GB2447221A - Acrylic polymer containing metal silicate - Google Patents

Abstract

A composition comprises a metal silicate dispersed throughout a film of a water-soluble, film-forming acrylic solution polymer. The metal silicate is preferably an alkali metal silicate, such as sodium, potassium, or lithium silicate. The acrylic solution polymer may have a molecular weight below 20000 and is especially styrene-co-methyl methacrylate, styrene-co-butyl acrylate, styrene-co-methyl acrylate, or styrene-co-ethylhexyl acrylate. The ratio of metal silicate to acrylic solution polymer may be 99:1 to 4:1. A composition for forming a gas barrier coating is also disclosed, which comprises a solution of a metal silicate and a water-soluble, film-forming acrylic solution polymer in a solvent. The solvent is typically aqueous and may contain a co-solvent, such as an alcohol. The composition may be applied to a plastics substrate and may be used to form a gas barrier film for packaging foodstuffs, pharmaceuticals, and other materials sensitive to the atmosphere.

Description

OXYGEN BARRIER COATINGS

The present invention relates to a coating composition having gas barrier properties, particularly the ability to block the passage of oxygen, and which may be used to coat and impart gas barrier properties to a variety of materials, notably packaging for foods and pharmaceuticals, where exposure to oxygen needs to be eliminated or restricted.

Synthetic plastics materials have long been used for the packaging of foods and other materials which need protection from handling and from moisture. However, in recent years, it has become appreciated that, in addition, many foods and other sensitive materials benefit from being protected from atmospheric oxygen. A wide variety of multilayer laminate structures has been developed to provide barrier properties and other performance characteristics suited to a pack's purpose. These laminates may be any combination of plastic, metal or cellulosic substrates, and may include one or more coating or adhesive layers. Laminates which include polymeric films having metals or inorganic compounds, such as silicon oxides, deposited thereon have been found to give good general barrier properties and are widely used. However, their properties tend to be very temperature dependent and they may lose their ability to prevent the ingress of oxygen altogether at high tempçratures, for example when the packaged material is retorted in order to sterilise and/or cook it. Moreover, the inorganic layer of these types of laminate is rather brittle and may crack or break when the laminate is flexed, resulting in a loss of the gas barrier properties.

As a result, a number of other laminated films have been proposed for this purpose. For example, EP 0 878 495 describes and claims a gas barrier laminated material comprising a substrate, an inorganic compound thin-film layer and a protective layer which are laminated in that order, where the protective layer is formed by coating on the inorganic compound thin-film layer a water-based coating composition containing a water-soluble polymer and at least one of (a) a metal alkoxide or a hydrolysate thereof and (b) a tin chloride, followed by heat drying. Other patents using similar techniques include EP 1 211 295 (JSR), EP 0 960 901 (Nakato) and US 6,337,370. Although good oxygen barrier performance is achieved, there are a number of drawbacks with this technology. These drawbacks include having to prepare the hydrolysed silane press-side (due to poor long term stability), the exothermic nature of the hydrolysis reaction and the potential hazards associated with having to handle the silane and hydrochloric acid or other acid. Furthermore, the water resistance of these coatings can be insufficient.

JP 2003170522 discloses a two-layer barrier, in which a polyethylene terephthalate film is first coated with a solution of lithium or sodium silicate and then with a solution of polyvinyl alcohol (PVA). This achieves an oxygen barrier of cc/m2/day at 40 C and 90% relative humidity (RH). JP 2002316381 also discloses a multi-layer barrier coating, which, in this instance, comprises: (a) an anchor coat (e.g. a urethane isocyanate): (b) a barrier coat comprising a metal silicate (e.g. lithium silicate) as well as, optionally, a nitrogen-containing compound and a water-soluble polymer (e.g. silicon-modified PVA); and (c) a top coat comprising a hydroxy-containing water-soluble polymer (e.g. PVA). However, because PVA is very hydrophilic, it can absorb large amounts of water and so, even if good gas barrier properties are achieved at the beginning, these properties soon deteriorate. Moreover, several layers are required in these cases, including at least a primer (anchor) layer, to ensure good adhesion of the gas barrier coating to the substrate.

JP 2006159801 A2 reports that BOPP (Biaxially Orientated PolyPropylene) films were coated with a polyethyleneimine based primer which was over coated with lithium silicate solution coating. Oxygen permeability of 11 cc/m2/day at 23 C at 90% RH was reported.

US 6649235 32 reports a similar process by coextruding polypropylene and polypropylene modified with maleic anhydride followed by corona treatment of the maleated side and over coating this surface with a barrier layer of polysilicates. In WO 9747678 Al, BOPP was primed with PVA and EVOH (Ethylene Vinyl Alcohol polymer) prior to application of the lithium potassium silicate layer. An oxygen

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transmission rate of 61 cc/m2/day at 23 c and 60% RH was recorded. Thus, the majority of the prior art deals with primer use and pure silicate coatings.

US 6071624 discloses that potassium silicate was required with lithium and sodium silicate coatings because lithium and sodium silicate coatings tend to effloresce, i.e., become covered by powdery crystalline material as a result of atmospheric exposure. In contrast, pure potassium silicate coatings do not effloresce, but suffer severe loss of barrier performance above 50% relative humidity. Pure lithium silicate coatings, on the other hand, exhibit little or no loss of barrier performance over the same relative humidity range.

JP 2003170522 A2 reported a modification to the above mentioned approach.

PET (PolyEthylene Teraphthalate) films were coated with a solution of lithium silicate and sodium silicate followed by an overcoat of PVA and Li2HPO4, which were then heat dried. An oxygen permeation value of 18 cc/m2/day was reported at 23 C and 90% RH. In similar patents, JP 2002316381 A2 and JP 2002283492 A2, a multilayer approach, with an anchor coat underpinning gas barrier coatings of alkali metal silicates and top coats of PVA containing composition were reported. As an example, PP (polypropylene) substrate was coated with an isocyanate anchor coat, followed by a coat of lithium silicate composition containing aminosilane and silane modified PVA and subsequently a final coat of PVA and clay composition. Oxygen permeation of 10.8 cc/m2/day was recorded.

Use of clay in combination with the metal silicates was also reported in JP 2001260264. BOPP substrate was coated with aminoethylated resin followed by coating with a 50:50 blend of lithium silicate and Na montmorillonite to dry film thickness of about 0.8 micron. This was thermally treated to provide a barrier film with good resistance to surface cracking.

None of these approaches appears to have achieved very low gas, especially oxygen, permeabilities and they all require the application of several coats.

JP2000 1 04020A2 discloses a gas barrier coating forming composition comprising (A) an aqueous dispersible polymer selected from a polyester polymer, an acrylic polymer and a polyurethane polymer, with (B) at least one or two water glass selected from sodium silicate, potassium silicate, lithium silicate, and ammonium silicate in certain weight ratios. The acrylic polymers used are acrylic emulsion polymers and, using these, we have found that the gas barrier properties are poor.

JP 20001 03986A2 discloses a composition for preparing a gas barrier coating comprising an aqueous solution containing (A) a water-soluble polymer having an amino group or a hydroxy group in the molecule (preferably a polyamide or polyvinyl alcohol-based polymer) and (B) a water glass (preferably lithium silicate). Although such a gas barrier coating does have improved barrier properties, we have found that the aqueous solution is rather unstable and that the gas barrier properties deteriorate significantly as the relative humidity increases.

We have now surprisingly discovered that these disadvantages may be overcome and a good gas barrier coating achieved in a single layer by applying a metal silicate in admixture with a water-soluble film-forming acrylic solution polymer.

Thus, the present invention consists in a composition comprising a metal silicate dispersed throughout a film of a water-soluble film-forming acrylic solution polymer.

The invention further consists in a composition for forming a gas barrier coating, comprising a solution of a metal silicate and a water-soluble film-forming acrylic solution polymer in a solvent. -The metal silicate employed is preferably a water-soluble silicate, for example an alkali metal silicate or polysilicate, such as lithium silicate, sodium silicate or potassium silicate. Commercially available silicates generally have a molar ratio of alkali metal oxide:silicon dioxide that varies, in some instances quite considerably, from the stoichiometric. This molar ratio can affect the properties of the silicate to a large extent and thus may affect the properties of the gas barrier coating of the present invention. Although the optimum molar ratio for any particular composition may be found by the person skilled in the art by simple experiment, in general, we prefer that the molar ratio of alkali metal oxide:silicon dioxide should be from 0.1 1 to 0.4:1, more preferably from 0.2:1 to 0.3:1 and most preferably around 0.25:1.

The other essential component of the composition of the present invention is a water-soluble film-forming acrylic solution polymer. It is important to note that these are very different in nature from the acrylic emulsion polymers used in JP2000104020A2, which, unlike the solution polymers used in the present invention, result in poor gas barrier performance. Examples of suitable acrylic solution polymers include copolymers of one or more ethylenically unsaturated hydrocarbons, such as styrene, with one or more hydrocarbyl, e.g. alkyl, acrylates, for example styrene-co- methyl methacrylate, styrene-co-butyl acrylate, styrene-co-methyl acrylate, and styrene-co-ethyl hexyl acrylate. These solution polymers preferably have a relatively low molecular weight, e.g. below 20,000. They are commonly prepared, as is well known in the art, in a solvent-based medium using solvent soluble free radical initiators.

Sufficient acid will be included so that, when the acid groups are neutralised with e.g. ammonia solution, the polymer becomes sufficiently hydrophilic for it to dissolve.

Thus, an acrylic solution is a homophasic system. Anionic solution polymers prepared in a similar way may also be used in the present invention.

Where the coating composition of the present invention is to be used in an environment of high relative humidity, the use of an acrylic solution polymer in accordance with the present invention is especially beneficial. Although other polymers, such as vinyl alcohol-based polymers, may give good barrier properties at low or moderate relative humidities, acrylic solution polymers maintain these properties in high relative humidity environments. Moreover, acrylic solution polymer-based compositions appear to exhibit greater stability than, for example those based on PVA, and so are preferred from this viewpoint also.

The coating composition of the present invention is usually and preferably applied in the form of a solution of the silicate 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). Where a co-solvent is present, this is preferably no more than 10% by weight of the entire composition, still more preferably no more than 5%. The preferred co-solvent is an alcohol, preferably ethanol.

The amount of acrylic solution polymer in the coating composition is preferably from I to 20% of the total solids comprising polymer and silicate, in other words, the ratio of silicate to polymer is preferably from 99:1 to 4:1, more preferably from 19:1 to 4:1. The concentration of silicate and polymer in the solution will depend on their solubility, the amount of solvent employed preferably being the minimum needed to achieve sufficient flowability to coat the substrate adequately. In general, the silicate will be employed in the form of a 3-25% by weight solution in water or water+co-solvent, and this will dictate the contents of the remaining components.

The coating composition of the present invention comprising the silicate, the acrylic solution polymer and a solvent therefor may be applied to a substrate by any conventional means. The solvent may then be removed, e.g. by heating, leaving a film comprising the silicate dispersed through the polymer on the substrate. The resulting gas barrier film may be a single ply film, or it may form part of more complex multilayer laminate structure which can include one or more additional substrates, adhesive coatings, layers of inks and varnishes, etc., as is well-known to those skilled in the art. It is preferred that the film of the present invention should be adhered to a further flexible plastics sheet.

There is no particular restriction on the nature of the flexible substrate, although it is preferably a plastics film, and any material suitable for the intended use may be employed. However, where the matter being packaged with the coating film of the present invention is a foodstuff or pharmaceutical, it will normally be preferred that the plastics film or other substrate should be food grade. Examples of suitable materials include: polyolefins, such as polyethylene or polypropylene; polyesters, such as polyethylene terephthalate, polybutylene terephthalate or polyethylene naphthenate; polyamides, such as nylon-6 or nylon-66; and other polymers, such as polyvinyl chloride, polyimides, acrylic polymers, polystyrenes, celluloses, or polyvinylidene chloride,. It is also possible to use copolymers of any compatible two or more of the monomers used to produce these polymers. We especially prefer the polyesters.

Where there is a further plastics sheet, this, too, should be flexible and may be selected from any of the materials exemplified in the preceding paragraph.

The thickness of the coating of the present invention will depend in part on the ability of the silicate to form a continuous, coherent coating layer. However, in general, we prefer that the coating should be from 50 nm to 3000 rim thick, more preferably from 200 to 2000 nm thick.

The invention thus also provides a process for forming a gas barrier film, which comprises applying to a substrate a composition comprising a solution of a metal silicate and a water-soluble film- forming acrylic solution polymer in a solvent, and removing the solvent.

The invention still further provides a packaged foodstuff, pharmaceutical or other material sensitive to the atmosphere, wherein the packaging comprises a gas barrier coating of the present invention.

The invention is further illustrated by the following non-limiting Examples.

EXAMPLES

Coatings were prepared in an aqueous solution with 5% (w/w) of ethanol. The oxygen transmission rates of the coated samples were determined on a Mocon Oxtran 2/21 gas permeability tester at 23 C and 50% relative humidity (few values at 80%).

The substrates used were Nuroll PET (a 12pm gauge polyethylene terephthalate substrate), OPP and OPA (Orientated polyamide). The coatings were applied with a No.2 K-bar unless otherwise mentioned and were dried in a warm flow of air (lab prints were dried with a hair dryer).

The laminates were prepared by applying an adhesive to the surface of a substrate film (e.g. PE -polyethylene) and applying the barrier coating to another substrate (e.g. PET) and bonding them together. The adhesive used was supplied by Henkel, UR 3855 along with catalyst hR 6055, and was prepared according to the manufacturers instructions and applied so as to achieve a final dry film weight of 4 gsm.

The laminates were then stored for 2 days at 50 C to ensure full cure of the isocyanate-based adhesive. The laminates were then tested for bond strength (NIl 5mm) and oxygen barrier as earlier described.

Substrate films were tested for the oxygen transmission rates through them at 23 C and at 50% RH. Values of 100 and 32 cc/m2/day were recorded for the PET and OPA substrates, whereas the transmission through OPP and PE was too high to be detected.

Pure polymer matrix (SCX 8082) was also drawn on the PET substrate and a value of 95 cc/m2/day was recorded at 23 C and 50% RH.

EXAMPLES 1 TO 31 & COMPARATIVE EXAMPLES 1 & 2 Oxygen transmission values of the coatings drawn with formulations carrying varibus silicates & polymers along with clay & wetting agent, drawn on different substrates & with different K bars and characterized at different relative humidity values. The results are shown in Tables I and 2.

EXAMPLES 32 TO 35

Oxygen transmission and the bond strength values of the laminates prepared with the inside coating of the barrier formulation. The results are shown in Table 3.

Table 1

Example Polymer Silicate Silicate! Total Substrate Bar Oxygen %RH No. polymer solids wt permeation ratio _________ cc/m2/day ______ I SCX 8082 LiSil L29 9 10 PET 2K 0.7 50 2 SCX 8082 LiSil L40 9 10 PET 2K 0. 1 50 3 SCX 8082 LiSil 665 9 10 PET 2K 0.6 50 4 SCX 8082 NaSil NaO 106 9 10 PET 2K 0.6 50 SCX 8082 NaSil Na0052 9 10 PET 2K 0.3 50 6 SCX 8082 KSiI K78 9 10 PET 2K 2 50 7 SCX 8085 LiSi! L29 9 10 PET 2K 2.5 50 8 SCX 8085 LiSil L40 9 10 PET 2K 0.1 50 9 SCX 8085 LiSil 665 9 10 PET 2K 2.3 50 SCX 8085 NaSilNaOlO6 9 10 PET 2K 0.2 50 11 SCX 8085 NaSil Na0052 9 10 PET 2K 1.2 50 12 SCX 8085 KSiI K78 9 10 PET 2K 2.5 50 13 DFC 3025 LiSil L29 9 10 PET 2K 3.2 50 14 DFC 3025 LiSil L40 9 10 PET 2K <0.1 50 DFC 3025 LiSil 665 9 10 PET 2K 0.6 50 16 DFC 3025 NaSilNaOlO6 9 10 PET 2K 0.3 50 17 DFC 3025 NaSjlNaOO52 9 10 PET 2K 0.6 50 18 DFC 3025 KSiI K78 9 10 PET 2K 3.6 50 19 SCX 8082 LiSil L40 4 10 PET 2K 2.2 50

Table 2

Example Polymer Silicate Silicate! Total Substrate Bar Oxygen %RH No. polymer solids wt permeation ratio _________ _________ cc/m2/day _____ SCX 8082 LiSil L40 2.3 10 PET 2K 79 50 21 SCX 8082 LiSil L40 1 10 PET 2K Gel 50 22 SCX 8082 NaSil Na0052 4 10 PET 2K 2.2 50 23 SCX 8082 NaSil Na0052 2.3 10 PET 2K 79 50 24 SCX 8082 NaSil Na0052 1 10 PET 2K Gel 50 SCX 8082 LiSil L40 9 10 PET OK 0.2 50 26 SCX 8082 LiSil L40 9 10 PET 1K 0.2 50 27 SCX 8082 LiSil L40 9 10 OPP 2K 5 50 28 SCX 8082 LiSil L40 9 10 OPP 2K 3.8 80 29 DFC 3025 LiSil L40 9 10 OPP 2K 9 50 SCX 8082 LiSiIL4O 9 10 OPA 2K 3.1 50 31 DFC 3025 LiSil L40 9 10 OPA 2K 5 50 Comp 1 SCX 1630 LiSil L40 18 10 PET 2K 47 50 Comp 2 DSM A2092 LiSil DIC 18 10 PET 2K 70 50 Note: (1) LiSil L29, LiSil L40, LiSil 665, NaSil NaO 106, NaSil Na0052, KSi1 K78 are silicates supplied by Ineos Silicas.

(2) LiSil DIC is a lithium silicate supplied by DIC.

(3) SCX 8082 and SCX 8085 are solution acrylics from Johnson Polymer.

(4) DFC 3025 is a direct food contact solution acrylic from Johnson Polymer.

(5) SCX 1630 is an emulsion acrylic from Johnson Polymer.

(6) DSM A2092 is an emulsion acrylic from DSM Neoresins.

Table 3

Example Polymer Silicate Silicate! Total Laminate structure Bar Oxygen permeability % Bond No. polymer solids cc!m2!day RH strength N __________ __________ __________ ratio wt % ____________________ 32 SCX 8082 LiSil L40 9 10 OPAlBarrier/OPP 2K 0.6 (27)1 50 0.8 33 SCX 8082 LiSil L40 9 10 OPA/Barrier/PE 2K 1.4 (28) 50 2.6 34 SCX 8082 LiSil L40 9 10 PET/Barrier/PE 2K 0.2 (98) 50 1.5 SCX 8082 LiSil L40 9 10 PETfBarrier!OPP 2K 0.1 (91) 50 1.5 Note: (1) The values in brackets are the permeation values through the laminates made without any barrier film.

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EXAMPLE 36

A formulation comprising 90% lithium silicate L40 and 10% polymer was prepared and coated onto a PET substrate as described in the above Examples. When using acrylic polymer SCX 8082, the system had a permeation of 0.10 cc/m2.day at 50% RH. A comparable system using PVA 396 in place of the SCX 8082 had an oxygen permeation of 0.14 cc/m2.day at 50% RH. At 75% RH, however, the acrylic system had a permeation of 1.8 cc/m2.day and the PVA system had a value of 3.7 cc/m2.day.

The PVA formulation also contained a large number of flakes, whereas the acrylic formulation was a substantially homogenous mixture.

EXAMPLE 37

The procedure described in Example 36 was repeated, but using a formulation comprising 80% lithium silicate L40 and 20% polymer. The acrylic system had a permeation of 2.2 cc/m2.day and the PVA system had an oxygen permeation of 0.32 cc/m2.ciay at 50% RH. At 75% RH, however, the acrylic system had a permeation of 8.3 ccfm2.day and the PVA system had a value of 28 cc/m2.day.

The PVA formulation also contained a large number of flakes, whereas the acrylic formulation was a substantially homogenous mixture.

Claims (23)

1. CLAIMS: I. A composition comprising a metal silicate dispersed

   throughout a film of a water-soluble film-forming acrylic solution polymer.
2. 2. A composition according to Claim 1, in which the metal silicate is an alkali metal silicate.
3. 3. A composition according to Claim 2, in which the metal silicate is sodium silicate, potassium silicate or lithium silicate.
4. 4. A composition according to Claim 2 or Claim 3, in which the molar ratio of alkali metal oxide:silicon dioxide is from 0.1:1 to 0.4:1.
5. 5. A composition according to Claim 4, in which the molar ratio is from 0.2:1 to 0.3:1, preferably around 0.25:1.
6. 6. A composition according to any one of the preceding Claims, in which the acrylic solution polymer is a copolymer of one or more ethylenically unsaturated hydrocarbons with one or more alkyl acrylates.
7. 7. A composition according to Claim 6, in which the acrylic solution polymer is styrene-co-methyl methacrylate, styrene-co-butyl acrylate, styrene-co-methyl acrylate, or styrene-co-ethyl hexyl acrylate.
8. 8. A composition according to any one of the preceding Claims, in which the acrylic solution polymer has a molecular weight below 20,000.
9. 9. A composition according to any one of the preceding Claims, in which the weight ratio of silicate to acrylic solution polymer is from 99:1 to 4:1.
10. 10. A composition according to Claim 9, in which the weight ratio of silicate to acrylic solutionpolymer is from 19:1 to 4:1.
11. 11. A composition for forming a gas barrier coating, comprising a solution of a metal silicate and a water-soluble film-forming acrylic solution polymer in a solvent.
12. 12. A composition according to Claim 11, in which the metal silicate is an alkali metal silicate.
13. 13. A composition according to Claim 12, in which the metal silicate is sodium silicate, potassium silicate or lithium silicate.
14. 14. A composition according to Claim 12 or Claim 13, in which the molar ratio of alkali metal oxjde:sjl icon dioxide is from 0.1:1 to 0.4:1.
15. 15. A composition according to Claim 14, in which the molar ratio is from 0.2:1 to 0.3:1,preferably around 0.25:1.
16. 16. A composition according to any one of Claims ii to 15, in which the acrylic solution polymer is a copolymer of one or more ethylenically unsaturated hydrocarbons with one or more alkyl acrylates.
17. 17. A composition according to Claim 16, in which the acrylic solution polymer is styrene-co-methyl methacrylate, styrene-co-butyl acrylate, styrene-co-methyl acrylate, or styrene-co-ethyl hexyl acrylate.
18. 18. A composition according to any one of Claims 11 to 17, in which the acrylic solution polymer has a molecular weight below 20,000.
19. 19. A composition according to any one of Claims II to 18, in which the weight ratio of silicate to acrylic solution polymer is from 99:1 to 4:1.
20. 20. A composition according to Claim 19, in which the weight ratio of silicate to acrylic solution polymer is from 19:1 to 4:1.
21. 21. A composition according to any one of Claims 11 to 20, in which the solvent is aqueous.
22. 22. A composition according to Claim 21, in which the solvent is water containing up to 10% by volume of a co-solvent.
23. 23. A composition according to Claim 22, in which the co-solvent is an alcohol.