WO2018091892A1

WO2018091892A1 - Coating method and product thereof

Info

Publication number: WO2018091892A1
Application number: PCT/GB2017/053443
Authority: WO
Inventors: Dermot O'hare; Jean-Charles BUFFET; Kanittika RUENGKAJORN; Katarina CERMELJ
Original assignee: Scg Chemicals Co., Ltd.; Scg Packaging Public Company Limited
Priority date: 2016-11-15
Filing date: 2017-11-15
Publication date: 2018-05-24

Abstract

A process for the preparation of a film is described, in which a substrate is coated with a coating mixture containing a polymer and an organic solvent-dispersible layered double hydroxide. The process of the invention is markedly simpler that conventional techniques for affording films having reduced permeability to degradative gases. The films obtainable by the process are particularly useful in packaging applications, notably in the food industry.

Description

COATING METHOD AND PRODUCT THEREOF

INTRODUCTION

[0001] The present invention relates to a process for the preparation of a film, as well as to films obtainable by the process and their uses in packaging applications. More specifically, the present invention relates to a process for the preparation of a film comprising an LDH- containing coating.

BACKGROUND OF THE INVENTION

[0002] Polymer films have been widely applied as packaging materials (e.g. in the food industry) due to their light weight, low cost and good processability (T. Pan, S. Xu, Y. Dou, X. Liu, Z. Li, J. Han, H. Yan and M. Wei, J. Mater. Chem. A, 2015, 3, 12350-12356). However, the effectiveness of polymer packaging materials in preventing product degradation depends on their impermeability to degradative gases such as 0₂ (Y. Dou, S. Xu, X. Liu, J. Han, H. Yan, M. Wei, D. G. Evans and X. Duan, Adv. Fund Mater., 2014, 24, 514-521 ) and water vapour.

[0003] In an endeavour to reduce the gas permeability of polymeric films used in packaging applications, inorganic materials have been incorporated directly into the polymeric films themselves (e.g. as fillers), or have been applied to the surface of such polymeric films (e.g. as a coating). Clays (such as montmorillonite) have been considered promising candidate materials for reducing the gas permeability of polymeric films. However, these materials suffer from the fact that they are naturally-occurring, and as such may be heavily contaminated with potentially harmful substances (e.g. heavy metals), thereby hampering their use in food packaging.

[0004] Aside from clays, layered-double hydroxides (LDHs) have been recognised as potentially useful materials for reducing the gas permeability of polymeric films. However, to date, research in the area of LDH coatings on polymeric films has focussed on the preparation of a complex "brick-mortar" structure obtained via layer-by-layer (LbL) assembly of LDH nanoplatelets and polymer on the film, in which a highly-ordered stack of alternating layers of LDH (brick) and polymer (mortar) is prepared by a series of alternating spin or dip coating steps using i) an LDH dispersion, and ii) a polymer solution. These assemblies have been rendered even more complex by infilling voids with C0₂ (to give a "brick-mortar-sand" structure) in an endeavour to further reduce the oxygen transmission rate (OTR) of the polymeric film. However, the elaborate and complex nature of such LbL techniques restricts their implementation on an industrial scale. [0005] In spite of the advances made by the prior art, there remains a need for improved means for reducing the gas permeability of polymeric films. In particular, there remains a need for an overall simpler coating technique allowing for the preparation of coated polymeric films having acceptable OTR and water-vapour transmission rate (WVTR) properties.

[0006] The present invention was devised with the foregoing in mind.

SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention there is provided a process for the preparation of a film, the process comprising the steps of:

a) providing a first substrate;

b) providing a coating mixture comprising an organic solvent, a polymer that is soluble in the organic solvent and a layered-double hydroxide that is dispersible in the organic solvent;

c) coating the first substrate with a layer of the coating mixture; and

d) drying the coated first substrate.

[0008] According to a further aspect of the present invention there is provided a film obtainable, obtained or directly obtained by a process as defined herein.

[0009] According to a further aspect of the present invention there is provided a film comprising:

a) a substrate; and

b) a coating layer provided on a least one surface of the substrate, wherein the coating layer comprises 2-70 wt% of an organic-solvent dispersible layered double hydroxide dispersed throughout a polymeric matrix.

[0010] According to a further aspect of the present invention there is provided a use of a film as defined herein in packaging.

[0011] According to a further aspect of the present invention there is provided a container comprising a film as defined herein. DETAILED DESCRIPTION OF THE INVENTION

Preparation of films

[0012] As discussed hereinbefore, the present invention provides a process for the preparation of a film, the process comprising the steps of:

a) providing a first substrate;

c) coating the first substrate with a layer of the coating mixture; and d) drying the coated first substrate.

[0013] The process of the invention provides a number of advantages over conventional techniques for reducing the gas permeability characteristics of polymeric films. When compared with techniques employing the use of an inorganic filler in the film itself, the present invention is advantageous in that it allows various different films to be coated with the same coating mixture. Hence, it not necessary for each polymeric film (e.g. PET, PL⁾, PE) to be purpose-made with the inclusion of an inorganic filler.

[0014] The use of LDH in the process of the invention also presents numerous advantages over prior art techniques employing clays. In contrast to clays (e.g. montmorillonite), LDHs are entirely synthetic materials, the composition, structure and morphology of which is governed by the manner in which they are prepared. As a consequence, the replacement of clays with LDHs in polymeric films for packaging applications considerably reduces - if not eliminates - the risk posed by potentially harmful contaminants (such as heavy metals), which present clear advantages for the food industry.

[0015] The process of the invention also presents a number of advantages over conventional LbL assembly techniques. As discussed hereinbefore, LbL techniques have been used to prepare complex "brick-mortar" structures, containing a highly-ordered stack of alternating layers of LDH (brick) and polymer (mortar) which is grown directly on a film by a series of alternating spin or dip coating steps using i) an LDH dispersion, and ii) a polymer solution, or is assembled separate from the film prior to being transferred onto it. In contrast to this approach, the present invention provides a considerably simpler technique for achieving coated polymeric films having acceptable OTR and WVTR properties. In particular, in the present process, both the LDH and the polymer are simultaneously applied to the film in a single step, whereas LbL processes require successive alternating separate steps for applying the LDH and polymer. This necessarily facilitates up-scaling of the present process, the coating mixture of which can be applied to the film from a single vessel in a production line in a single application step. Moreover, the present process provides a greater degree of flexibility in the manner in which the coating mixture may be applied to the film on an industrial scale. As a non-limiting example, the present process may be implemented using a roller-and-bath apparatus, in which the coating mixture is licked onto a roller being in contact with a bath, and is then transferred onto a film also being in contact with the roller, thereby allowing vast quantities of film to be continuously coated in a short period of time. Such cost-effective techniques are entirely incompatible with LbL techniques, the complex structures of which can only be achieved by sequential dip or spray coating techniques.

[0016] Aside from the above-discussed advantages, the use of an organic solvent-dispersible LDH in step b) of the present process results in an overall more lipophilic film having reduced WVTR properties. Moreover, the use of an organic solvent-dispersible LDH facilitates up- scaling of the process to an industrial scale. In particular, the use of organic solvent-dispersible LDHs permits the use of organic solvent-based coating mixtures. When compared with water- based coating mixtures, organic solvent-based coating mixtures can be dried at lower temperatures for shorter periods of time, thereby reducing operation costs and environmental impact.

[0017] In an embodiment, the first substrate is selected from polyethylene terephthalate (PET), polyethylene (PE), biaxialiy oriented polypropylene film (BOPP), polypropylene (PP), polyvinyl dichloride (PVDC), polyamide, nylon, and polylactic acid (PLA). Suitably, the first substrate is PET.

[0018] In an embodiment, the polymer is selected from one or more of cellulose acetate (CA), polyurethane (PU), polyacrylic acid (PAA), polyvinyl alcohol (PVOH), polyamide, and a copolymer comprising vinyl alcohol (e.g. polyethylene vinyl alcohol (EVOH) and polyethylene vinyl acetate (EVA)). Particularly suitably, the polymer is cellulose acetate.

[0019] The organic solvent used in step b) may be any organic solvent. Particularly suitable organic solvents are acetone, ethanol and ethyl acetate.

[0020] In another embodiment, the coating mixture of step b) is prepared by:

b1 ) dissolving the polymer in the organic solvent to provide a polymeric solution; b2) dispersing the layered-double hydroxide in the organic solvent to provide a layered-double hydroxide dispersion; and

b3) mixing together quantities of the polymeric solution and layered-double hydroxide dispersion to yield the coating mixture. Preparing the coating mixture in this manner allows for a greater degree of control over its composition. Coating mixtures used in the prior art have been prepared by blending together polymerisable acrylic monomers, other polymers and inorganic materials (e.g. clays) in the presence of a solvent and then conducting radical polymerisation of the resulting blend under elevated temperature to yield the polymeric coating mixture. As a consequence, coating mixtures prepared by such in-situ polymerisation techniques are likely to contain a variety of polymeric products, each having different properties (e.g. molecular weight). This necessarily makes it different to prepare multiple batches of coating mixture to the exact same specification. In contrast to this approach, the coating mixtures of the present process can be prepared by mixing together quantities of i) a LDH dispersion, and ii) a polymeric solution prepared by dissolving one or more polymers in a solvent, and thus having a pre-determined properties (e.g. viscosity). The present process also eliminates the risk of generating potentially unwanted (or harmful) side products by uncontrolled radical polymerisation of a complex blend of ingredients.

[0021] The organic solvents used in steps b1 ) and b2) may be the same or different. Suitably, the organic solvents used in steps b1 ) and b2) are the same. More suitably, the organic solvents used in steps b1 ) and b2) are ethyl acetate, ethanol or acetone.

[0022] In an embodiment, the LDH has a platelet morphology, wherein the largest dimension of the platelet is 0.01 -10 μιη. Suitably, the largest dimension of the platelet is 0.01 -1 μιτι.

[0023] In an embodiment, the coating mixture comprises 1 -15 wt% of layered double hydroxide. Suitably, the coating mixture comprises 1 -10 wt% of layered double hydroxide. More suitably, the coating mixture comprises 2-6 wt% of layered double hydroxide.

[0024] In an embodiment, the aspect ratio of the layered double hydroxide is at least 10, wherein aspect ratio is the average diameter of the layered double hydroxide platelet divided by the average thickness of the layered double hydroxide platelet.

[0025] In an embodiment, the coating mixture comprises 1 -20 wt% of polymer. Suitably, the coating mixture comprises 1 -10 wt% of polymer.

[0026] In an embodiment, the coating mixture has a viscosity of 1 -1000 cP.

[0027] In an embodiment, the coating mixture has a total solids content (polymer and LDH) of 1 -30 wt%. Suitably, the coating mixture has a total solids content of 5-15 wt%. More suitably, the coating mixture has a total solids content of 8-15 wt%.

[0028] The film prepared by the process of the invention may have a laminated structure. In such cases, after step c) and prior to step d), the coated first substrate is contacted with a second substrate, such that the layer of coating mixture is provided between the first and second substrates. In such an embodiment, the wet coating mixture serves as an adhesive to adhere the second substrate to the first substrate. In such embodiments, the polymeric matrix may also comprise a curing agent for the adhesive.

[0029] Alternatively, a laminated structure may be achieved by using a separate, dedicated adhesive layer. Hence, the process may further comprise the steps of:

e) applying a layer of adhesive to the dried coated first substrate resulting from step d), such that the layer of adhesive is provided on top of the layer applied during step c); and

f) contacting the layer of adhesive applied in step e) with a second substrate.

[0030] The second substrate may be selected from polyethylene terephthalate (PET), polyethylene (PE), biaxiai!y oriented polypropylene film (BOPP), polypropylene (PP), polyvinyl dichloride (PVDC), polyamide, nylon, and polylactic acid (PLA). The second substrate and the first substrate may be the same or different.

[0031] The adhesive may be selected from cellulose acetate, polyvinyl alcohol) (PVOH), polyvinyl acetate, polyvinyl dichloride (PVDC), polyurethane, an acrylic-based adhesive, an epoxy resin and mixtures thereof. Alternatively, the adhesive may be a copolymer based on one or the aforementioned polymers and one or more additional comonomers, such as ethylene (e.g. polyethylene vinyl alcohol). Suitably, the adhesive is food-grade. Suitably, the adhesive may also comprise a curing agent.

[0032] In an embodiment, the adhesive may be a polyurethane and/or acrylic-based adhesive.

[0033] In an embodiment, the process comprises a step e') of coating the dried layer of coating mixture resulting from step d) with a further layer of coating mixture, and then drying the further layer of coating mixture. Step e') may be repeated multiple times to afford a substrate containing a plurality of individually coated layers. It will be appreciated that each coating layer may be the same or different.

[0034] In an embodiment, the layered double hydroxide has a structure according to formula (I) shown below:

[M^z+ _{1 x}MV⁺,(OH)₂]^a+(Xⁿ )_m-_ibH₂0-c(solv)

(I) wherein

M is at least one charged metal cation;

M' is at least one charged metal cation different from M; z is 1 or 2;

y is 3 or 4;

0<x<0.9;

0<b≤10;

0≤c≤10;

X is at least one anion;

n is the charge on anion X;

solv is an organic solvent capable of hydrogen-bonding to water; a is equal to z(1 -x)+xy-2; and

m≥ a/n,

and wherein the layered double hydroxide comprises one or more organic species capable of rendering the layered double hydroxide dispersible in the organic solvent of step b).

[0035] The use of LDHs having a structure according to formula (I) presents a number of advantages over the use of conventional LDHs. Perhaps most notably, the LDHs of formula (I) comprise one or more organic species within their structure (either between the cationic layers or on the surface of the LDH) which renders them dispersible in the organic solvent of step b). Whereas conventionally prepared LDHs are non-dispersible in organic solvents (and are only dispersible in water), the presence of the one or more organic species in the LDHs of formula (I) increases their lipophilicity so as to render the LDH readily dispersible in organic solvents. The increased lipophilicity of the LDH also gives rise to a film having better WVTR properties.

[0036] In an embodiment, the one or more organic species is a surfactant. It will be understood that the term surfactant means any compound having a hydrophilic portion capable of ionic or covalent association with the surface (internal or external) of the LDH, and a lipophilic portion. Suitably, the organic species is an anionic surfactant. Exemplary surfactants include sodium dodecyl sulphate and sodium dodecylbenzenesulfonate.

[0037] The organic species may be a (4-22C)fatty acid, or a salt thereof (e.g. an (8-22C)fatty acid, or a salt thereof). Exemplary organic species include butyric acid, caproic acid, lauric acid, myristic acid, palmitic acid stearic acid, arachidic acid, oleic acid, linoleic acid, maleic acid, and salts thereof. Particularly suitably, the organic species is selected from stearic acid, lauric acid, or a salt thereof (e.g. sodium salts).

[0038] In formula (I), X may be an anion selected from at least one of a halide (e.g. chloride), an inorganic oxyanion, a surfactant, an anionic chromophore, and/or an anionic UV absorber (for example 4-hydroxy-3-10 methoxybenzoic acid, 2-hydroxy-4 methoxybenzophenone-5- sulfonic acid (HMBA), 4-hydroxy-3-methoxy-cinnamic acid, p-aminobenzoic acid and/or urocanic acid).

[0039] Inorganic oxyanions may be of the general formula X'_mO_n(OH)p^"g, wherein m = 1 -5; n = 2-10; p = 0-4, q = 1 -5; X' = B, C, N, S, P, and may include carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, borate, nitrate, phosphate, sulphate.

[0040] Surfactants include anionic surfactants (such as sodium dodecyl sulphate and sodium dodecylbenzenesulfonate) and 4-22C)fatty acids and salts thereof (such as butyric acid, caproic acid, lauric acid, myristic acid, palmitic acid stearic acid, arachidic acid, oleic acid, linoleic acid, maleic acid, and salts thereof).

[0041] Particularly suitably, the anion X is one or more of an inorganic oxyanion selected from carbonate, bicarbonate, nitrate or nitrite, or a surfactant (e.g. lauric acid, sodium laurate, sodium dodecyl sulphate or sodium stearate).

[0042] In an embodiment, the anion X is the organic species. It will be appreciated that in some circumstances, the anion X and the organic species may be one and the same. For example, when the anion X is a surfactant (e.g. sodium stearate), it may itself assume the role of the organic species responsible for conferring organic solvent dispersibility to the LDH. In such scenarios, the anion X, being the organic species, may be located between the cationic layers of the LDH.

[0043] Alternatively, anion X may confer no organic solvent dispersibility to the LDH, in which case the organic species has a different identity to anion X. For example, anion X may be carbonate, and the organic species may have any one or more of the identities recited hereinbefore. In such circumstances, the organic species may be introduced into the LDH structure by modification of a pre-prepared LDH (e.g. modification of a carbonate-containing LDH such as Mg₃AI-C0₃).

[0044] In an embodiment, when z is 2, M is Mg, Zn, Fe, Ca, Sn, Ni, Cu, Co, or a mixture of two or more of these, or when z is 1 , M is Li. Suitably, z is 2 and M is Ca, Mg, Zn or Fe. More suitably, z is 2 and M is Ca, Mg or Zn.

[0045] In an embodiment, when y is 3, M' is Al, Ga, In, Fe, or a mixture thereof, or when y is 4, M' is Sn, Ti or Zr or a mixture thereof. Suitably, y is 3. More suitably, y is 3 and M' is Al.

[0046] The solvent 'solv' may be any organic solvent that is capable of hydrogen-bonding (as a donor or acceptor) to water. Hydrogen bond donor groups include R-OH, R-NH₂, R₂NH, whereas hydrogen bond acceptor groups include ROR, R₂C=0 RN0₂, R₂NO, R₃N, ROH, RCF₃. Exemplary organic solvents include ethyl acetate, acetone, ethanol, m-cresol, o-cresol, p- cresol, n-butanol, sec-butanol, n-pentanol, n-hexanol, cyclohexanol, diethyl ether, diisopropyl ether, di-n-butyl ether, methyl tert-butyl ether (MTBE), tert-amyl methyl ether, cyclopentyl methyl ether, anisole, butyl carbitol acetate, cyclohexanone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), methyl isoamyl ketone, methyl n-amyl ketone, isophorone, isobutyraldehyde, furfural, methyl formate, methyl acetate, isopropyl acetate, n-propyl acetate, isobutyl acetate, n-butyl acetate, n-amyl acetate, n-hexyl acetate, methyl amyl acetate, methoxypropyl acetate, 2-ethoxyethyl acetate, 2-butoxyethyl acetate, n-butyl propionate, n- pentyl propionate, triethylamine, 2-nitropropane, aniline, Ν,Ν-dimethylaniline, nitromethane, and mixtures of two or more thereof. Suitably, the organic solvent 'solv' is ethyl acetate, ethanol or acetone.

[0047] In an embodiment, x has a value according to the expression 0.18<x<0.9. Suitably, x has a value according to the expression 0.18<x<0.5. More suitably, x as a value according to the expression 0.18< <0.4.

[0048] In an embodiment, the LDH of formula (I) is a Zn/AI, Mg/AI, Zn,Mg/AI, Mg/AISn, Ca/AI, Ni/AI or Cu/AI layered double hydroxide.

[0049] In a particular embodiment, M is Ca, Mg, Zn or Fe, M' is Al, and X is carbonate, bicarbonate, nitrate, nitrite, an organic species defined herein, or a mixture thereof. Suitably, M is Ca, Mg or Zn, M' is Al, and X is carbonate, bicarbonate, nitrate, nitrite, an organic species defined herein, or a mixture thereof. More suitably, M is Ca, Mg or Zn, M' is Al, and X is carbonate, an organic species defined herein, or a mixture thereof.

[0050] The LDH of formula (I) may be prepared by a process comprising the steps of:

i) precipitating a layered double hydroxide having the formula (II) from an aqueous solution containing cations of the metals M and M', the anion(s) X^n~, and optionally an ammonia-releasing agent;

[M^z+ ₁-,M'y⁺ _x(OH)₂]^a+(Xⁿ-)_m 3H₂0

(II) wherein M, M', z, y, x, a, b, n, m and X are as defined for formula (I) ii) ageing the layered double hydroxide precipitate obtained in step (i) in the reaction mixture of step (i);

iii) collecting the aged precipitate resulting from step (ii), then washing it with water and optionally a solvent 'solv' as defined hereinbefore; and

iv) drying/filtering the washed precipitate.

[0051] The ammonia-releasing agent used in step i) may increase the aspect ratio of the resulting LDH platelets. Suitable ammonia-releasing agents include hexamethylene tetraamine (HMT) and urea. Suitably, the ammonia-releasing agent is urea. The amount of ammonia- releasing agent used in step i) may be such that the molar ratio of ammonia-releasing agent to metal cations (M + M') is 0.5:1 to 10:1 (e.g. 1 :1 to 6:1 or 4:1 to 6:1 ).

[0052] The solvent 'solv' optionally included in the water washing step iii) may have any of the identities recited hereinbefore. When a 'solv' is included in the water washing step, the LDH is washed with a mixture of water and 'solv', in which case c in formula (I) is 0<c≤10.

[0053] In an embodiment, in step i), the precipitate is formed by contacting aqueous solutions containing cations of the metals M and M', the anion X"^", and optionally an ammonia-releasing agent, in the presence of a base being a source of OH^" (e.g. NaOH, NH₄OH, or a precursor for OH^" formation). Suitably the base is NaOH. In an embodiment, the quantity of base used is sufficient to control the pH of the solution at 6.5-14. Suitably, the quantity of base used is sufficient to control the pH of the solution at 7.5-13. More suitably, the quantity of base used is sufficient to control the pH of the solution at 9-1 1 .

[0054] In an embodiment, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) for a period of 5 minutes to 72 hours at a temperature of 25-180 °C.

[0055] Suitably, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) for a period of 0.5 to 72 hours. More suitably, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) for a period of 5 to 48 hours. Most suitably, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) for a period of 12 to 36 hours.

[0056] Suitably, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) at a temperature of 80-150°C. More suitably, in step ii), the layered double hydroxide precipitate obtained in step i) is aged in the reaction mixture of step i) at a temperature of 90-140 °C.

[0057] Step ii) may be performed in an autoclave.

[0058] In an embodiment, in step iii), the aged precipitate resulting from step ii) is collected, then washed with water and optionally a solvent 'solv' until the filtrate has a pH in the range of 6.5-7.5. Suitably, step iii) comprises washing the aged precipitate resulting from step ii) with a mixture of water and 'solv' at a temperature of 50-100°C. More suitably, the 'solv' is selected from ethyl acetate, ethanol and acetone. More suitably, the quantity of 'solv' in the washing mixture is 5-95% (v/v), preferably 30-70% (v/v). [0059] The LDH resulting from step iv) may be used directly in the process of the invention. Alternatively, the LDH resulting from step iv) may be subjected to one or more post-preparation treatments.

[0060] In an embodiment, the LDH resulting from step iv) may be subjected to a first post- preparation treatment process comprising the steps of:

v. dispersing the LDH resulting from step iv) in a 'solv' solvent as defined hereinbefore to produce a slurry;

vi. maintaining the slurry resulting from step v); and

vii. isolating the layered double hydroxide resulting from step vi).

[0061] In an embodiment, the slurry produced in step v) and then maintained in step vi) contains 1 -100 g of water-washed wet precipitate per 1 L of 'solv'. Suitably, the slurry produced in step v) and maintained in step vi) contains 1 -75 g of water-washed wet precipitate per 1 L of 'solv'. More suitably, the slurry produced in step v) and maintained in step vi) contains 1 -50 g of water-washed wet precipitate per 1 L of 'solv'. Most suitably, the slurry produced in step v) and maintained in step vi) contains 1 -30 g of water-washed wet precipitate per 1 L of 'solv'.

[0062] In step vi), the slurry produced in step v) is maintained for a period of time. Suitably, the slurry is stirred during step vi).

[0063] In an embodiment, in step vi), the slurry is maintained for a period of 0.5 to 120 hours (e.g. 0.5 to 96 hours). Suitably, in step vi), the slurry is maintained for a period of 0.5 to 72 hours. More suitably, in step vi), the slurry is maintained for a period of 0.5 to 48 hours. Even more suitably, in step vi), the slurry is maintained for a period of 0.5 to 24 hours. Yet more suitably, in step vi), the slurry is maintained for a period of 0.5 to 10 hours. Most suitably, in step vi), the slurry is maintained for a period of 1 to 8 hours.

[0064] In an embodiment, the first post-preparation treatment process comprises a step viii) of contacting the layered double hydroxide isolated in step vii) with at least one 'solv' as defined herein. In certain embodiments, it may be advantageous to perform one or more additional 'solv' treatment steps on the precipitate isolated in step vii). In an embodiment, in step viii), the isolated layered double hydroxide is washed with at least one 'solv' (e.g. using Buchner apparatus). Alternatively, step viii) comprises the steps of :

viii¹) dispersing the isolated layered double hydroxide in a 'solv' to form a slurry;

viii²) maintaining the slurry for a period of 0.5 to 72 hours;

viii³) isolating the layered double hydroxide resulting from step viii²); and

viii⁴) optionally repeating steps viii¹). to viii³) a further 1 -10 times (e.g. once or twice). [0065] Hence, step viii) may comprise performing additional dispersion-maintaining-isolation cycles in order to remove residual water from the layered double hydroxide.

[0066] The 'solv' used in the first post-preparation treatment process is suitably ethanol, acetone, ethyl acetate or a mixture of two or more thereof.

[0067] In an embodiment, the LDH resulting from step iv) may be subjected to a second post- preparation treatment process being a solvothermal treatment process.

[0068] Suitably, the solvothermal treatment process comprises heating the LDH resulting from step iv) in at least one solvent 'solv', or a mixture of at least one solvent 'solv' and the filtrate of step iv), under increased pressure. More suitably, the solvothermal treatment process is carried out in an autoclave at a temperature of 70-180 °C for a period of 2-72 hours. Even more suitably, the solvothermal treatment process is carried out in an autoclave at a temperature of 80-160 °C for a period of 2-48 hours.

[0069] When anion X in formula (II) does not confer organic solvent-dispersibility to the LDH (i.e. when anion X is not the organic species), the process of preparing the LDH may comprise an additional step wherein the organic species is introduced into (or onto) the LDH.

[0070] In an embodiment, the dried LDH isolated from steps iv), vii) and viii) may be contacted with an organic species as defined hereinbefore. The dried LDH isolated from steps iv), vii) and viii) may be mixed with the organic species (which itself may be provided neat, or in a solvent). Alternatively, the dried LDH isolated from steps iv), vii) and viii) may be milled in the presence of the organic species.

[0071] In an embodiment, the organic species may be introduced into the LDH structure by adding the organic species to the slurry of step v) or viii).

[0072] Step c) of the present process may be performed by various different techniques.

[0073] In one embodiment, the coating mixture may be applied to the substrate in step c) by spraying, dip coating or spin coating.

[0074] Alternatively, the coating mixture may be applied to the substrate in step c) using a bath-and-roller assembly. Such assemblies will be understood to comprise a rotating roller being in partial contact with a bath containing a coating mixture. As the roller rotates, the coating mixture coats the surface of the roller, and is transferred onto a substrate passing over the surface of the roller. Additional rollers may be present to meter the quantity of coating mixture applied to the substrate, or to remove excess coating mixture. Such assemblies may additionally comprise a Mayer rod, or other means, to ensure uniform distribution of the coating mixture across the surface of the substrate. Films

[0075] As discussed hereinbefore, the present invention also provides a film obtainable, obtained or directly obtained by a process as defined herein

[0076] As discussed hereinbefore, the present invention also provides a film comprising:

a) a substrate; and

[0077] The films of the invention have improved OTR and WVTR properties with respect to prior art films. In particular, the increased lipophilicity of the organic-solvent dispersible LDHs used in the film allows improved WVTR properties (i.e. lower WVTR values) to be attained.

[0078] It will be understood that the films of the invention are distinguished from LbL-prepared films by virtue of the fact that they do not contain a plurality of alternating layers of polymer and LDH. Rather, the films of the invention contain a single layer of LDH dispersed throughout a polymeric matrix. The LDH may be randomly dispersed throughout the polymeric matrix.

[0079] In an embodiment, the coating layer comprises 3-60 wt% of layered double hydroxide. Suitably, the coating layer comprises 5-50 wt% of layered double hydroxide.

[0080] In an embodiment, the LDH is as described in any of the paragraphs appearing hereinbefore in relation to the process for preparing the film.

[0081] In an embodiment, the polymeric matrix comprises an organic solvent-soluble polymer as described in any of the paragraphs appearing hereinbefore in relation to the process for preparing the film.

[0082] In an embodiment, the substrate is as described in any of the paragraphs appearing hereinbefore in relation to the process for preparing the film.

[0083] In an embodiment, the coating layer has a thickness of 0.1 -10 μηι (e.g. 1 -10 μηι).

[0084] In an embodiment, the film comprises multiple coating layers. Suitably, the film comprises 1 -10 individually coated layers. Suitably, the film comprises 1 -4 individually coated layers.

[0085] In an embodiment, the coating layer comprises: a) 5-50 wt% of layered double hydroxide;

b) 50-95 wt% of polymeric matrix; and

c) 0-2 wt% of organic solvent.

[0086] In an embodiment, the coating layer comprises:

a) 5-50 wt% of layered double hydroxide;

b) 50-95 wt% of cellulose acetate; and

c) 0-2 wt% of ethyl acetate.

[0087] The film may have a laminated structure. Hence, in one embodiment, the substrate is a first substrate, and the film comprises a second substrate disposed on top of the coating layer, such that the coating layer is located between the first and second substrates. In such embodiments, the coating layer serves as an adhesive to adhere the second substrate to the first substrate.

[0088] Alternatively, the film comprises a layer of adhesive provided between the coating layer and the second substrate. In such embodiments, a dedicated adhesive layer adheres the second substrate to the coated first substrate.

[0089] The second substrate may be as described in any of the paragraphs appearing hereinbefore in relation to the process for preparing the film.

[0090] The adhesive may be as described in any of the paragraphs appearing hereinbefore in relation to the process for preparing the film.

Applications of the film

[0091] As discussed hereinbefore, the present invention also provides a use of a film as defined herein in packaging.

[0092] As discussed hereinbefore, the present invention also provides a container comprising a film as defined herein.

[0093] The advantageous OTR and WVTR properties of the films of the invention render them useful in the field of packaging, particularly in the food industry. Accordingly, the films of the invention may be used in packaging or in a container that is intended to package or contain a foodstuff. EXAMPLES

[0094] The present invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:

Figure 1 shows FTIR spectra of Mg₃AI-LR-LDHs prepared by the urea method (Example 1.2) with [urea]:[Mg+AI] ratio of (a) 6:1 , (b) 5:1 , (c) 4:1 , (d) 3:1 , (e) 2:1 and (f) 1 :1 . All samples were aged at 90 °C for 18 hours.

Figure 2 shows PXRD patterns of Mg₃AI-LR-LDHs prepared by the urea method (Example 1.2) with [urea]:[Mg+AI] ratio of (a) 6:1 , (b) 5:1 , (c) 4:1 , (d) 3:1 , (e) 2:1 and (f) 1 :1. All samples were aged at 90 °C for 18 hours, Δ denotes the (009) peaks belonging to the Mg₃AI-(LR+LA)-LDH phase and ^* denotes sample holder peaks.

Figure 3 shows TEM images of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1 .2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 °C for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

Figure 4 shows FTIR spectra of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1.2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 °C for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

Figure 5 shows PXRD patterns of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1 .2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 °C for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

Figure 6 shows the transmittance (500 nm wavelength) of various organo-LDH dispersions (Example 3) in ethanol, ethyl acetate, toluene and o-xylene.

Figure 7 provides a schematic flow diagram outlining the coating process.

Figure 8 shows SEM images of (a) PET/CA + ST1 -24 5%, (b) PET/CA + ST2-48 5%, (c) PET/CA + ST2-48 3% coatings on PET film (top view).

Figure 9 shows oxygen transmission rate values (OTR) of various uncoated and coated films.

Figure 10 shows water vapour transmission rate values (WVTR) of various uncoated and coated films.

Figure 1 1 shows X-ray powder crystallography of various coated and uncoated films. Figure 12 shows transmittance (500 nm wavelength) data of coated films for four different LDHs at four different LDH loadings (0 wt% loading corresponds to the PET/CA film without LDH).

Materials and methods

Elemental analysis (EA)

[0095] Elemental analyses for carbon, hydrogen and nitrogen were performed at the School of Human Sciences, London Metropolitan University, using the quantitative combustion technique.

Fourier transform infrared (FTIR) spectroscopy

[0096] FTIR spectra were recorded on a Bio-Rad FTS-6000 instrument. Samples were mounted on a DuraSampllR Diamond ATR stage, and transmittance was recorded in the range 700 - 4,000 cm^-1 with 50 scans at 4 cm^-1 resolution.

Gas barrier testing

[0097] Oxygen barrier testing was measured on an Oxygen Permeation Analyzer 8001 , with values obtained at 23 °C at 0% relative humidity (RH).

Powder X-ray diffraction (PXRD)

[0098] PXRD data were recorded on a PANalytical X'pert Pro diffractometer operating at 40 kV/40 mA in reflection mode with Cu K_a radiation (λ = 1.542 A). Samples were placed into steel sample holders, which were spun at 15 rpm during recording, and analysed from 2 ° to 70 ° with a slit size of 1 °.

Scanning electron microscopy (SEM)

[0099] SEM images were acquired on a Jeol JSM-6100 microscope with an accelerating voltage of 20 kV. The powder samples were mounted on adhesive carbon tape attached to metal stubs. Before analysis, the powder samples and the coated films were sputter-coated with a 10 nm platinum layer to facilitate imaging. Solid state nuclear magnetic resonance (NMR) spectroscopy

[00100] ²⁷AI DP MAS and ¹³C CP MAS solid state NMR spectra were obtained at 104.2 and 100.5 MHz, respectively (9.4 T) on a Bruker Avance III HD 400 spectrometer. For ²⁷AI DP MAS NMR spectroscopy, in order to obtain quantitative MAS spectra, a single pulse excitation was applied using a short pulse length (0.23 με). 10,000 scans were acquired with a 0.1 s delay and a MAS rate of 10 kHz using 4 mm O.D zirconia rotors. The ²⁷AI NMR chemical shift is referenced to an aqueous solution of AI(N0₃)3 (0 ppm). ¹³C CP MAS NMR spectra were measured using 4 mm O.D zirconia rotors and a MAS rate of 10 kHz using a cross-polarisation sequence with a variable X-amplitude spin-lock pulse (O. B. Peersen, X. L. Wu, I. Kustanovich and S. O. Smith, J. Magn. Reson., 1993, 104, 334-339) and spinal64 proton decoupling. 5,000 transients were acquired using a contact time of 2.5 ms, an acquisition time of 25 ms (2,048 data poits zero filled to 16 K) and a recycle delay of 5 s. The ¹³C spectra were referenced to adamantane (the upfield resonance was taken to be δ = 29.5 ppm on a scale where 5(TMS) = 0) as a secondary reference (W. L. Earl and D. L. Vanderhart, J. Magn. Reson., 1982, 48, 35-54)

Surface area analysis

[00101] The gas adsorption isotherm for nitrogen adsorption onto the powder sample surface was measured using a CE Instruments Sorptomatic 1990. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method.

Tap density measurement

[00102] A few grams of the loose powder were transferred to a measuring cylinder and tapped 6,000 times, its volume being measured every 100 taps. Kawakita equation was used to obtain the Carr's index.

Thermogravimetric analysis (TGA)

[00103] A few milligrams of sample were placed in a platinum crucible suspended from a platinum wire, which was weighed empty first. Sample mass was recorded under a continuous flow of N₂ from 30 to 600 °C at a rate of 10 O/minute. At the start of the program several minutes at 30 °C were included to allow the sample mass to settle, and at the final temperature to ensure mass loss was complete. Transmission electron microscopy (TEM)

[00104] TEM images were acquired on a Jeol JEM-2100 microscope at an accelerating voltage of 200 kV, and at t e Materials Department, University of Oxford on a Jeol 4000EX microscope at 200 kV. A few milligrams of material were dispersed in ethanol using sonication, and then cast onto carbon films on copper grids (400 mesh, Agar Scientific) and allowed to air dry. Image analysis was carried out using ImageJ software.

Ultraviolet-visible (UV-vis) spectroscopy

[00105] UV-vis measurements were taken on a Cary Series UV-vis spectrometer. The absorbance was measured in the range from 800 to 200 nm. The dispersion was shaken and a small amount transferred into a quartz cuvette. The films were positioned diagonally into a quartz cuvette.

Example 1 - Synthesis of LDHs

1 .1 Synthesis of Mg₃AI-LR-LDH

[00106] Mg3AI-L.R-L.DH was synthesised by the co-precipitation method. Mg(N0₃)2 · 6H2O (9.62 g, 37.5 mmol) and AI(N0₃)₃ · 9H₂0 (4.69 g, 12.5 mmol) were dissolved in 100 mL deionised (Dl) water (Solution A). Laurie acid (2.40 g, 12.0 mmol) was dissolved in 150 mL Dl water at 80 °C and the pH was adjusted to 10 by dropwise addition of 4 M NaOH (Solution B). Solution A was added to Solution B dropwise over the period of 1 hour with vigorous stirring and the pH of the precipitation was maintained at -10 using 4 M NaOH solution. The resulting reaction mixture was aged in an oil bath heated at 105 °C for 18 hours with stirring at 700 rpm. After ageing, the LDH product was washed with 1 L of 1 :1 mixture of ethanol and Dl water at 80 °C, followed by drying overnight in a vacuum oven at room temperature.

1 .2 Synthesis of Mg₃AI-LR-LDH (urea method)

[00107] Mg(N0₃)₂ · 6H₂0 (9.62 g, 37.5 mmol), AI(N0₃)₃ · 9H₂0 (4.69 g, 12.5 mmol) and urea (15.01 g, 250.0 mmol) were dissolved in 200 mL Dl water (Solution C). Laurie acid (2.40 g, 12.0 mmol) was dissolved in 50 mL Dl water at 80 °C and the pH was adjusted to 10 by dropwise addition of 4 M NaOH (Solution D). Solution C was added to Solution D during vigorous stirring. The resulting reaction mixture was aged in an oil bath heated at 90°C or 105 °C for 18 hours with stirring at 700 rpm. After ageing, the LDH product was washed with 1 L of 1 :1 mixture of ethanol and D I water at 80 °C, followed by drying overnight in a vacuum oven at room temperature.

1 .3 Synthesis of Mq₃AI-LR-LDH (urea hvdrothermal method)

[00108] Mg3AI-L.R-L.DH was synthesised by two different homogeneous precipitation methods Mg(N0₃)₂ · 6H2O (9.62 g, 37.5 mmol), AI(N0₃)₃ · 9H₂0 (4.69 g, 12.5 mmol) and urea (15.01 g, 250.0 mmol) were dissolved in 200 mL Dl water (Solution C). Laurie acid (2.40 g, 1 2.0 mmol) was dissolved in 50 mL Dl water at 80 °C and the pH was adjusted to 10 by dropwise addition of 4 M NaOH (Solution D). Solution C was added to Solution D during vigorous stirring. 40 mL of the reaction mixture was transferred to a 50 mL Teflon inner vessel within a stainless steel outer vessel, followed by ageing in an oven preheated at 1 05 °C for 18 hours. After ageing, the LDH product was washed with 1 L of 1 :1 mixture of ethanol and Dl water at 80 °C, followed by drying overnight in a vacuum oven at room temperature.

1 .4 Optimisation of urea method

[00109] Mg₃AI-LR-LDH was synthesised by the urea method.

[00110] When the initial form of the molecule to be intercalated was sodium laurate (pH 10), in Solution C, the amounts of urea were varied (Table 1 ). The reaction mixture was aged in an oil bath heated at three different temperatures (90, 105 or ~\ 25 °C) for three different ageing periods (5 minutes, 1 8 or 24 hours) with stirring at 700 rpm.

Table 1 Quantities of urea used for different [urea]:[Mg+AI] ratios.

[urea]:[Mg+AI] murea/ g riurea mmol

1 :1 3.00 50.0

2:1 6.00 100.0

3:1 9.00 150.0

4:1 1 2.01 200.0

5:1 1 5.01 250.0

6:1 1 8.02 300.0 [00111] When the the initial form of the molecule to be intercalated was lauric acid (pH 3), Mg(N0₃)₂ · 6H₂0 (9.62 g, 37.5 mmol) and AI(N0₃)₃ · 9H₂0 (4.69 g, 12.5 mmol) were dissolved in 150 mL Dl water and heated to 100 °C (Solution A). Urea (18.01 g, 300.0 mmol) was dissolved in 50 mL of water (Solution B). Lauric acid (2.40 g, 12.0 mmol) was dissolved in 50 mL Dl water at 80 °C (Solution C). Solution A and then Solution B were added to Solution C during vigorous stirring. The resulting reaction mixture was aged in an oil bath heated at 125 °C for 24 hours with stirring at 700 rpm.

[00112] After ageing, all LDH products were washed with 1 L of 1 :1 mixture of ethanol and Dl water at 80 °C, followed by drying overnight in a vacuum oven at room temperature.

1 .5 Synthesis of stearate-modified LDH

[00113] A mixed metal solution was prepared from 9.6 g of Mg(N0₃)₂-6H₂0, 4.7 g of AI(N0₃)₃-9H₂0 (4.68 g, 12.5 mmol) in 50 mL of de-carbonated water (Solution A). A second solution contained 2.65 g of Na2C0₃ in 50 mL of deionised water. (Solution B). The solution A was added drop-wise (58 mL/minutes) to the Solution B. The system was kept at constant pH 10 by using 4 M NaOH and aged for 16 hours at room temperature. Then, the slurry was washed by de-carbonated water until the pH was close to 7 and followed by washing by using ethanol. The slurry was washed with 1000 ml of ethanol and then re-dispersed in 600 ml of this solvent for 1 hour. Then the obtained LDH solid was filtered, rinsed with 400 mL of ethanol, and dried in a vacuum oven for 24 hours.

[00114] Zn stearate (80 mg) was dissolved in 20 mL of xylene at 70 °C. 200 mg of the LDH in 10 mL of xylene was added into Zn stearate solution. The mixture was stirred at 70 °C for 5 min. After cooling to room temperature, the solid was filtered and dried in the vacuum oven at room temperature.

1 .6 Synthesis of lau rate-modified LDH

[00115] Mg(N0₃)₂-6H₂0 (9.60 g, 37.4 mmol) and AI(N0₃)₃-9H20 (4.68 g, 12.5 mmol) were dissolved in 50 mL of distilled water (Solution A). A second solution was made containing Na₂C0₃ (2.65 g, 25.0 mmol) and NaOH (4g, 100 mmol) dissolved in 200 mL distilled water (Solution B). Solution A was added quickly to Solution B and stirred for 30 minutes. The LDH was washed twice with water and once with acetone by centrifuge-washing cycles. Six centrifuge tubes were used at 9000 rpm for five minutes. The resulting LDH slurry was dispersed in 200 mL acetone for 17 hours. The LDH slurry was then filtered, washed with 100 ml. acetone and dispersed in 100 mL acetone for one hour. This procedure was repeated three times. The resulting LDH was dried overnight in a vacuum oven.

[00116] 200 mg of the obtained LDH was dispersed in 10 mL of ethanol. 36 mg of sodium laurate was dissolved in ethanol at 70 °C. Then the LDH slurry was quickly added to the laurate solution and kept stirring at 70°C for 5 minutes. The final product was collected by filtration and dried in a vacuum oven overnight.

Example 2 - Post-preparation LDH treatment processes

2.1 Solvent treatment process

[00117] Mg₃AI-LR-LDH was synthesised with [urea]:[Mg+AI] ratios of 5:1 and 6:1 for initial form of the molecule to be intercalated being sodium laurate and lauric acid, respectively. In both cases, the ageing was performed in an oil bath heated at 125 °C for 24 hours. After ageing, the LDH was washed with 1 L of 1 :1 mixture of ethanol and Dl water at 80 °C and the wet cake was dispersed in 250 mL of a solvent until a well dispersed slurry was obtained. The slurry was filtered but was not allowed to dry, followed by re-dispersion in 500 mL of the solvent and left to stir at 800 rpm for 4 hours. Finally, the resulting LDH was filtered, washed with 250 mL of the solvent and dried overnight in a vacuum oven at room temperature. Two different solvents were used, ethanol and acetone.

2.2 Solvothermal treatment process 1 (ST1 )

[00118] Mg₃AI-LR-LDH was synthesised for [urea]:[Mg+AI] ratios of 5:1 and 6:1 for initial form of the molecule to be intercalated being sodium laurate and lauric acid, respectively. In both cases, the ageing was performed in an oil bath heated at 125 °C for 24 hours. For the wet cake starting material form, the organo-LDH product was filtered without washing and ~½ of the wet cake was dispersed in 40 mL of a 5:3 mixture of mother liquor (filtrate) and ethanol. For dried and non-dried forms, the organo-LDH product was washed with 1 L of 1 :1 mixture of ethanol and Dl water at 80 °C, followed by overnight drying in a vacuum oven for the dried form only. 4.00 g of dry powder or ~¼ of washed wet cake was dispersed in 40 mL of a 5:3 mixture of Dl water an ethanol in the case of dried and non-dried forms, respectively. The dispersion of any of the three starting material forms was transferred into a 50 mL Teflon inner vessel within a stainless steel outer vessel. The mixture was put into a pre-heated oven at 90, 100 or 150 °C for 4, 6, 8 or 24 hours. Following ST1 , the autoclaves were placed into cold water to cool and each organo-LDH product was washed with 200 mL of ethanol, followed by drying overnight in a vacuum oven at room temperature. 2.3 Solvothermal treatment process 2 (ST2)

[00119] For ST2, the same procedure was followed as for ST1 with the following differences: only the dried and non-dried starting material forms were considered, the volume of the organo- LDH dispersion was reduced by ½ while keeping the LDH concentration constant, and the solvent used was pure ethanol. Furthermore, the oven temperature was kept at 150 °C while the treatment time was 4, 6, 8, 24 or 48 hours.

Example 3 - Preparation of organo-LDH dispersions

[00120] 20 mg of finely ground organo-LDH sample was added to 2 ml_ of the chosen solvent in a glass vial and sonicated for 20 minutes.

Example 4 - Characterisation of LDHs and LDH dispersions

[00121] Figure 1 shows FTIR spectra of Mg₃AI-LR-LDHs prepared by the urea method (Example 1 .2) with [urea]:[Mg+AI] ratio of (a) 6:1 , (b) 5:1 , (c) 4:1 , (d) 3:1 , (e) 2:1 and (f) 1 :1 . All samples were aged at 90 °C for 18 hours.

[00122] Figure 2 shows PXRD patterns of Mg₃AI-LR-LDHs prepared by the urea method (Example 1 .2) with [urea]:[Mg+AI] ratio of (a) 6:1 , (b) 5:1 , (c) 4:1 , (d) 3:1 , (e) 2:1 and (f) 1 :1. All samples were aged at 90 °C for 18 hours, Δ denotes the (009) peaks belonging to the Mg₃AI- (LR+LA)-LDH phase and ^* denotes sample holder peaks.

[00123] Figure 3 shows TEM images of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1 .2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 °C for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

[00124] Figure 4 shows FTIR spectra of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1 .2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 °C for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

[00125] Figure 5 shows PXRD patterns of Mg₃AI-LR-LDHs: (a) U-LR product (Example 1 .2), and product after solvent treatment (Example 2.1 ) with (b) acetone and (c) ethanol, (d) product after ST1 at 150 Ό for 24 hours (Example 2.2), and product after ST2 at 150 °C for (e) 24 and (f) 48 hours (Example 2.3).

[00126] Figure 6 shows the transmittance (500 nm wavelength) of various organo-LDH dispersions (Example 3) in ethanol, ethyl acetate, toluene and o-xylene. Example 5 - Preparation of orqano-LDH/cellulose acetate (LDH/CA) coatings

[00127] Figure 7 provides a schematic flow diagram outlining the coating process.

[00128] (LDH/CA) coatings were fabricated on -25 μηη thick polyethylene terephthalate (PET) film. Mg₃AI-L.R-L.DH dispersion in ethyl acetate and CA solution in ethyl acetate were prepared individually, and then mixed together during vigorous stirring to prepare coating mixtures with a total solid loading of 12 wt%. The total solid loading was kept constant, and the organo-LDH and CA proportions were varied so as to obtain organo-LDH loadings of 0, 1 , 3 and 5 wt%. The coating mixtures were stirred for 4 hours at 500 rpm. The coating was carried out on a K Control Coater (RK PrintCoat Instruments Ltd) with the meter bar number 3 at a speed of 5 m min ¹. After the application of the coating on PET film, the solvent was evaporated at room temperature.

Example 6 - Characterisation of coated and uncoated PET films

[00129] Figure 8 shows SEM images of (a) PET/CA + ST1 -24 5%, (b) PET/CA + ST2-48 5%, (c) PET/CA + ST2-48 3% coatings on PET film (top view). "PET/CA + ST1 -24 5%" refers to a PET film that has been coated with an LDH/CA coating containing 5 wt% of sodium laurate- LDH (produced by the urea technique), wherein the as-prepared LDH was subjected to ST1 (in a 5:3 water:ethanol mixture) at ^ 50 °C for 24 hours. "PET/CA + ST2-48 5%" refers to a PET film that has been coated with an LDH/CA coating containing 5 wt% of sodium laurate-LDH (produced by the urea technique), wherein the as-prepared LDH was subjected to ST2 (in ethanol) at 150 °C for 48 hours. "PET/CA + ST2-48 3%" refers to a PET film that has been coated with an LDH/CA coating containing 3 wt% of sodium laurate-LDH (produced by the urea technique), wherein the as-prepared LDH was subjected to ST2 (in ethanol) at 150 Ό for 48 hours.

[00130] Figure 9 shows oxygen transmission rate values (OTR) of various uncoated and coated films.

[00131] Figure 1 0 shows water vapour transmission rate values (WVTR) of various uncoated and coated films.

[00132] Figure 1 1 shows X-ray powder crystallography of various coated and uncoated films.

[00133] Figure 12 shows transmittance data of coated films for four different LDHs at four different LDH loadings (0 wt% loading corresponds to the PET/CA film without LDH). [00134] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims

1. A process for the preparation of a film, the process comprising the steps of:

a) providing a first substrate;

c) coating the first substrate with a layer of the coating mixture; and

d) drying the coated first substrate.

2. The process of claim 1 , wherein the first substrate is selected from polyethylene

terephthalate (PET), polyethylene (PE), biaxia !y oriented polypropylene film (BOPP), polypropylene (PP), polyvinyl dichloride (PVDC), polyamide, nylon, and polylactic acid (PLA).

3. The process of claim 1 or 2, wherein the polymer is selected from one or more of cellulose acetate (CA), polyurethane (PU), polyacrylic acid (PAA), polyvinyl alcohol (PVOH), polyamide, and a copolymer comprising vinyl alcohol (e.g. polyethylene vinyl alcohol (EVOH) and polyethylene vinyl acetate (EVA)), preferably the polymer is cellulose acetate.

4. The process of any one of claims 1 , 2 or 3, wherein the layered double hydroxide has a structure according to formula (I) shown below:

(I) wherein

M is at least one charged metal cation;

M' is at least one charged metal cation different from M;

z is 1 or 2;

y is 3 or 4;

0<x<0.9;

0<fo≤10;

0≤c≤10;

X is at least one anion;

n is the charge on anion X; solv is an organic solvent capable of hydrogen-bonding to water;

a is equal to z(1 -x)+xy-2; and

m≥ a/n,

5. The process of claim 4, wherein when z is 2, M is Mg, Zn, Fe, Ca, Sn, Ni, Cu, Co, or a mixture of two or more of these, or when z is 1 , M is Li.

6. The process of claim 4 or 5, wherein when y is 3, M' is Al, Ga, In, Fe, or a mixture thereof, or when y is 4, M' is Sn, Ti or Zr or a mixture thereof.

7. The process of any one of claims 4, 5 and 6, wherein M' is Al.

8. The process of any one of claims 4 to 7, wherein the layered double hydroxide of formula (I) is a Zn/AI, Mg/AI, ZnMg/AI, CaAI, Ni/AI or Cu/AI layered double hydroxide.

9. The process of any one of claims 4 to 8, wherein the organic species is a surfactant.

10. The process of any one of claims 4 to 9, wherein the organic species is an anionic surfactant (e.g. sodium dodecyl sulphate or sodium dodecylbenzenesulfonate).

1 1 . The process of any one of claims 4 to 10, wherein the organic species is a (4-22C)fatty acid, or a salt thereof (e.g. butyric acid, caproic acid, lauric acid, myristic acid, palmitic acid stearic acid, arachidic acid, oleic acid, linoleic acid, maleic acid, and salts thereof), preferably stearic acid, lauric acid, or a salt thereof.

12. The process of any one of claims 4 to 1 1 , wherein X is an anion selected from at least one of a halide, an inorganic oxyanion, or an anionic organic species as defined in claim 9, 10 or 1 1 .

13. The process of any one of claims 4 to 12, wherein the layered double hydroxide is prepared by a process comprising the steps of:

i. precipitating a layered double hydroxide having the formula (II) from an aqueous solution containing cations of the metals M and M', the anion Xⁿ, and optionally an ammonia-releasing agent: [M^z+ ₁._xM'y⁺,(OH)₂]^a+(Xⁿ-)_m foH₂0

(II) wherein M, M', z, y, x, a, n, m, b and X are as defined for formula (I); ii. ageing the layered double hydroxide precipitate obtained in step (i) in the reaction mixture of step (i);

iii. collecting the aged precipitate resulting from step (ii), then washing it with water and optionally a solvent 'solv' as defined in claim 4; and iv. drying/filtering the washed precipitate.

14. The process of claim 13, wherein the ammonia-releasing agent is selected from

hexamethylene tetraamine (HMT) and urea.

15. The process of claim 13 or 14, wherein in step iii) the aged precipitate resulting from step (ii) is washed with a mixture of water and one or more of ethanol, acetone and ethyl acetate.

16. The process of any one of claims 13, 14 or 15, wherein the process of preparing the LDH further comprises the steps of:

v. dispersing the LDH resulting from step iv) in a 'solv' solvent as defined in claim 4 to produce a slurry;

vi. maintaining the slurry resulting from step v); and

vii. isolating the layered double hydroxide resulting from step vi).

17. The process according to claim 16, wherein in step vi), the slurry is maintained for a period of 0.5 to 96 hours.

18. The process of claim 16 or 17, wherein the slurry is stirred during step vi).

19. The process of any one of claims 16, 17 and 18, further comprising the step of:

viii. contacting the layered double hydroxide isolated in step vii) with a solvent 'solv' as defined in formula (I), wherein the solvent 'solv' is as defined in claim 4, and optionally contains an organic species as defined in any of claims 4 to 1 1 .

20. The process of any one of claims 13, 14 or 15, wherein the layered double hydroxide resulting from step iv) is solvothermally treated.

21 . The process of claim 20, wherein the solvothermal treatment comprises heating the layered double hydroxide resulting from step iv) in either:

a) at least one solvent 'solv', or

b) a mixture of the filtrate of step iv) and at least one solvent 'solv',

under increased pressure.

22. The process of claim 20 or 21 , wherein the solvothermal treatment comprises heating the layered double hydroxide to a temperature of 50 -250 °C in an autoclave.

23. The process of claim 22, wherein the layered double hydroxide is solvothermally treated for 1 -72 hours.

24. The process of any of claims 4 to 23, wherein the solvent 'solv' is selected from

acetone, ethanol, ethyl acetate, or a mixture of two or more thereof.

25. The process of any preceding claim, wherein the layered double hydroxide has a

platelet morphology, wherein the largest dimension of the platelet is 0.01 -10 μηη.

26. The process of claim 25, wherein the largest dimension of the platelet is 0.01 -1 μηη.

27. The process of any preceding claim, wherein the aspect ratio of the layered double hydroxide is at least 10, wherein aspect ratio is the average diameter of the layered double hydroxide platelet divided by the average thickness of the layered double hydroxide platelet.

28. The process of any preceding claim, wherein the coating mixture comprises 1 -15 wt% of layered double hydroxide.

29. The process of any preceding claim, wherein the coating mixture comprises 2-6 wt% of layered double hydroxide.

30. The process of any preceding claim, wherein the coating mixture comprises 1 -20 wt% of polymer.

31 . The process of any preceding claim, wherein the coating mixture comprises 1 -10 wt% of polymer.

32. The process of any preceding claim, wherein the coating mixture has a viscosity of 1 -1000 cP.

33. The process of any preceding claim, wherein after step c) and prior to step d), the

coated first substrate is contacted with a second substrate, such that the layer of coating mixture is provided between the first and second substrates.

34. The process of any one of claims 1 to 32, further comprising the steps of:

f) contacting the layer of adhesive applied in step e) with a second substrate.

35. The process of claim 33 or 34, wherein the second substrate is selected from

polyethylene terephthalate (PET), polyethylene (PE), biaxiaily oriented polypropylene film (BOPP), polypropylene (PP), polyvinyl dichloride (PVDC), polyamide, nylon, and polylactic acid (PLA).

36. The process of claim 34, wherein the adhesive is selected from one or more of cellulose acetate, polyvinyl alchohol) (PVOH), polyvinyl acetate, polyvinyl dichloride (PVDC), polyurethane, an acrylic-based adhesive, and an epoxy resin.

37. A film obtainable by the process of any preceding claim.

38. A film comprising:

a) a substrate; and

b) a coating layer provided on a least one surface of the substrate,

wherein the coating layer comprises 2-70 wt% of an organic-solvent dispersible layered double hydroxide dispersed throughout a polymeric matrix.

39. The film of claim 38, wherein the organic-solvent dispersible layered double hydroxide is randomly dispersed throughout the polymeric matrix.

40. The film of claim 38 or 39, wherein the coating layer comprises 5-50 wt% of layered double hydroxide.

41 . The film of one of claims 38, 39 or 40, wherein the layered double hydroxide is as defined in any of claims 4 to 27.

42. The film of any one of claims 38 to 41 , wherein the polymeric matrix comprises a

polymer as defined in claim 3.

43. The film of any one of claims 38 to 42, wherein the substrate is as defined in claim 2.

44. The film of any one of claims 38 to 43, wherein the coating layer comprises:

a) 5-50 wt% of layered double hydroxide;

b) 50-95 wt% of polymeric matrix; and

c) 0-2 wt% of organic solvent.

45. The film of any one of claims 38 to 44, wherein the coating layer has a thickness of 0.1 -10 μηι (e.g. 1 -10 μηι).

46. The film of any one of claims 38 to 45, wherein the substrate is a first substrate, and the film comprises a second substrate disposed on top of the coating layer, such that the coating layer is located between the first and second substrates.

47. The film of any one of claims 38 to 46, wherein the film comprises a layer of adhesive provided between the coating layer and the second substrate.

48. The film of any one of claims 38 to 47, wherein the adhesive is selected from one or more of cellulose acetate, polyvinyl alchohol) (PVOH), polyvinyl acetate, polyvinyl dichloride (PVDC), polyurethane, an acrylic-based adhesive, an epoxy resin.

49. Use of a film as claimed in any one of claims 37 to 48 in packaging.

50. A container comprising a film as claimed in any one of claims 37 to 48.