GB2494376A - Coating a substrate with a foam lacquer containing filler and collapsing the foam, to produce high barrier coating - Google Patents

Abstract

Foam is produced by agitating a mixture of a soluble lacquer, gas and sub-micron sized filler particles. The foam is deposited on a substrate. The foams bubbles are collapsed into the substrate, to generate a layer of dried lacquer and filler. Preferably, the lacquers solute is polyvinyl alcohol or water based styrene or acrylic. The filler is preferably clay (greatest dimension ¯200nm) comprising or containing sodium montomorillonite, silicon dioxide, talc, bentonite or kaolinite. Preferably, the deposited lacquer is subjected to infrared (IR) light after deposition. The substrate is preferably calendered paper, tracing paper, or a cellulose or metal based material. Multiple layers may be applied to both surfaces, preferably with variation in particle sizes. The apparatus is also claimed. The high barrier coating comprises oriented, interlocking layers.

Description

Bubbles Barrier The present invention concerns laminated materials which are capable of protecting substances from outside contamination from moisture, and gases such as Oxygen. Such materials can also protect against migration of molecules from the interior of sealed packages and are particularly suitable for storing foodstuffs such as potato crisps, biscuits and cakes but can find many other applications. As some 100,000,000 packets of potato crisps alone are eaten in the United Kingdom alone each week it is obviously desirable for the packaging of crisps and similar products to be biodegradable. However it will be appreciated that for packaging materials intended for foodstuff the necessity of a long shelf life conflicts with the desire to make the material biodegradable. Thus the core of one problem faced by attempts to develop a biodegradable food packaging material is that of providing a material having a sufficient bather to oxygen, light and moisture and which can also be mass produced. However it will be appreciated that the subject matter of the present invention, which is to be described more fully hereinafter, is not limited to biodegradable materials nor to the food packaging industry.

One example of a prior specification in this field is Patent Application no. PCTIEP2009/00 176 directed to "PACKAGING LAMINATE, METhOD FOR MANUFACTURING OF THE

PACKAGING LAMINATE AND PACKAGING CONTAINER PRODUCED

THEREFROM".This specification discloses a non-foil laminated material for liquid food packaging comprising a core layer of paper or paperboard, outermost liquid-tight, heat sealable layers of polyfin and applied onto the inner side of the core layer an oxygen bather layer formed by a dried layer of a liquid composition initially formed from a polymer binder dispersed or dissolved in a liquid medium.

Another example of prior art in the field of laminated bather layers onto transparent plastic films can be found in a recent paper entitled "LAYER-BY-LAYER ASSEMBLY OF THIN FILM OXGEN BARRIER" by Woo-sik Jang et al available on-line from www. Sciencedirect. Corn.

This paper describes the manufacture of such films which may be used for packaging applications. This paper describes building up laminated films using a layer-by layer method(LbL) which is an aqueous technique in which films are produced by alternate exposure of a polar or charged substrate to water-based solutions of mixtures containing charged or polar ingredients to form pairs of complementary layers with each pair of two layers being referred to as bi-layers. Whilst this process can manufacture laminated barrier materials with very good barrier properties it is repetitive and relatively laborious.

In a published co-pending patent application no. PCT/0B2010/002053 to the same inventor as the present application there is described a biodegradable laminated material for packaging food stuffs and a method of manufacturing the same in which a cellulosic layer preferably of highly calendered paper is coated with a barrier layer which may comprise a thin film of barrier material. The material of this barrier layer may take several different forms though is preferably of aluminium. Other possible materials for the barrier layer include aluminium oxide, silicon oxide. Additionally this barrier layer may be ceramic selected from oxides such as those of calcium, zinc, or boron or a mixture of aluminium or one of its oxides with one of these ceramics.

Such a layer is preferably between 0.01 and 0.15 microns thick. One particular feature of this prior application is that this barrier layer is treated with an additional layer of a lacquer having added organic or inorganic sub-micron sized filler particles. A preferred lacquer in this prior specification is an acrylic lacquer such as that formed by a polymerisation of a mixture of water and monomers of acrylic and styrene plus surfactants, stabilisers etc. Such copolymers are BIM BASSS3 manufactured by BIM BA AB of Finland and a MICHELMANN product of the USA.

These are essentially water-based styrene/acrylie copolymers. The solute in the lacquer can also be based on a polyvinyl alcohol. Another potential lacquer is a recently announced amorphous PVOH called Nichigo 0-polymer by the same company. One reason for the presence of the sub-micron filler particles. The purpose of the added filler particles is to improve the barrier qualities of the deposited barrier layer by providing a more tortuous path through the barrier layer. As will be described hereinafter the same principle for improving the qualities of a bather layer or in fact a cellulose or fibre based substrate is used in some embodiments of the present invention.

The present invention is concerned with providing improvements to the techniques described in the aforesaid prior documents and also in respect to what is described in the prior application.

Thus the present invention is directed to providing an alternative technique for coating a substrate with a barrier layer which is relatively simple to implcmcnt for mass production and also to offer the possibility of excellent barrier protection in combination with otherwise gas and/or water permeable substrates such as cellulosic materials, and other natural fibrous materials.

In accordance with one aspect of the present invention there is provided A method of manufacturing a laminated material, the method comprising the steps mixing a soluble lacquer with organic or inorganic sub-micron sized filler particles, introducing a gas into the mixture of lacquer and particles, and agitating the mixture of gas, lacquer and particles to generate foam, depositing the foam on a substrate and collapsing the bubbles onto the substrate to generate on the substrate a barrier layer of the dried lacquer and the filler particles.

Preferably the solute in the lacquer is a water based styrene/acrylic or a polyvinyl alcohol.

In accordance with another aspect of the present invention there is provided apparatus for manufacturing laminated material comprising means for supplying a mixture of a soluble lacquer, organic or inorganic sub-micron sized filler particles and a gas to means for agitating said mixture to generate foam, means for supplying the foam to the surface of a substrate and means for collapsing the bubbles so that layers of dried lacquer and filler particles are deposited on the substrate to provide a barrier layer.

In order that the present invention may be more readily understood embodiments thereof will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view through a cross section through an array of bubbles deposited as foam onto a substrate; Figure 2 is a diagrammatic plan view of the substrate of Figure 1 illustrating how the outlines ofjust 2 layers of collapsed bubbles overlap; Figure 3 is a diagrammatic sectional view of a system for applying a layer of foam to a flexible substrate; Figures 4 and 5 are figures similar to Figure 4 showing other systems for applying foam to substrates; Figure 6 is a section through a system for collapsing applied layers of bubbles; Figure 7 is a diagrammatic section through an embodiment of a system for generating foam for use in the systems shown in any one of Figures 3 to 5, and Figure 8 is a cross section through a whisk which can be used in the embodiment of Figure 7.

Referring now to Figure 1 this shows an array consisting of layers of bubbles 1 deposited on a substrate 2. In actuality the the layers will not be so regular and the bubbles will vary in size. It is believed that a suitable mean bubble size for carrying out the present invention will be approximately 0.5 mm to 1.5mm.

In accordance with the present invention each bubble will contain within its walls a plurality of sub-micron sized particles. Preferably these particles will be sub micron-sized platelets of crystalline substances such as sodium montmorillonite clay or other suitable clay-based materials which can for example include silicon oxide. Other suitable materials for the filler include Talc, Bentonite and Kaolinite. Such particles will be referred to in this specification as filler particles.

Typical filler particles such as those of sodium montomorillite clay are asymmetric and have an average diameter of approximately 200nm (nanometres), though the total diameter range can extend from a minimum of lOnm to l000nms. The filler particles will usually be much thinner with a thickness of between 2 to 8 nm though there may of course be extreme outliers. It will also be appreciated that the life time of each bubble will be very short with the result that the sub-micron sized filler particles in the bubbles will be deposited as the bubbles collapse onto the surface of the substrate and will form, together with the remaining dried bubble material as a barrier layer on the substrate. Thus the filler particles from any one individual bubble will be deposited largely within the diameter of the bubble with many of the deposited particles lying parallel. It is believed that any filler particles within a bubble will tend to align their longest axis with the adjacent bubble walls and that as each bubble normally collapses downwardly a substantial percentage of the filler particles contained within its walls will tend to have their major axis parallel to the surface of the substrate on which they are deposited. The combination of the dried material from which the bubble were formed together with the successive layers of deposited filler particles provides a barrier layer to the migration of molecules such as water and oxygen.

Ways in which filler particle orientation on deposition may be improved so as to provide a more tortuous path will be discussed hereinafter.

In order to form the bubbles shown in Figure 1 it will be necessary for the material from which the bubbles are generated to be a liquid which will foam. However it is not just the foaming qualities of the liquid which are important but also what is left behind on the substrate 2 after the bubbles have collapsed and the substrate has dried. It is for this reason that the present invention utilises what are generally know as lacquers. These usually comprise a liquid carrier agent which acts as a solute and which after application normally dries to leave a shiny protective layer as a residue. There three basic types of lacquer, namely what are known as Urushiol lacquers, styrene -based lacquers and nitrocellulose-based lacquers. Whilst the solute in a lacquer is preferably water if the final result is to be bio-degradable other solutes may also be used.

Turning now to Figure 2 of the accompanying drawings it will be seen, because of the manner in which the bubbles were initially stacked on the substrate 2 as shown in Figure 1 that the patterns formed by the collapsed bubbles in one layer, and of course the filler particles deposited because of that collapse, will naturally be staggered with respect to the bubbles of the preceding and succeeding layers. Successive overlapping layers will provide the required tortuous path which is so important in providing a barrier to migrating molecules. Figure 2 illustrates this by showing, from above, just two overlapping layers of collapsed bubbles. In practise there will be more than two layers of bubbles. The number of layers of deposited bubbles can be varied as will be described hereinafter and this number can be changed in dependence on the material of the substrate and the desired barrier qualities of the final laminate.

Turning now to Figure 3 of the accompanying drawings this shows a system for depositing layers of bubbles such as shown in Figure 1 onto the substrate 2. In Figure 3 a foamed mixture made up from bubbles of lacquer and filler particles is shown at 4 being deposited on the substrate 2. This substrate may be a highly calendered paper-based material, such as described in the already mentioned co-pending application, with a upper metal or ceramic based deposited barrier layer onto which the foamed lacquer is applied in order to improve bather qualities of the laminate. The already described deposition of multiple overlapping layers of particles and lacquer residue can provide even better barrier properties than the method described in the aforesaid copending patent application. Alternatively the deposited metal and/or ceramic layer may be dispensed with so that the final laminated material will rely for its barrier properties on the combination of the substrate, the filler particles and the dried lacquer. In fact it is believed that this bubble deposition technique is suitable for giving excellent barrier properties to an increased range of substrates with or with out the addition of a metallised bather layer. Thus in addition to Glassine and KrafI paper, a wide range of other materials including tracing paper, linerboard, moulded fibre cases, folding cardboard and cellulose-based films may also be treated in the same manner.

In the system shown in Figure 3 foamed lacquer containing filler particles is supplied by a supply pipe 5 and pass through a slot 6 into a chamber 7 through which the substrate 2 passes. As the substrate 2 passes through the chamber 7 it is coated with the foam. Prior to entering the chamber 7 the substrate passes over a support table 8 and under a curved entrance lip into the chamber and exits via a profiled curved surface shown at 9 onto a second support table 10. The positions of the two support tables maybe adjustable to accommodate changes in thickness of the substrate 2 and also to adjust tension in the substrate. In this embodiment the height of the bubble layer exiting chamber 7 by the height between surface 9 and the surface of substrate 2.

Figure 4 shows another embodiment of a system for depositing foam onto substrate 2.

Structural integers in this figure and Figure 3 have been given the same reference numerals and will not be described further. Figure 4 has an additional support roller 11 for the substrate 2, a different orientation for foam supply pipe S and edge sensors 12 for detecting the edges of the substrate 2 which can be used to control the width of the deposited foam.

In a third embodiment shown in Figure 5 pressurised foamed lacquer is supplied vie pipe 5 to an applicator tube 13 having a narrow extrusion slot 14. The substrate 2 passes over this slot through the nip between the applicator tube 13 and an adjustable back-up roller 15.

As already mentioned an important consideration in providing the best possible barrier layer using foam application in the orientation of the filler particles when they are finally deposited on substrate 2 by the collapsing of the bubbles. Given the nature of the bubble walls and the manner in which the asymmetric filler particles are likely to fall when the bubbles collapse it is desirable to have the bubbles collapsing in an orderly manner after their deposition on the substrate 2. The bubble layer may be forcible compressed by passing it and the substrate though a nip or a narrow slot lying parallel to the plane of the substrate or by spraying the surface of the substrate 2 with known defoaming agents. Another way is to provide a thermal shock to the bubbles after they are deposited. Yet another possibility is to charge the surface and the bubbles in the foam so that the bubbles are attracted onto the surface of the substrate.

onto the substrate by the use of heated rollers. Additionally Infra Red radiation can be used both to dry the substrate and to destroy the bubble structure of the deposited bubble layer either before or after the foam-carrying substrate passes through a slot or nip. Controlling the width of any slot or nip will determine the number of individual layers of bubbles which will be collapsed onto the substrate and accordingly the barrier properties of the final laminate.

One embodiment of a system for collapsing system bubbles is shown in Figure 6. In this figure a roller 16 is mounted above a layer of foam which has been deposited on substrate 2. A tank 17 holds a defoaming agent. There are many examples of defoaming agents which can be used.

These include blends of polyglycol high quality fatty acid esters, Alkoxane copolymer or water based emulsions. Actual examples of such materials are Dispelair DP 104 and Dispelair DP 150 manufactured by Blackburn Chemicals, Drewplus (RTM) S-4271 which is silicon based and manufactured by Ashland Chemicals, and Surfynol DF 210 a fatty acid-based surfactant manufactured by Air Products. The roller is vertically adjustable in relation to substrate 2 and is set at such a height that at its lowest point it lies below the upper most layer of foam after it has been deposited by, for example, by one of the embodiments 3 to 5. The roller 16 is rotated so that its surface moves in the same direction as the substrate. As the roller rotates it picks up the defoaming agent from the tank 16 and applies it to the foam layer 1, It will be apparent that if the system used in Figure 8 is to be used changes may need to be made to the embodiments shown in Figures 3, 4 and 5 to ensure that the requisite height of the foam layer is achieved before the foam layer reaches the roller 16. The numeral 18 represents an infra-red lamp for rapidly drying the collapsed bubbles.

Referring now to Figure 7 of the accompanying drawings this shows a system for generating foam. In this figure 20 indicates a holding tank for holding a lacquer containing sub-micron filler particles such the clay-based particles of the kind which have already been mentioned. Known methods for generating such particles includes processes using ball or planetary mill which operate by the attrition of larger portions of the material from which the particles are to be formed. Some other methods such as lithography, vacuum coating and spray coating form sub-micron particles in situ and accordingly are not thought to be appropriate for the present invention. Other methods are known as "wet chemistry" techniques and involve building ions or molecules into larger structures but are at present not suitable for the generation of filler particles on the scale required for mass production. Whilst much of the thrust of the present invention and in the pending application which has been briefly described is the provision of bio-degradable packaging for food products which require mass production on a very Large scale the present application is also concerned with other fields. Thus it can be used to manufacture very high quality barrier coatings which are translucent and are not necessarily bio-degradable and which may in addition be made conductive by the use of conductive filler particles. Such barrier substances can be of high commercial value for relativeiy small volumes on manufacture. Accordingly techniques such as gas phase synthesis, gas vapour synthesis and laser ablation can be considered provided that the particles so generated can be dispersed throughout an appropriate liquid carrier medium in a uniform manner. The holding tank 20 also contains the liquid medium of one of the types already described and in addition may contain one or more agents for assisting in the foaming of the gas bubbles to be applied to the substrate 2.

The mixture in tank is driven by a pump through a pipe 21 to the inlet of a mechanical whisk 22 which whips the medium in a foam having the appropriate bubble size. The whisk, which is shown in greater detail in Figure 8, is driven by a variable speed motor 23 and is provided by pressurised gas via a suitable gas supply via a pipe 24. The output of the whisk 22 is pressurised foam and as is shown is conveyed for deposition on the substrate 2 by a pipe 5 corresponding to the respective pipes 5 shown in Figures 3 to 5. This pipe 5 can have a pressure control valve for adjusting the pressure with which the foam is supplied to the substrate 2. The whisk 22 is supplied with coolant by a pipe 25.

Turning now to Figure 8 of the drawings the pressurised gas supplied by pipe 21 is fed into the whisk 22 at a gas inlet 26. The whisk 22 has an outer casing 27 which has another inlet, indicated at 28, for receiving the mixture to be whisked via pipe 21 which leads from the holding tank 20, Within casing 27 are ball bearings 29 and 30, a hollow shaft 31 carrying an internal gas supply pipe 32 leading from gas inlet 26 into the actual whisk mechanism 22. This whisk mechanism comprises a tubular body 36 carrying a large number of radially extending fins which can move freely between opposed sets of fins 37 carried on the inner wall 38 of a static cooling chamber 39.

The pressurised gas from gas inlet 26 is carried by pipe 32 to where the gas can mix with the supply of lacquer from inlet 28 in the annular chamber formed by the outer wall 40 of the whisk mechanism 22 and the inner wall of the cooling chamber 39 of the cooling chamber. The cooling chamber has an inlet 41 for a suitable cooling medium and an outlet 42 for the cooling medium. It will be appreciated that in order to generate bubbles of the requisite size the whisk mechanism will be rotated at a very high speed and will generated substantial heat which will have to be dissipated. Whisk mechanisms of this type are well know in the food industry. Finally outlet 43 of the whisk is connected to the already mentioned pipe 5.

With regard to the gas provided by the gas supply to the whisk 22 this may vary in accordance with the nature of the solute and may for example be compressed air or a suitable inert gas such as nitrogen.

The various embodiments described hereinbefore have been shown as depositing the bubbles on a single side of a substrate which is moved once through a deposition chamber. Of courses in many cases it will be desirable to apply the foam to both sides of the substrate to improve it barrier qualities. The lacquer residue, once the deposited bubbles have collapsed and dried, may also be used in known heat sealing processes to manufacture packages without the need for an additional heat sealing layer made from a petroleum-based substance. In addition, given the need to maximise barrier properties a substrate may be pass through one or more additional stages of bubble application so that additional deposits of bubbles can be made at each stage. The appropriate use of one or more additional stages may provide a still further degree of improvement in barrier quality by selecting the mean average size of the filler particles so that this mean average size differs between each successive stage.

One of the mechanisms by which the filler particles are believed to improve barrier properties is as follows. It is known from electron microscopy that vapour deposited barrier layers such as aluminium, aluminium oxide or the other materials described earlier in this specification, even on very smooth substrates, still result in a number of very small holes and fissures through which water and gas molecules will eventually pass. Accordingly a first stage deposition of filler particles with a mean size which is less than the mean size of the filler particles deposited in a subsequent layer of foam will increase the chances of the smallest defects in the surface of the substrate being at least partially filled at the first foam deposition and accordingly that the deposition of filler particles at the second stage will be more effective. Given that the manufacture of smaller filler particles is more expensive than that of relatively larger particles due to the greater abrasion required the overall cost of manufacturing the laminate will be reduced. It will of course be apparent that there is no absolute requirement that the filler particles deposited in successive stages are to be of the same material nor that the carrier medium for the filler particles remains the same.

Claims (1)

1. <claim-text>CLAIMS1)A method of manufacturing a laminated material, the method comprising the steps mixing a soluble lacquer with organic or inorganic sub-micron sized filler particles, introducing a gas into the mixture of lacquer and particles, agitating the mixture of gas, lacquer and particles to generate foam, depositing the foam on a substrate and collapsing the bubbles onto the substrate to generated successive layers of the dried lacquer and the filler particles.</claim-text> <claim-text>2)A method according to claim I wherein the solute in the lacquer is a water based styrene/acrylic.</claim-text> <claim-text>3)A method according to claim 1 wherein the solute in the lacquer is based on polyvinyl alcohol.</claim-text> <claim-text>4)A method according to any preceding claim wherein the average size of the deposited bubbles is between 0.5 and 1.5 mm.</claim-text> <claim-text>5) A method according to any one of the preceding claims wherein the bulk of said particles are selected from clay based materials and have an average greatest dimension which does not exceed 200nm.</claim-text> <claim-text>6)A method according to claim 5 wherein the clay based material is selected from the group of sodium montomorillonite, clay based materials containing a silicon oxide, Talc, Bentonite and Kaolinite.</claim-text> <claim-text>7)A method according to any preceding claims wherein the ratio of weight of the particles is between 4 and 50% of the weight of the solute.</claim-text> <claim-text>S)A method according to any of the preceding claims wherein the substrate is selected from one of the following calendered or super calendered paper, tracing paper, or cellulose-based materials.</claim-text> <claim-text>9)A method according to any one of the preceding claims wherein the deposited lacquer is subjected to inlia red light after deposition.</claim-text> <claim-text>I 0)A method according to any one of the preceding claims wherein the bubbles are deposited on a substrate having an additional barrier layer which includes a metal based material. -11-</claim-text> <claim-text>11) A method according to any one of the preceding claims and including the step of depositing a a layer of heat sealable material onto at least one side of the laminated material.</claim-text> <claim-text>12) A method according to any one of the preceding claims wherein the steps of depositing bubbles on one side of the substrate, collapsing the bubbles and drying them are repeated at least once.</claim-text> <claim-text>13) A method according to any one of the preceding claims wherein at least one step of depositing bubbles is carried out on both surfaces of the substrate.</claim-text> <claim-text>14) A method according to claim 12 and claim 13 when dependent on claim 12 wherein when if two separate layers of bubbles are deposited on a surface of the substrate the average size of the filler particles is varied between the two deposition steps.</claim-text> <claim-text>15) Apparatus for manufacturing laminated material comprising means for supplying a mixture of a soluble lacquer, organic or inorganic sub-micron sized filler particles and a gas to means for agitating said mixture to generate foam, means for supplying the foam to a substrate and means for collapsing the bubbles so that dried lacquer and filler particles are deposited on the substrate.</claim-text>