprising a plurality of layers of differing substances at least one of which is a substrate and at least another of which is a layer comprised of PVOH, characterised in that said layer comprised of PVOH has been formed in situ directly or indirectly on the substrate by chemical grafting.

26. A container or a laminate according to claim 24 or 25, wherein said PVOH is bonded to cellulose or PE of said stiffening layer or said substrate.

27. A method for making a material or a packaging container comprising bonding a layer comprised of PVOH to a substrate or stiffening layer, characterised in that the PVOH layer is bonded directly or indirectly to the substrate or stiffening layer by chemical grafting.