WO2012175621A1

WO2012175621A1 - Method for producing coated vacuum metallized substrates with high vapour and oxygen barrier properties

Info

Publication number: WO2012175621A1
Application number: PCT/EP2012/061990
Authority: WO
Inventors: Jacky VANDEN ECKER; Paul VAN EMMERICK
Original assignee: Ar Metallizing N.V.
Priority date: 2011-06-21
Filing date: 2012-06-21
Publication date: 2012-12-27
Also published as: NL2006977C2; CN103796832A; EP2723570A1

Abstract

The present invention relates to a process for preparing a metallised sheetlike substrate suitable as flexible packaging material having a moisture vapour transmission rate (MVTR) of below 20 g/m² d and an oxygen transmission rate (OTR) of below 25 cm³/m² d bar, comprising the steps of: a) depositing on a non-woven sheetlike substrate at least a first coating composition selected from the group consisting of i) film forming protein in combination with a polymer binder; ii) a polyvinylidene chloride solution, emulsion or dispersion, and iii) an aqueous dispersion of a clay and a polymer binder and combinations thereof, to form a first coating layer, and b) applying a metal layer by vacuum deposition on the first coating layer; and to the products thus obtainable.

Description

METHOD FOR PRODUCING COATED VACUUM METALLIZED SUBSTRATES WITH HIGH VAPOUR AND OXYGEN BARRIER

PROPERTIES Field of the Invention

The present invention relates to metalized substrates, more specifically to non-woven sheet like materials, such as paper, having a Moisture vapour transmission rate (MVTR) of below 20 g/m² d and an Oxygen transmission rate (OTR) of below 25 cm³/m² d bar.

Background of the Invention

Flexible food packaging for perishable foodstuffs and other sensitive materials such as for instance tobacco has traditionally been prepared from aluminium foil, laminates comprising aluminium foil and polyolefin materials, such as polyethylene (PE), poplypropylene (PP), or condensation polymers such as polyethylene terephthalate (PET) or polyamide (PA) foils. These films are often also coated with barrier coatings such as polyvinylidene chloride to improve oxygen barrier properties, and may be provided with a heat sealing material such as wax or hot melt adhesive.

A wide variety of multilayer laminate structures has been developed to provide barrier properties and other performance characteristics suited for packaging purposes.

However, the use of aluminium foil and polymeric laminates or metallized polymers has come under pressure due to the high energy requirement, as well as the difficulty to recycle laminates, leading to a large amount of difficult to recycle waste, which also is not or only slowly

biodegradable, and hence gives environmental concerns.

Materials that are readily biodegradable, in particular cellulosic materials, usually do not have required low moisture vapour and oxygen transmission rates to permit the application as flexible food packaging. Applicants have now surprisingly prepared a non-woven cellulose- based material that offers high barrier properties versus oxygen and waste vapour, while at the same time offering very high, if not complete recyclability and high biodegradability.

SUMMARY OF THE INVENTION

The present invention relates to a process for preparing a metallised sheetlike substrate having a moisture vapour transmission rate (MVTR) of below 20 g/m² d and an oxygen transmission rate (OTR) of below 25 cm³/m² d bar, suitable as flexible packaging material, comprising the steps of: a) depositing on a non-woven sheetlike substrate a first coating composition selected from the group consisting of i) film forming protein in combination with a polymer binder; ii) a polyvinylidene chloride solution, emulsion or dispersion, and iii) an aqueous dispersion of a clay and a polymer binder, to form a first coating layer, and b) applying a metal layer by vacuum deposition on the first coating layer.

The process according to the invention is particularly suitable for the processing of sheet-like non-woven materials. More preferably, these are composed of fibrous, preferably at least in part cellulosic material such as tissue, paper, or cardboard, wherein paper is most preferred. The shape of the substrates according to the present invention preferably is in the form of a sheet material, such as for example a film or paper sheet. Typical substrates include one-side coated papers, siliconized papers, craft papers as well as in particular uncoated, highly calendered papers.

The present process step a) comprises coating of the substrate surface with solvent-based or aqueous solutions or dispersions prior to the

metallization step.

The first coating layer has several functions: it serves as a base coat to ensure the smoothness of the substrate surface prior to the metallization process. It however also serves as first barrier layer that reduces the gas and water vapour transition through the material. The first coating layer may be applied to the substrate by any suitable means and process, such as spraying or rolling of a dispersion, emulsion or solution of the coating material. The application may be performed in a single, double, or multiple layers, preferably in a single or double layer.

The application step of the liquid coating material is then typically followed by a suitable drying and/or curing step to remove and solvent or carrier fluid in the case of a dispersion or emulsion or dispersion. This step is preferably performed by providing a heat source and/or gas flow over the coated surface to remove and solvent or carrier fluid in the case of a

dispersion or emulsion. The heat source may be a radiation heat source such as infrared light, and/or preferably provided by the flow of hot air or other suitable gasses in a drying oven or bed. A particularly preferred way of coating application is a process wherein the coating is transferred to the substrate by means of one or more optionally engraved cylinder(s) in a high- speed rotating coating machine, ensuring the right amount of drying energy and humidification steps are applied to the substrate. The engraved cylinder(s) are preferably housed in a closed chamber, more preferably in overpressure, and advantageously use doctor blades for the application. An example of a coating process is disclosed for instance in US 3,1 13,888.

After or during the drying step, the substrate may be further subjected to a smoothing operation, such as calendaring to further increase surface smoothness and to reduce open pores.

The first coating composition may i) comprise a film forming protein. The protein preferably is selected from the group consisting of whey protein isolate, whey protein concentrate, hydrolyzed whey protein, soy protein isolate or concentrate, beta-lactoglobulin, alpha-lactalbumin, milk casein, egg white protein, wheat gluten, cottonseed protein, peanut protein, rice protein, or pea protein, or any combination thereof.

These proteins may be in a denatured or native, undenatured form, or mixtures of thereof, and provide the substrate with a high moisture barrier, in particular if combined with a polymer crosslinker or binder. The latter may advantageously comprise a polyol compound, including but not limited to saccharides such as sucrose, maltose, trehalose, cellobiose, and/or lactose; modified or unmodified starch, or a polymer polyol such as polyvinylalcohol or ethylenevinyl alcohol. The polvinylalcohol may preferably be derived from a hydrolysed polyvinylacetate polymer. The benefit of this coating composition is that it is fully biodegradable, and allows full recyclability of the thus prepared flexible packaging material in the case of cellulosic, i.e. paper or cardboard substrates, and were found by the applicant to result in very high gas barrier, i.e. high oxygen and water vapour barrier properties when combined with the metal layer.

A further possible coating composition ii) comprises a copolymer of vinylidene chloride, usually referred to as polyvinylidene chloride or PVDC. This PVDC composition provides a water vapour barrier layer, which combined with the metal layer of step b), giving low gas permeability, thereby yielding a structure having excellent water and gas barrier properties, and were found by the applicant to result in very high gas barrier, i.e. high oxygen and water vapour barrier properties when combined with the metal layer.

Yet a further coating composition iii) for the first coating layer comprises dispersed clay, especially nanoparticulate clay, and a hydrophilic polymer, such as polyvinyl alcohol (PVA) or ethylene-vinyl alcohol copolymer (EVOH), preferably in combination with a polymeric acrylic dispersion.

The clay used is preferably nanoparticulate. A nanoparticulate clay is a clay with particles having at least one dimension in the nanometre range, i.e. of less than 100 nm. Typical nanoparticulate clay particles have a maximum dimension of less than 100 nm, for example a maximum dimension of less than 50 nm, such as a maximum dimension of less than 20 nm.

Preferably a portion of the clay mineral has been intercalated or exfoliated during the dispersion process. There is no restriction on the type of clay used in this invention provided it is sufficiently dispersible in an aqueous medium and that it is capable of being intercalated or exfoliated during dispersion. In an exfoliated form the aspect ratio of the clay, i.e. the ratio between the length and thickness of a single clay 'sheet' will have an impact on the level of oxygen barrier achieved. The greater the aspect ratio, the more the rate of oxygen diffusion through the dry coating and laminate will be reduced. Clay minerals with aspect ratios between 20 and 10,000 are preferably used. Particularly preferred are those minerals having an aspect ratio greater than 100.

The term "aspect ratio" refers to the proportional relationship between the average length of the major axis divided by the average width of the minor axis, i.e. particle length and width, i.e. L/W, wherein L and W are length and thickness of the clay platelets, respectively.

Examples of suitable clays include kaolinite, montmorillonite, atapulgite, illite, bentonite, halloysite, kaolin, mica, diatomaceous earth and fuller's earth, calcined aluminium silicate, hydrated aluminium silicate, magnesium

aluminium silicate, sodium silicate and magnesium silicate. Commercial examples of suitable materials are Cloisite Na+ (available from Southern Clay), Bentone ND (available from Elementis).

Of these, the montmorillonite clays, are preferred, nanoparticulate clays being most preferred.

The coating composition is applied in the form of a solution or

dispersion of the clay and the polymer in a suitable solvent. The solvent is preferably aqueous, and is more preferably water, optionally containing a small quantity of a miscible co-solvent, such as an alcohol, for example ethanol, n-propanol or isopropanol or a ketone such as acetone.

If desired, in addition to the hydrophilic polymers, PVA and/or EVOH, other polymers or resins may be included in the coating composition, provided these co-resins are themselves compatible in the final composition. Examples of such polymers and resins include solution acrylics, acrylic emulsions, polyesters, alkyds, sulphopolyesters, polyurethanes, vinyl acetate emulsions, poly( vinyl butyral), polyvinyl pyrrol idone), poly(ethylene imine), polyamides, polysaccharides, proteins, epoxy resins and the likes. Suitable amounts and suitable application for clay comprising coatings are well within the skills of an artisan.

It is also possible to include sol-gel precursors in these compositions, e.g. a hydrolysate of tetraethyl orthosilicate. Suitable amounts and suitable application for clay comprising coatings are well within the skills of an artisan.

The first coating composition may also comprise combinations of i) to iii) the above, e.g. protein compositions, clay and/or PVDC, as well as optional polymeric binders.

Such coatings have been found to provide excellent gas barrier properties, but surprisingly were found by the applicant to result in very high gas barrier, i.e. high oxygen and water vapour barrier properties when combined with the metal layer.

The first coating layer typically has a dry film thickness of between 10 μιτι and 0.5 μιτι, preferably between 8 and 0.9 μιτι, more preferably between 6 and 1 .5 μιτι, and most preferably between 5 and 2 μιτι. The coating layer.

The first coating typically is typically applied at a solids content of between 0.5 and 6 g/m², preferably between 0.6 and 4.5 g/m², more

preferably between 1 .5 and 3 g/m².

After application of the first coating layer, the coated substrate is then subjected to a high vacuum metallization process to coat the exposed coating layer surface with a thin metallic surface of a metal. In this high vacuum metal izing process, the metal is vaporized and deposited on the substrate.

Aluminium is particularly well suited for this invention, but other metals such as silver, tin, zinc, gold, platinum, titanium, gold, lead, nickel and tantalum, as well as alloys such as chromium titanium may also preferably be employed.

The metallic layer may serve a number of functions besides its decorative purpose, since it creates a brilliant metallic surface. Applicants have found that the combination of a first coating layer according to the invention, followed by the metal layer serves as a barrier coating which permits a simple paper MVTR and OTR values that are otherwise not achievable for coated paper materials.

With respect to the subject invention, the Water Vapor Transmission

Rate (MVTR) is determined at 38°C and 90% relative humidity (RH) according to the ASTM F1249 Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor.

The oxygen transmission rate (OTR) is expressed in cm³/m²/day, and is determined at 23°C and at 50% relative humidity (RH) according to the ASTM F1927 Standard Test Method for Determination of Oxygen Gas Transmission Rate, Permeability and Permeance at Controlled Relative Humidity Through Barrier Materials Using a Coulometric Detector.

Without wishing to be bound to any particular theory, it is believed that the first coating layer and the metal layer act synergistically, whereby the metal layer effectively blocks or reduces open pores left in the first coating layer such that hitherto unreported gas and water vapour barrier properties are obtained.

More beneficially, these values are achieved without the need to resort to a polymeric polyolefin- or PET based synthetic polymer film or to aluminium foil. Moreover, the paper coating is fully recyclable, since the thin layers of metal and first coating layer will not disturb the degradability or recyclability of the paper. Yet further, since the metal layer is very thin, the first coating layer also effectively acts as a base coating which fills minor surface imperfections and provides a smooth surface to receive the metal deposit and thereby further increases the barrier properties.

The deposited metal adheres to the surface of the first coating. The resulting metal coating thickness typically is in a range of from 40-3000 Angstrom, preferably at least 50, more preferably at least 100 Angstrom, but may generally be in the range of from 10 to 250 nm. The resulting metal -coated substrates may be prone to oxidation, which may cause adherence problems in subsequent treatments and applications, for instance label printing. Furthermore, the surface is prone to scratching or damages. Accordingly, in order to convert the metallised substrate obtained in step b) into an easily overprintable substrate, a topcoat, mostly a clear coat, may advantageously be applied to the metallised substrate in optional step c). Suitable clear coats are for instance disclosed in US3,677,792 and

WO00/77300. Preferably, the metallised substrate may be subjected to calendaring to increase smoothness prior to, after the base coat application, or after the metallization step.

Advantageously, the three coating layers, i.e. first coating layer, metal layer and top coat may together synergistically provide an even higher barrier effect. To achieve this matter, the desired water vapour and oxygen

permeability barrier may be distributed between the three layers. In this case, the topcoat and first coating layer should be chosen such that both impart a barrier effect, e.g. the first coating layer and topcoat are chosen that the desired barrier properties are achieved, for instance by combining any of coating compositions i) or ii) as first coating layer with a coating composition iii) as topcoat.

The overprintable top coat is applied to the surface of the metallic layer to enhance its printability and to reduce potential corrosion.

The top coat may be a solvent based coating, a water based coating or otherwise based coating. The topcoat is chosen such that it preferably does not affect the metal layer, the base coat, or the substrate in any significant way, thus avoiding for instance delamination or corrosion of the metal layer.

The application process of the coating to a substrate is preferably done by means of one or more optionally engraved cylinder(s), ensuring the right amount of drying energy and humidification steps are applied to the substrate. The machine may use corona treatment to regulate the surface energy of the substrate prior to coating. The engraved cylinder(s) are preferably housed in a closed chamber, more preferably in overpressure, and advantageously use doctor blades for the application. The application may be performed in a single, double, or multiple layers, preferably in a single or double layer.

The topcoat preferably provides the sheet with physical characteristics that result in a final sheet having the desired characteristics of good

appearance, high gloss, high metal adhesion, satisfactory printability, high wet rub, high dry and wet flexibility, and other factors such as low wet expansivity, curl stability, corrosion resistance, excellent ink retention, and/or short wash off / deglueing. Yet further, the topcoat should be overprintable, i.e. allow application of printing inks in any suitable printing process, without causing compatibility issues or defaults, such as wetting problems. The overprintability of topcoats is an important issue in the area of packaging and labelling in applications such as ink printing, hot foil stamping and thermal transfer printing.

Clear coats usually applied for metallized substrates are known to be difficult to overprint, due to the inherent low surface tension, and the potential presence of waxes and silicone additives as flowing aids and/or defoamers in the formulations. Accordingly, the top coat employed in the present process is overprintable, i.e. providing good adhesion to inks when printed onto the topcoat and properly cured, and substantially without surface defects.

The top coat typically has a dry thickness of between 10 μιτι and 0.5 μιτι, preferably less than 5 μιτι, more preferably less than 4 μιτι, and most preferably less than 3 μιτι. The top coat preferably has a dry thickness of at least 0.5 μιτι, more preferably at least 0.6 μιτι, and most preferably at least 0.7 μιτι. The top coat typically is applied at a solids content of between 6 and 0.5 g/m², preferably between 3 and 0.6 g/m², more preferably between 2 and 0.85 g/m².

In a second aspect, the present invention also relates to the products obtainable in this process, and to their use in flexible packaging for perishable goods. The products according to the present invention are good to highly biodegradable, and can be sealed easily, e.g. hot sealing.

The products obtained in the process according to the present invention have a Moisture vapour transmission rate (MVTR) of below 20 g/m² d and an Oxygen transmission rate (OTR) of below 25 cm³/m² d bar.

Preferably, the products have a MVTR of below 19, more preferably below 18, yet more preferably below 15, yet more preferably below 13, again more preferably below 10, and again more preferably below 8 and most preferably below 5 g/m² d. Preferably, the products have a OTR of below 18, more preferably below 15, yet more preferably below 12, again more preferably below 10, more preferably below 8 and most preferably at or below 6 cm³/m² d. In a preferred embodiment of the present process, the substrate obtained in step b) or c) may be further laminated at the non-coated side to obtain a laminated backside barrier. This may be advantageously be performed with a polymeric film, such as for instance polyethylene, or coating layers as obtainable from polyester dispersions. The thus obtained substrate even has higher barrier properties, thereby achieving an MVTR values of from 1 to 3, and an OTR of less than 0.2.

According to one embodiment of the invention, metallised paper sheet materials are produced with superior gas and vapour barrier performance characteristics which can be tailored to specific end use applications.

For example, the metallised paper can be produced with a very shiny, high gloss surface appearance. The present invention preferably relates to the product obtainable by the subject process, i.e. a metalized product comprising a first barrier coating layer, a metallic layer deposited directly onto the surface of the first coating layer, and optionally a topcoat layer applied onto the surface of the metal layer.

The product may advantageously be subjected to other processing, depending on the desired end use; for example, used as a packaging wrap or bag. The present invention hence preferably relates to a substrate according the invention, comprising I) a non-woven sheetlike substrate; II) a first coating layer comprising i) at least one protein composition in combination with a polymer binder; ii) a polyvinylidene chloride solution, emulsion or dispersion, and/or iii) a clay and a polymer binder, and III) a vacuum deposited metal layer adhering to the first coating layer, and optionally IV) a top coat deposited on the metal layer, and further optionally V) a backside barrier coating on the non-metallized side of the sheetlike substrate.

The present invention further relates to a flexible packaging for perishable goods comprising a substrate as set out above, and the use of the substrate or packaging for the packaging of perishable goods, including but not limited to food, tobacco, coffee and/or tea. The packaging of substrate may ideally also allow encapsulating goods that carry fragrances, e.g. fruit- scented and aromatised tea or scented and aromatised chewing gum, and provide effectively a gas barrier to keep the fragrances from escaping through packaging, or becoming subjected to oxidation.

While the present invention has been described with reference to specific preferred embodiments, it should be appreciated that variations are possible without departing from the scope of the invention. Therefore, the invention is not intended to be limited by the description in the specification but only by the language of the claims and equivalents thereof.

The invention is further illustrated in the following, non-limiting calculated examples (Table 1 ):

Example 1 : Comparison between three packaging materials

Table 1 : Comparison between three sheetlike substrates with different films

The example clearly shows the superior gas barrier properties of the products obtainable in a simple process, similar and better than polyolefins and/or poly-condensation polymer films presently used for flexible packaging

Claims

1 . A process for preparing a metallised sheetlike substrate suitable as flexible packaging material having a moisture vapour transmission rate (MVTR) of below 20 g/m² d and an oxygen transmission rate (OTR) of below 25 cm³/m² d bar, comprising the steps of:

a) depositing on a non-woven sheetlike substrate at least a first coating composition comprising i) a film forming protein in combination with a polymer binder; ii) a polyvinylidene chloride solution, emulsion or dispersion, and/or iii) an aqueous dispersion of a clay and a polymer binder, to form a first coating layer, and

b) applying a metal layer by vacuum deposition on the first coating layer.

2. A process according to claim 1 , wherein the substrate is a sheet-like non- woven cellulosic material.

3. A process according to claim 2, wherein the substrate comprises fibrous material such as tissue, paper or cardboard.

4. A process according to any one of the previous claims, wherein the metal is selected from the group of aluminium, silver, tin, zinc, gold, platinum, titanium, gold, lead, nickel and tantalum, and/or alloys or combinations thereof.

5. A process according to any one of claims 1 to 4, further comprising adding a further step c) of applying a top coat on the substrate obtained in step b).

6. A process according to any one of claims 1 to 5, further comprising step d) of applying a backside coat on the backside of the substrate prior to or after steps a, b or c.

7. A metalized and top coated sheetlike non-woven substrate obtainable by the process according to any one of claims 1 to 6.

8. A substrate according to claim 7, comprising

I) a non-woven sheetlike substrate;

II) a first coating layer comprising

i) at least one protein composition in combination with a polymer binder; ii) a polyvinylidene chloride solution, emulsion or dispersion, and/or iii) a clay and a polymer binder, and

III) a vacuum deposited metal layer adhering to the first coating layer; and optionally

IV) a top coat deposited on the metal layer, and optionally

V) a backside barrier coating on the non-metallized side of the sheetlike substrate.

9. A flexible packaging for perishable goods comprising a substrate according to claim 7 or 8.

10. Use of a substrate or packaging according to any one of claims 7 to 9 for the packaging of perishable goods, including but not limited to food, tobacco, coffee and/or tea.