Composite material

Abstract

A composite material comprising at least one layer A comprising a first polysaccharide derivatised with compose a fatty acid ester moiety and at least one layer B comprising an underivatized polysacch

Description

06 11 23 The present invention relates to a biodegradable composite material, of particular use as a packaging material or as a coating for a packaging material. Also provided are processes for preparing the composite material, and substrates coated with the composite material. BACKGROUND OF INVENTION Traditional polymer materials of use in packaging are typically prepared using petroleumbased starting materials, which provide materials such as films with suitable structural and barrier properties. PET (polyethylene terephthalate) is a thermoplastic polymer used in various food and drink containers, including meat trays, and is currently formed from ethylene glycol and dimethyl terephthalate. Ethylene glycol is made from ethene found in natural gas, and dimethyl terephthalate is produced from para-xylene, which is itself derived from crude oil. LDPE (low-density polyethylene) and PP (polypropylene) are thermoplastics also used in packaging applications, and are made from ethylene and propylene, respectively, both of which are formed by steam cracking of hydrocarbons. The use of such materials is not sustainable in the long term because of the finite nature of fossil fuel supplies, and associated environmental concerns due to the materials being non-biodegradable or breaking down to more microplastic particles. Biopolymer alternatives to petroleum-based starting materials are attractive from a sustainability and environmental perspective, but tend to lack the required structural and barrier properties to be of use as packaging materials, particular packaging for food. Biopolymers based on polysaccharides are particularly attractive due the abundance of polysaccharide-based materials in nature, and seaweed in particular has been recognized as a sustainable feedstock. However, due to their hydrophilic nature, use as a direct replacement for petroleum-derived plastics, which demonstrate highly water-resistant properties, is not attainable. A range of polysaccharide-based materials, such as starch mixtures, agar mixtures, methylcellulose, cellophane, sodium alginate and pectin, and their application in food packaging has been reported. However, while the oxygen barrier of the polysaccharide materials was sufficient to increase food shelf life, the materials exhibited high water vapour permeability rates and poor water resistance, as described in Cazon et al. (2017), Kibar (2017) and Di az-Montez (2022). 06 11 23 Ways to improve the water resistance ability of polysaccharides and other biopolymers through addition of hydrophobic ingredients (for example lipids, oils or waxes), incorporation of fillers, cross-linking or multilayer composite films formation have been reported. However, the obtained film properties were still not comparable with synthetic non-degradable or degradable counterparts in terms of water barrier properties, as described in Abdullah et al. (2022) Ghiasi et al. (2020) and Khalil et al. (2019). In summary, although a number of sustainable materials have been developed, none have the required water barrier and oxygen barrier properties, while also being sufficiently biodegrable. Thus, there is a need to develop biopolymer materials to replace the use of petrochemical-based plastic materials, which derive from a sustainable source and are biodegradable, while also having the required structural and barrier properties. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a composite material comprising: at least one layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and at least one layer (B) comprising a second polysaccharide which is un-derivatised. In one embodiment, at least a portion of (and preferably substantially all or all of) a surface of layer (A) is in contact with at least a portion of (and preferably substantially all or all of) a surface of a layer (B). In another embodiment, at least a portion of (and preferably substantially all or all of) a surface of layer (A) is bonded to at least a portion of (and preferably substantially all or all of) a surface of layer (B) using an adhesive. In another aspect of the present invention is provided a substrate with a surface having a coating comprising a composite material as described herein. Preferably, the substrate is a packaging material. In another aspect of the present invention is provided a process for preparing a composite material comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; c) applying layer (B) to layer (A), then applying heat and / or pressure. 06 11 23 In another aspect of the present invention is provided a process for preparing a composite material, comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; c) applying layer (A) to layer (B), then applying heat and / or pressure. In another aspect of the invention is provided a process for preparing a composite material, comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and b) treating at least a portion of a surface of layer (A) to form a layer (B) comprising a second polysaccharide which is un-derivatised. In another aspect of the invention is provided a process for preparing a composite material, comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; and b) treating at least a portion of a surface of layer (B) to form a layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. In another aspect of the present invention is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) preparing a composite material according to a process as described herein; and b) applying the composite material to at least a portion of a surface of the substate using pressure and optionally heat. In another aspect of the present invention is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a’) treating at least a portion of a surface of the substrate to form a first layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b’) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. 06 11 23 In another aspect of the present invention is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a’) treating at least a portion of a surface of the substrate to form a first layer (B) comprising: a second polysaccharide which is un-derivatised; b’) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. In another aspect of the present invention is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) applying layer (A) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. In another aspect of the present invention is provided a process for preparing a substrate having a coating, the coating comprises a composite material, the process comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) applying layer (B) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Embodiments and preferences described below with respect to the composite material apply equally to individual layers (A) and (B), to the substrate having a coating comprising the composite material, to the processes for preparing the composite material, and to the processes for preparing a substrate having a coating, wherein the coating comprises a composite material. BRIEF DESCRIPTION OF FIGURES Figure 1 illustrates various arrangements of composite materials containing layer (A) and layer (B), with and without an adhesive layer. 06 11 23 Figure 2 illustrates various arrangements of composite materials containing different numbers of layers (A) and (B), with and without adhesive layers. Figure 3 illustrates various arrangements for layering a composite material containing layer (A) and layer (B) onto a substrate, with and without adhesive layers. Figures 4 and 5 illustrate various arrangements where one or both sides of a substrate are coated with composite material, with and without adhesive layers. Figure 6 shows the results of a degradation study for layers (A) and (B) (Example 9). Figure 7 shows the results of an anaerobic digestion study (Example 10). Figure 8 shows the FTIR spectrum of agar palmitate obtained as per Example 1, overlayed with spectra for agar and palmitic acid. DETAILED DESCRIPTION The present invention relates to layered materials (composite materials) which have excellent water barrier properties and excellent oxygen barrier properties, while also being biodegradable. The present inventors have developed improved methods for derivatising polysaccharides to contain fatty acid ester moieties by converting at least a proportion of the polar hydroxyl groups of the polysaccharide into long chain fatty acid esters. The resulting derivatised polysaccharides demonstrate excellent water barrier properties, but tend to exhibit inferior oxygen barrier properties. The present inventors have found that by layering these highly water-resistant functionalised polysaccharide materials with other unmodified polysaccharide materials that show good oxygen barrier properties, layered composite materials can be formed which exhibit both water and oxygen barrier properties, therefore have great potential for use in packaging applications. Advantageously, the materials of the present invention are biodegradable. The composite materials of the invention have utility in their own right, particularly when in the form of a film, as a replacement for PET, LDPE and PP in packaging applications, but can also be used as coatings for substrates such as card, carton board and other paper materials, which provide additional mechanical support. The composite material of the present invention comprises at least one layer (A) and at least one layer (B). 06 11 23 Layer (A) comprises a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. In one embodiment, the first polysaccharide is selected from the group consisting of cellulose (including natural cellulose, microcrystalline cellulose and nanocellulose; and including derivatives thereof, such as hydroxypropylcellulose, methylcellulose, carboxymethylhydroxypropylcellulose, hydroxypropylcellulose and hydroxypropylhydroxylethylcellulose), starch (including derivatives thereof such as hydroxypropyl starch), agar, carrageenan, alginic acid, an alginate salt (e.g. sodium alginate, calcium alginate or potassium alginate), fucoidan, laminarin, agarose, agaropectin, ulvan, xanthan gum and pectin. Carrageenan includes kappa, lota and lambda forms, although suitably kappa or iota form is used. In one embodiment, the first polysaccharide is derived from seaweed and is selected from the group consisting of agar, carrageenan, alginic acid, an alginate salt (e.g. sodium alginate, calcium alginate or potassium alginate), fucoidan, laminarin, agarose, agaropectin and ulvan. In a preferred embodiment, the first polysaccharide is selected from the group consisting of cellulose, starch and agar. In one embodiment, the first polysaccharide is cellulose or starch. In another embodiment, the first polysaccharide is agar. The first polysaccharide has been derivatised to comprise fatty acid ester moieties. The fatty acid ester moieties are typically formed by an esterification reaction between least a proportion of the hydroxyl groups on the first polysaccharide with an activated fatty acid compound, such as an acyl chloride or anhydride. Alternatively, the fatty acid ester moieties can be formed by reacting the first polysaccharide with a fatty acid or fatty acid ester in the presence of a coupling reagent / activator Suitable coupling reagents / activators include EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), N,N ' -dicyclohexylcarbodiimide, p-toluenesulfonyl chloride, methanesulfonyl chloride, 1,1'-carbonyldiimidazole, N,N'-diisopropylcarbodiimide, oxalyl chloride, thionyl chloride, acetic anhydride, trifluoroacetic anhydride, trifluoromethanesulfonyl chloride or any combination thereof. The esterification reaction can further comprise a base such as triethylamine, pyridine, 4-dimethylamine pyridine (DMAP) or imidazole. Suitable solvents for the esterification reaction include dimethylacetamide, dimethylformamide, formamide, toluene, dimethylsulfoxide, pyridine, chloroform, dichloromethane, dimethylacetamide / lithium chloride, imidazole, or any combination thereof. Such derivatisation methods are known in the art and described in US2,211,338A, CA2087488A1, US5,589,577A and US2009 / 0299053A1 all of which are incorporated herein by reference in their entirety. Earlier applications filed by the present inventors GB2203196.7 and GB2206804.3 describe methods for preparing a fatty acid derivative of a polysaccharide derived from seaweed, and are incorporated herein by reference in their entirety. An 06 11 23 exemplary method for preparing a fatty acid ester functionalised first polysaccharide using a fatty acid acyl chloride is described in General Method A. Example 1 describes the preparation of agar functionalised with palmitic acid ester moieties, and the FTIR of the resulting derivatised polysaccharide is shown in Figure 8. In one embodiment, the first polysaccharide is functionalised with no more than 7 molar equivalents per repeat unit, preferably no more than 5 molar equivalents, of activated fatty acid compound (or fatty acid or fatty acid ester). In one embodiment, the first polysaccharide is functionalised with between 1 and 7 molar equivalents, preferably between 1 and 5 molar equivalents, preferably between 3 and 5 molar equivalents of activated fatty acid compound (or fatty acid or fatty acid ester). In a preferred embodiment, the fatty acid ester moieties are unbranched. In a preferred embodiment, the fatty acid ester moieties are saturated. In one embodiment, the fatty acid ester moieties are of formula: -C(0)0(CH2)io-isCH3, for example -C(O)O(CH2)wCH3 (“C12” e.g. derived from activated lauric acid), -C(O)O(CH2)iiCH3, -C(O)O(CH2)i2CH3 (“C14” e.g. derived from activated myristic acid), - C(O)O(CH2)i3CH3, -C(O)O(CH2)i4CH3 (“C16” e.g. derived from activated palmitic acid), - C(O)O(CH2)i5CH3, -C(O)O(CH2)i6CH3 (“C18” e.g. derived from activated stearic acid), - C(O)O(CH2)i7CH3 or-C(O)O(CH2)i8CH3(“C20” e.g. derived from activated arachidic acid). In particular, the fatty acid ester moieties are of formula -C(O)O(CH2)i4CH3 (“C16” e.g. derived from activated palmitic acid) or -C(O)O(CH2)i5OH3 or -C(O)O(CH2)i6CH3 (“018” e.g. derived from activated stearic acid). In one embodiment, the first polysaccharide is derivatised using an activated fatty acid of formula CH3(CH2)io-i8C(0)LG wherein LG is a leaving group, for example CH3(CH2)ioC(0)LG (activated lauric acid), CH3(CH2)nC(O)LG, CH3(CH2)i2C(O)LG (activated myristic acid), CH3(CH2)i3C(O)LG, CH3(CH2)i4C(O)LG (activated palmitic acid), CH3(CH2)i5C(O)LG, CH3(CH2)i6C(O)LG (activated stearic acid), CH3(CH2)i7C(O)LG or CH3(CH2)i8C(O)LG (activated arachidic acid). Layer (A) may also contain one or more additives in order to modify the structural or functional properties of the layer. In one embodiment, layer (A) comprises an additive selected from the group consisting of a plasticiser, a filler, a surfactant and an antioxidant. Suitable plasticisers include a vegetable oil (such as sunflower oil, palm oil, rapeseed oil, coconut oil, linseed oil, castor oil, peanut oil, tung oil or soybean oil) or a derivative thereof (such as epoxidized linseed oil, epoxidized tung oil, epoxidized castor oil, acetylated castor 06 11 23 oil, epoxidized soybean oil, an epoxidized soybean oil fatty ester or methyl epoxy soyate), diisononyl-phthalate, mineral oil, limonene, tributyl citrate, diethyl adipate, dibutyl sebacate, acetyl tributyl citrate, triethyl citrate, glycerol, glycerol triacetate, bis(2-ethylhexyl)adipate, glycerol diacetate, poly(ethylene glycol) monolaurate, poly(ethyleneglycol), 1,4-butanediol, dimethyl phthalate, diethyl phthalate, di-(2-ethylhexyl)phthalate, di-isodecyl phthalate, or any combination thereof. Preferred plasticisers are vegetable oils (such as sunflower oil, palm oil, rapeseed oil, coconut oil, linseed oil, castor oil, peanut oil, tung oil or soybean oil) and derivatives thereof (such as epoxidized linseed oil, epoxidized tung oil, epoxidized castor oil, acetylated castor oil, epoxidized soybean oil, an epoxidized soybean oil fatty ester or methyl epoxy soyate). In one embodiment, the plasticiser is soybean oil. Suitably, when present, the plasticiser is present in an amount between about 1 wt.% and about 80 wt.%, such as between about 1 wt.% and about 50 wt.%, between about 1 wt.% and about 25 wt.%, between about 1 wt.% and about 10 wt.%, between about 2 wt.% and about 8 wt.%, such as about 5 wt.% (wherein the wt.% refers to the weight relative to the total weight of the layer A components). In one embodiment, layer (A) further comprises a vegetable oil (such as sunflower oil, palm oil, rapeseed oil, coconut oil, linseed oil, castor oil, peanut oil, tung oil or soybean oil) or a derivative thereof (such as epoxidized linseed oil, epoxidized tung oil, epoxidized castor oil, acetylated castor oil, epoxidized soybean oil, an epoxidized soybean oil fatty ester or methyl epoxy soyate). Suitably, layer (A) does not contain any added water as plasticizer (or in any other capacity), due to the hydrophobic nature of the layer. Thus, in one embodiment, layer (A) does not contain water, or contains essentially no water. Suitable fillers include microcrystalline cellulose, nanocrystalline cellulose and a mineral clay such as Montmorillonite clay. Suitable surfactants include polysorbate 20, polysorbate 40, polysorbate 80, sorbitan monopalmitate, sorbitan monolaurate, sorbitan monooleate, Triton X-100, monolaurin, and combinations thereof. Suitable antioxidants include phenolic compounds such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), alpha-tocopherol, alpha-tocopherol acetate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenyldiphosphonite, tris (2,4-ditert-butylphenyl) phosphite and bis-(2,4-di-tert.-butylphenol)pentaerythritol diphosphite. 06 11 23 In one embodiment, layer (A) comprises a first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide which has been derivatised to comprise palmitic ester and / or stearic ester moieties. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof. Suitably, layer (A) does not contain water. Layer (A) in standalone form (e.g. in film form) can be prepared using a process such as extrusion (including sheet extrusion and film extrusion), extrusion film blowing, hot pressing or solution / dispersion casting. Extrusion is suitably sheet / film extrusion which can be carried out using a sheet extruder or a blown film extruder. General Method B-1 describes an exemplary method for forming layer (A) by extrusion. A suitable process for preparing layer (A) by hot pressing is described in General Method B-2, and in Examples 2a and 2b. A suitable process for preparing layer (A) by solution / dispersion casting is described in General Methods Y-2, and C-1. Preferably layer (A) in standalone form (e.g. in film form) is prepared by hot pressing or by solution / dispersion casting, in particular by hot pressing. Suitable solvents for solution / dispersion casting include esters (ethyl acetate, ethyl propanoate, ethyl butylate, propyl propanoate, butyl butylate, isobutyl butylate, 2-ethoxyethylacetate), ketones (acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 4-heptanone), ethers (tetrahydrofuran, 2-methyl-tetrahydrofuran, ethylene glycol dimethyl ether, methyl tert-butyl ether), aromatics (benzene, toluene, xylenes, propylbenzene, p-cymene, pyridine), chlorinated (dichloromethane, chloroform), water, DMSO, N-methyl morpholine and mixtures thereof. In a preferred embodiment, the solvent is ethyl propanoate, 3-pentanone, toluene, 2-methyl THF, 4-methyl-2-pentanone or a mixture thereof. In one embodiment, layer (A) is formed from a solution / dispersion by evaporation (e.g. as in General Method C-1). Thus, in one embodiment is provided a process for preparing layer (A), wherein layer (A) comprises a first polysaccharide which has been derivatised to comprise fatty acid ester moieties, wherein layer (A) is formed by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting. Preferably layer (A) is formed by hot pressing or by solution / dispersion casting, in particular is formed by hot pressing. Layer (B) comprises a second polysaccharide which is un-derivatised. Un-derivatised in the context of the present invention means that the polysaccharide has not been derivatised to comprise fatty acid ester moieties. In one embodiment, the second polysaccharide has not been chemically modified. In one embodiment, the second polysaccharide is selected from the group consisting of agar, alginic acid, an alginate salt (e.g. sodium alginate, calcium 06 11 23 alginate or potassium alginate), carrageenan, starch (including derivatives thereof such as hydroxypropyl starch) and xanthan gum. Carrageenan includes kappa, lota and lambda forms, although suitably kappa or iota form is used. Suitably the second polysaccharide is selected from the group consisting of alginic acid, an alginate salt (e.g. sodium alginate, calcium alginate or potassium alginate), carrageenan and starch, and in particular is alginic acid or an alginate salt (such as sodium alginate). The base polysaccharide in layer (A) and layer (B) can be the same, but in layer (A) the polysaccharide has been derivatised to comprise fatty acid ester moieties, whereas in layer (B) the polysaccharide is un-derivatised. Layer (B) may also contain one or more additives in order to modify the structural or functional properties of the layer. In one embodiment, layer (B) comprises an additive selected from the group consisting of a plasticiser, a filler, a surfactant and an antioxidant. Suitable plasticisers include water, glycerol, sorbitol, mannitol, citric acid, polyethylene glycol, 1,4-butanediol, 1-butanol, tributyl citrate, diethyl adipate, dibutyl sebacate, acetyl tributyl citrate, triethyl citrate, glycerol triacetate, bis(2-ethylhexyl)adipate, glycerol diacetate, poly(ethylene glycol) monolaurate, xylitol, sucrose, glucose and fructose, or any combination thereof. Preferred plasticisers include water and glycerol. In one embodiment, the plasticiser comprises water and one or more compounds selected from the group consisting of glycerol, sorbitol, mannitol, citric acid, polyethylene glycol, 1,4-butanediol and 1-butanol. Suitably, when present, the plasticiser is present in an amount between about 1 wt.% and about 200 wt.%, such as between about 1 wt.% and about 100 wt.%, or between about 1 wt.% and about 40 wt.% (wherein the wt.% refers to the weight relative to the total weight of the layer B components). In one embodiment, layer (B) further comprises water and / or glycerol. Suitable fillers include microcrystalline cellulose, nanocrystalline cellulose and a mineral clay such as Montmorillonite clay. Suitable surfactants include polysorbate 20, polysorbate 40, polysorbate 80, sorbitan monopalmitate, sorbitan monolaurate, sorbitan monooleate, Triton X-100, monolaurin, and combinations thereof. Suitable antioxidants include phenolic compounds such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), alpha-tocopherol, alpha-tocopherol acetate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis(2,4-di-tert-butylphenyl) 4,4'- biphenyldiphosphonite, tris (2,4-ditert-butylphenyl) phosphite and bis-(2,4-di-tert.-butylphenol)pentaerythritol diphosphite. 06 11 23 In one embodiment, layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (B) further comprises a plasticizer which is water and / or glycerol. Layer (B) in standalone form (e.g. in film form) can be prepared using a process such as extrusion (including sheet extrusion and film extrusion), extrusion film blowing, hot pressing or solution / dispersion casting. Extrusion is suitably sheet / film extrusion which can be carried out using a sheet extruder or a blown film extruder. Preferably layer (B) in standalone form (e.g. in film form) is prepared using solution / dispersion casting. Suitable processes for preparing layer (B) by solution / dispersion casting are described in General Methods Y-1, C-1 and C-2, and in Examples 3a, 3b, 3c and 3d. Suitable solvents for solution / dispersion casting include water, N-methyl morpholine N-oxide, DMSO, esters (ethyl acetate, ethyl propanoate, ethyl butylate, propyl propanoate, butyl butylate, isobutyl butylate, 2-ethoxyethylacetate), ketones (acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 4-heptanone), ethers (tetrahydrofuran, 2-methyl-tetrahydrofuran, ethylene glycol dimethyl ether, methyl tertbutyl ether), chlorinated (dichloromethane, chloroform), and mixtures thereof. In a preferred embodiment, the solvent is water. In one embodiment, layer (B) is formed from a solution / dispersion by evaporation (e.g. as in General Method C-1). Thus, in one embodiment is provided a process for preparing layer (B), wherein layer (B) comprises a second polysaccharide which is un-derivatised, wherein layer (B) is formed by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting. General Methods Y-1, C-1 and C-2 describe a method for forming layer (B) by solution / dispersion casting. Preferably layer (B) is formed by solution / dispersion casting. It should be noted that the designation of “first” and “second” polysaccharide for layers (A) and (B), respectively, is intended to indicate that the polysaccharides are part of two separate layers (i.e. although the base polysaccharide in layers (A) and (B) can be the same, in layer (A) the polysaccharide has been modified to comprise fatty acid ester moieties, while the polysaccharide in layer (B) is un-derivatised). In embodiments where this is clear, the “first” and “second” designations can be removed without changing the scope of the embodiment. Layers (A) and (B) combine to form the composite material of the invention. 06 11 23 In one embodiment, layer (A) comprises a first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide which has been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch; wherein layer (A) optionally further comprises a plasticizer which is a vegetable oil or a vegetable oil derivative, in particular a vegetable oil; optionally layer (A) does not contain water; and wherein layer (B) optionally further comprises a plasticizer which is water and / or glycerol. In one embodiment, the material is formed by layering layer (A) directly on top of layer (B) (or vice versa), such as shown in Figure 1: AB. Although in arrangement AB the layers are in direct contact along an entire surface, embodiments are also envisaged in which only a portion of the layers are in direct contact, as shown in Figure 1: AB - partial overlap. Thus, in one embodiment, at least a portion of a surface of layer (A) is in contact with at least a portion of a surface of a layer (B). Suitably, substantially all or all of a surface of layer (A) is in contact with substantially all or all of a surface of layer (B). In the context of the present invention “substantially all” means the majority of a surface e.g. 95% surface coverage, 96% surface coverage, 97% surface coverage, 98% surface coverage or 99% surface coverage. In one embodiment, the material is formed by layering layer (A) on top of layer (B), wherein a layer of adhesive is interposed between the layers, as shown in Figure 1 (A(adhesive)B). Again, the layers can totally overlap (A(adhesive)B - total overlap) or partially overlap (A(adhesive)B - partial overlap). Thus, in one embodiment, at least a portion of a surface of layer (A) is bonded to at least a portion of a surface of layer (B) using an adhesive. Suitably, substantially all or all of a surface of layer (A) is bonded to substantially all or all of a surface of layer (B) using an adhesive. Detailed processes for forming the material are described below and in the Examples. In further embodiment, the material is formed by a mixture of direct contact between layer (A) and layer (B), and a layer of adhesive interposed between at least a portion of layer (A) and layer (B), see for example Figure 1 (A(adhesive - partial)B - total overlap) and (A(adhesive -partial)B - partial overlap). 06 11 23 Suitable adhesives include a starch, a starch derivative, a starch ester, casein, protein, cellulose, natural rubber, a polyamide, a polylactide, an acrylate, a styrene-acrylate, a cyanoacrylate, an epoxy resin, a phenol, a polyurethane, a polyvinyl acetate, a polyvinyl alcohol, vinyl acetate ethylene, a urea-formaldehyde and a styrene-butadiene dispersion. The composite material of the invention comprises at least one layer (A) and least one layer (B) i.e. the minimum number of layers is a single layer of (A) and a single layer of (B). In one embodiment is provided a composite material comprising or consisting of: a single layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised. Layer (A) and layer (B) can be directly in contact with one another (i.e. layer (A) is layered directly on top of layer (B), or vice versa) such that the two layers are in complete contact along an entire surface, or part of a surface. Thus, in one embodiment, at least a portion of a surface of layer (A) is in contact with at least a portion of a surface of layer (B), and preferably substantially all or all of a surface of layer (A) is shown in contact with at substantially all or all of a surface of layer (B), as shown in Figure 1: AB - total overlap. In another embodiment, layers (A) and (B) are bonded together using an adhesive, placed between the layers. Thus, in one embodiment is provided a composite material comprising or consisting of: a single layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; a single layer (B) comprising: a second polysaccharide which is un-derivatised; and a layer of adhesive positioned between layer (A) and layer (B). In one embodiment, at least a portion of a surface of layer (A) is bonded to at least a portion of a surface of layer (B) by the adhesive. Preferably, substantially all or all of a surface of layer (A) is bonded to substantially all or all of a surface of layer (B) by the adhesive, as shown in Figure 1: A(adhesive)B - total overlap. In all of the embodiments above, layers (A) and (B) optionally comprise one or more additives as described hereinabove. Multilayer composite materials are also provided, such as containing three layers in the arrangement ABA or BAB; containing four layers in the arrangement ABAB (which is considered to be the same as BABA); containing five layers in the arrangement ABABA or BABAB; containing six layers in the arrangement ABABAB (which is considered to be the same as BABABA); containing seven layers in the arrangement ABABABA or BABABAB; or containing eight layers in the arrangement ABABABAB (which is considered to be the same as BABABABA). In all of the above arrangements the layers can be completely overlapping or can partially overlap, and the layers can be directly bonded together or can be bonded together using an adhesive. Mixed arrangements of total overlap / partial overlap / direct 06 11 23 bonding / adhesive within a single material are also contemplated. Figure 2 shows an arrangement ABA (direct bonding with no adhesive, and the layers ABA completely overlap); the arrangement ABA but with a single layer of adhesive between one layer of (A) and layer (B) (labelled as “A(adhesive)BA”); a four layer arrangement ABAB (with no adhesive layers); and a five layer arrangement ABABA, with four layers of adhesive between each AB / BA pair of layers. Although not illustrated, as mentioned above, embodiments where layers are partially overlapping, and embodiments where layers are bonded using a mixture of direct contact and adhesive are also contemplated. Thus, in one embodiment is provided a composite material as described herein, comprising or consisting of three layers in the arrangement ABA, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material as described herein, comprising or consisting of three layers in the arrangement BAB, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material as described herein, comprising or consisting of four layers in the arrangement ABAB, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material as described herein, comprising or consisting of five layers in the arrangement ABABA, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material as described herein, comprising or consisting of five layers in the arrangement BABAB, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material as described herein, comprising or consisting of six layers in the arrangement ABABAB, optionally comprising a layer of adhesive between one or more of the layers. In one embodiment is provided a composite material comprising or consisting of: a single layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein 06 11 23 the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch; and optionally comprising a layer of adhesive between layer (A) and layer (B). Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and layer (B) comprises a plasticiser which is water and / or glycerol. In one embodiment is provided a composite material comprising or consisting of three layers in the arrangement ABA, optionally comprising a layer of adhesive between one or more of the layers, wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. In one embodiment is provided a composite material comprising or consisting of three layers in the arrangement BAB, optionally comprising a layer of adhesive between one or more of the layers; wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. 06 11 23 In one embodiment is provided a composite material comprising or consisting of four layers in the arrangement ABAB, optionally comprising a layer of adhesive between one or more of the layers; wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. In one embodiment is provided a composite material comprising or consisting of five layers in the arrangement ABABA, optionally comprising a layer of adhesive between one or more of the layers; wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. In one embodiment is provided a composite material comprising or consisting of five layers in the arrangement BABAB, optionally comprising a layer of adhesive between one or more of the layers; wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and 06 11 23 layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. In one embodiment is provided a composite material comprising or consisting of six layers in the arrangement ABABAB, optionally comprising a layer of adhesive between one or more of the layers; wherein layer (A) comprises: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and, a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide has suitably been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch. Suitably, layer (A) further comprises a plasticizer which is a vegetable oil or a derivative thereof, and suitably layer (B) comprises a plasticiser which is water and / or glycerol. The composite materials of the invention have excellent water and oxygen barrier properties, while still being biodegradable, as demonstrated in Examples 8-12. The composite material of the invention has utility in its own right, for example when in the form of a film. The composite material of the invention can also be applied to a surface of a substrate, which provides additional mechanical support for the material, allowing it to be used in packaging applications which require a more rigid material. The substrate typically comprises, or is based on a carbohydrate material. Suitable substrates include paper, cardboard, corrugated board, paperboard, carton board, fibre and fabric. The substrate itself may already be used as a packaging material, but when combined with the composite material of the invention is enhanced by having greater water barrier properties and / or greater oxygen barrier properties. In one embodiment, a surface of a substrate has a coating comprising the composite material as described herein. Part of a single surface of the substrate can be coated, or an entire surface can be coated. If the substrate is a material in a form with two major surfaces (e.g. in 2D sheet form, or in 3D form such as in the form of a container with an inner and an outer 06 11 23 surface (e.g. a tray, bowl or cup)), then one or both of the (major) surfaces can be coated, and each surface can be partially coated or fully coated. In one embodiment, the substrate is in sheet form. In another embodiment, the substrate is in the form of a container with an inner and outer surface (e.g. a tray, bowl or cup). Thus, in one aspect of the invention is provided a substrate with a surface having a coating comprising a composite material, wherein the composite material comprises: at least one layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and at least one layer (B) comprising a second polysaccharide which is un-derivatised. Embodiments and preferences described above in relation to the composite material of the invention apply equally to the coated substrate aspect of the invention. As the composite material of the invention contains two different layers (A) and (B), either one of layer (A) or layer (B) can be in closest proximity to the surface of the substrate being coated. For example, where the composite material contains two layers (a single layer (A) and a single layer (B)), there are two alternative arrangements: the first in which layer (A) is closest to the substrate (designated “S”): SAB (as shown in Figure 3); and the second in which layer (B) is closest to the substrate: SBA (as shown in Figure 3). The composite material can be directly applied to a surface of the substrate or can be attached to a surface of the substrate using a layer of adhesive. Suitable adhesives include a starch, a starch derivative, a starch ester, casein, protein, cellulose, natural rubber, a polyamide, a polylactide, an acrylate, a styrene-acrylate, a cyanoacrylate, an epoxy resin, a phenol, a polyurethane, a polyvinyl acetate, a polyvinyl alcohol, vinyl acetate ethylene, a ureaformaldehyde and a styrene-butadiene dispersion. In embodiments where the composite material also contains an adhesive layer (an example is shown in Figure 3: S(adhesive)B(adhesive)A), the adhesives used can be the same or different. In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of two layers in the arrangement SAB, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: S(adhesive)AB, S(adhesive)A(adhesive)B and SA(adhesive)B. In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of two layers in the arrangement SBA, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: S(adhesive)BA, S(adhesive)B(adhesive)A and SB(adhesive)A. In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of three layers in the arrangement SBAB, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: S(adhesive)BAB, S(adhesive)B(adhesive)AB, S(adhesive)B(adhesive)A(adhesive)B, S(adhesive)BA(adhesive)B, SB(adhesive)A(adhesive)B, SBA(adhesive)B and SB(adhesive)AB. 06 11 23 In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of three layers in the arrangement SABA, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: S(adhesive)ABA, S(adhesive)A(adhesive)BA, S(adhesive)A(adhesive)B(adhesive)A, S(adhesive)AB(adhesive)A, SA(adhesive)B(adhesive)A, SAB(adhesive)A and SA(adhesive)BA. In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of four layers in the arrangement SBABA, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: S(adhesive)BABA, S(adhesive)B(adhesive)ABA, S(adhesive)B(adhesive)A(adhesive)BA, S(adhesive)B(adhesive)A(adhesive)B(adhesive)A, S(adhesive)BA(adhesive)BA, S(adhesive)BA(adhesive)B(adhesive)A, S(adhesive)B(adhesive)ABA, S(adhesive)B(adhesive)AB(adhesive)A, S(adhesive)B(adhesive)A(adhesive)BA, S(adhesive)BABA, S(adhesive)B(adhesive)ABA, S(adhesive)B(adhesive)A(adhesive)BA, SB(adhesive)A(adhesive)B(adhesive)A, SBA(adhesive)BA, SBA(adhesive)B(adhesive)A, SB(adhesive)ABA, SB(adhesive)AB(adhesive)A and SB(adhesive)A(adhesive)BA. In one embodiment is provided a substrate (S) with a surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of four layers in the arrangement SABAB, optionally comprising a layer of adhesive between 06 11 23 one or more of the layers. Specific examples include: S(adhesive)ABAB, S(adhesive)A(adhesive)BAB, S(adhesive)A(adhesive)B(adhesive)AB, S(adhesive)A(adhesive)B(adhesive)A(adhesive)B, S(adhesive)AB(adhesive)AB, S(adhesive)AB(adhesive)A(adhesive)B, S(adhesive)A(adhesive)BAB, S(adhesive)A(adhesive)BA(adhesive)B, S(adhesive)A(adhesive)B(adhesive)AB, S(adhesive)ABAB, S(adhesive)A(adhesive)BAB, S(adhesive)A(adhesive)B(adhesive)AB, SA(adhesive)B(adhesive)A(adhesive)B, SAB(adhesive)AB, SAB(adhesive)A(adhesive)B, SA(adhesive)BAB, SA(adhesive)BA(adhesive)B and SA(adhesive)B(adhesive)AB. When the substrate is a material in a form with two major surfaces (e.g. in sheet form, or in container form with an inside and an outside surface) the coating comprising the composite material of the invention can be applied to both surfaces (including a portion of both surfaces, a portion of one surface and substantially all or all of the other surface, or preferably substantially all or all of both surfaces). The composite material applied to one surface can be different to the composite material applied to the other surface, but suitably the composite materials are the same. Suitably, the composite materials are applied in the same arrangement on each surface i.e. the same type of layer (either layer A or layer B) is in closest proximity to the each surface being coated. Thus, in one embodiment is provided a substrate (S) with a first surface having a coating comprising a composite material as described herein and a second surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of layers in the arrangement BASAB, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: BAS(adhesive)AB, BAS(adhesive)A(adhesive)B, BASA(adhesive)B, B(adhesive)AS(adhesive)AB, B(adhesive)AS(adhesive)A(adhesive)B, B(adhesive)ASA(adhesive)B, BA(adhesive)S(adhesive)AB, BA(adhesive)S(adhesive)A(adhesive)B, BA(adhesive)SA(adhesive)B, B(adhesive)A(adhesive)S(adhesive)AB, B(adhesive)A(adhesive)S(adhesive)A(adhesive)B and B(adhesive)A(adhesive)SA(adhesive)B. In another embodiment is provided a substrate (S) with a first surface having a coating comprising a composite material as described herein and a second surface having a coating comprising a composite material as described herein, wherein the composite material comprises or consists of layers in the arrangement ABSBA, optionally comprising a layer of adhesive between one or more of the layers. Specific examples include: ABS(adhesive)BA, ABS(adhesive)B(adhesive)A, ABSB(adhesive)A, A(adhesive)BS(adhesive)BA, A(adhesive)BS(adhesive)B(adhesive)A, AB(adhesive)S(adhesive)BA, AB(adhesive)SB(adhesive)A, A(adhesive)B(adhesive)S(adhesive)B(adhesive)A A(adhesive)B(adhesive)SB(adhesive)A. A(adhesive)BSB(adhesive)A, AB(adhesive)S(adhesive)B(adhesive)A, A(adhesive)B(adhesive)S(adhesive)BA, and 06 11 23 Figure 4 shows the arrangement ABSBA, where the same composite material is used to coat both surfaces, and the composite material is applied to each surface in the same orientation (i.e. the layer (B) surface is applied to each surface, and in the embodiment illustrated the material is applied directly to the surface with no adhesive. An arrangement BA(adhesive)S(adhesive)AB is also shown, where the composite material itself (AB / BA) does not contain any adhesive, but the material is bonded to both surfaces of the substate by an adhesive layer. The third arrangement illustrated in Figure 4 shows a substrate where one surface is coated with a first composite material, and the other surface is coated with a second (in this case different) composite material. The first surface is coated with composite material BA, where the composite material does not contain any adhesive, but the material is bonded via layer (A) to the first surface using a layer of adhesive. The second surface is coated with composite material BAB, where the material contains a layer of adhesive between each of the layers. The composite material is bonded directly to the second surface via layer (B), without using a layer of adhesive. Also disclosed are substrates as described herein with a single layer (A) or (B) (i.e. not a composite layer). Thus, in a further aspect of the disclosure is provided a substrate with a surface having a coating comprising a material, wherein the material comprises: a single layer (A) comprising a polysaccharide which has been derivatised to comprise fatty acid ester moieties. Embodiments and preferences described above in relation to the layer (A) and the substrate apply equally to this aspect of the invention. In a further aspect of the disclosure is provided a substrate with a surface having a coating comprising a material, wherein the material comprises: a single layer (B) comprising an un-derivatised polysaccharide. Embodiments and preferences described above in relation to the layer (B) and the substrate apply equally to this aspect of the invention. 06 11 23 Also contemplated is a substrate (S) with a first surface having a coating comprising a single layer (A) comprising a polysaccharide which has been derivatised to comprise fatty acid ester moieties; and a second surface having a coating comprising a single layer (B) comprising an un-derivatised polysaccharide. This arrangement BSA (or ASB), optionally comprises a layer of adhesive between one or more of the layers and the surface of the substrate. Specific examples include: BSA, BS(adhesive)A, B(adhesive)SA, and B(adhesive)S(adhesive)A. Embodiments and preferences described above in relation to the layers (A) and (B) and the substrate apply equally to this aspect of the invention. The material can be directly applied to a surface of the substrate or can be attached to a surface of the substrate using a layer of adhesive. Suitable adhesives include a starch, a starch derivative, a starch ester, casein, protein, cellulose, natural rubber, a polyamide, a polylactide, an acrylate, a styrene-acrylate, a cyanoacrylate, an epoxy resin, a phenol, a polyurethane, a polyvinyl acetate, a polyvinyl alcohol, vinyl acetate ethylene, a ureaformaldehyde and a styrene-butadiene dispersion. Where the substrate is in sheet form, the adhesive can be applied to both surfaces (or a portion of both surfaces). Figure 5 shows the arrangement SB, in which layer (B) is applied to one surface, and is directly bonded to the surface without using adhesive. Also shown is the arrangement ASA and A(adhesive)S(adhesive)A, in which both sides of the substrate are coated with layer (A), without the use of adhesive and with the use of adhesive, respectively. In one embodiment is provided a substrate with a surface having a coating comprising a composite material, wherein the composite material comprises: at least one layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and at least one layer (B) comprising a second polysaccharide which is un-derivatised; wherein layer (A) comprises a first polysaccharide selected from the group consisting of cellulose, starch and agar, wherein the first polysaccharide which has been derivatised to comprise palmitic ester and / or stearic ester moieties; and layer (B) comprises a second polysaccharide selected from the group consisting of alginic acid, an alginate salt, carrageenan and starch; wherein, layer (A) optionally further comprises a plasticizer which is a vegetable oil or a derivative thereof; optionally layer (A) does not contain water; and wherein layer (B) optionally further comprises a plasticizer which is water and / or glycerol. 06 11 23 Suitably, the substrate is selected from the group consisting of paper, cardboard, corrugated board, paperboard, carton board, fibre and fabric. Both the substrate and the composite material of the invention are preferably biodegradable. Thus, in one embodiment, the substrate is biodegradable. In one embodiment, the composite material of the invention is biodegradable. In one embodiment, the substrate having a coating comprising the composite material of the invention, is biodegradable. Both the substrate and the composite material of the invention are preferably compostable. Thus, in one embodiment, the substrate is compostable. In one embodiment, the composite material of the invention is compostable. In one embodiment, the substrate having a coating comprising the composite material of the invention, is compostable. Biodegradability can be assessed using the “Degradability in soil environment” study in Evaluation Methods and Example 9, and using the “Anaerobic digestion” study in Evaluation Methods and Example 10. Compostability can be assessed by an analogous method using compost instead of soil. Various processes for preparing the composite materials and coated substrates of the invention will now be described. All embodiments and preferences described above with respect to the composite material (layers (A) and (B), adhesives, arrangements etc.) and the substrate (nature of the substrate and coating arrangements etc) apply equally to the following processes. Suitable processes for preparing individual (standalone) layers (A) and (B) are described hereinabove. The standalone layers (A) and (B) can be directly bonded to each other to form a composite material e.g. by using heat and / or pressure, or can be bonded using a layer of adhesive. In one embodiment, the layers are directly bonded together by coextrusion or by hot pressing. An exemplary method for forming composite material ABA by hot pressing is described in General Method D-1, and in Example 4. Suitable adhesives are described hereinabove. Thus, in one embodiment is provided a process for preparing a composite material (e.g. AB) comprising the steps of: 06 11 23 a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; c) applying layer (B) (formed in step b)) to layer (A) (formed in step a)), then applying heat and / or pressure. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting (as described above), in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Step b) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by solution / dispersion casting. Step c) is suitably carried out by hot pressing. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. In another embodiment is provided a process for preparing a composite material (e.g. BA) comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; c) applying layer (A) (formed in step b)) to layer (B) (formed in step a)), then applying heat and / or pressure. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting (as described above), in particular by solution / dispersion casting. Step b) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Step c) is suitably carried out by hot pressing. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. In both of the embodiments above, further layers (A) and (B) can be applied before step c), to form composite materials with multiple layers e.g. ABA, BAB, ABAB etc. Considering the use of an adhesive to bond together layers (A) and (B), in one embodiment is provided a process for preparing a composite material (e.g. AB) comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) applying a layer of adhesive (to the layer formed in step a)); c) forming a layer (B) comprising: a second polysaccharide which is un-derivatised, d) applying layer (B) (formed in step c)) to the layer of adhesive (formed in step b)). 06 11 23 Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Step c) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solvent / dispersion casting, in particular by solution / dispersion casting. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. In another embodiment is provided a process for preparing a composite material (e.g. BA) comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) applying a layer of adhesive (to the layer formed in step a)); c) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties, d) applying layer (A) (formed in step c)) to the layer of adhesive (formed in step b)). Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by solution / dispersion casting. Step c) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solvent / dispersion casting, in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. Alternatively, a standalone layer of (A) or (B) can be formed as described above, and then a subsequent layer (B or A) can be formed in situ on the preformed layer, for example by solution / dispersion casting, as described in General Method W and in Example 4a. Suitable solvents for solution / dispersion casting include esters (ethyl acetate, ethyl propanoate, ethyl butylate, propyl propanoate, butyl butylate, isobutyl butylate, 2-ethoxyethylacetate), ketones (acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 4-heptanone), ethers (tetrahydrofuran, 2-methyl-tetrahydrofuran, ethylene glycol dimethyl ether, methyl tertbutyl ether), aromatics (benzene, toluene, xylenes, propylbenzene, p-cymene, pyridine), chlorinated (dichloromethane, chloroform), water, and mixtures thereof. The solution / dispersion can be applied to the preformed layer by any suitable means including by doctor blade coating, bar coating, spiral wire bar coating, brush coating, spray coating, dip coating, curtain coating, spin coating or by roll coating, and subsequent evaporation of the solvent. Thus, in one aspect, the present invention provides a process for preparing a composite material (e.g. AB), comprising the steps of: 06 11 23 a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and b) treating at least a portion of a surface of layer (A) (formed in step a)) to form a layer (B) comprising a second polysaccharide which is un-derivatised. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Step b) is suitably carried out by solution / dispersion casting. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. In another aspect, the present invention proves a process for preparing a composite material (e.g. BA), comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; and b) treating at least a portion of a surface of layer (B) (formed in step a)) to form a layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by solution / dispersion casting. Step b) is suitably carried out by solution / dispersion casting. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. Also provided are processes for preparing composite materials comprising forming third and subsequent layers, such as ABA, BAB, ABAB etc., by repeating the steps for forming AB / BA composite materials already described. As described above, the composite material of the invention can be applied to the surface of a substrate to form a coating. The composite material can be pre-formed using the processes described directly above and then applied to the substrate. Alternatively, the composite material can formed directly on the surface of the substrate (i.e. the coating is formed and applied to the substrate in situ), e.g. by forming the individual layer(s) directly and sequentially onto the surface of the substrate. Considering the method of applying a pre-formed composite material, in one embodiment is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) preparing a composite material as described herein; and 06 11 23 b) applying the composite material to at least a portion of a surface of the substate using pressure and optionally heat. In step b), the composite material can be applied and bonded to the substrate using a process such as hot pressing, coextrusion, flow lamination, hot or cold rolling, or calender rolling Suitable application pressures for rolling and extrusion processes are from 0 bar(g) to 200 bar(g). Suitable processing temperatures are from 30 °C to 220 °C, and suitable equipment includes but is not limited to single-screw extruders, twin-screw extruders, extrusion dies, flow through forming plates and static forms, hydraulic presses (heated or not), screw presses (heated or not), flow lamination lines, single, twin, and multiple roller rolling lines, calender rollers. Preferably step b) is carried out by hot pressing, such as in Example 5. Alternatively, the pre-formed composite material can be bonded to the surface of the substrate using a layer of adhesive. Suitably adhesives are described hereinabove. Thus, in one embodiment is provided a process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) preparing a composite material as described herein; b) treating at least a portion of a surface of the substrate to form a layer of adhesive; c) applying the composite material (formed in step a)) to the layer of adhesive (formed in step b). Considering the method of forming the composite material in situ when coating the substrate, in one embodiment is provided a process for preparing a substrate having a coating (e.g. SAB), wherein the coating comprises a composite material, the process comprising the steps of: a’) treating at least a portion of a surface of the substrate to form a first layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b’) treating at least a portion of the surface of layer (A) (formed in step a’) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. Step a) is suitably carried out by solution / dispersion casting. Step b) is suitably carried out by solution / dispersion casting. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. Also provided is the reverse embodiment, an in situ process for preparing a substrate having a coating (e.g. SBA), wherein the coating comprises a composite material, the process comprising the steps of: 06 11 23 a’) treating at least a portion of a surface of the substrate to form a first layer (B) comprising: a second polysaccharide which is un-derivatised; b’) treating at least a portion of the surface of layer (B) (formed in step a’) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Step a) is suitably carried out by solution / dispersion casting. Step b) is suitably carried out by solution / dispersion casting. Layers (A) and (B) optionally comprise one or more additives as described hereinabove. An example of this embodiment is described in Example 6. Mixed methods involving at least one layer which is preformed and at least one layer which is formed in situ are also contemplated. Thus, in one embodiment is provided a process for preparing a substrate having a coating (e.g. SAB), wherein the coating comprises a composite material, the process comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) applying layer (A) (formed in step a)) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (A) (formed in step b)) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by hot pressing or by solution / dispersion casting, and preferably by hot pressing. Step b) is suitably carried out by hot pressing. Step c) is suitably carried out by solution / dispersion casting. In one embodiment is provided a process for preparing a substrate having a coating (e.g. SBA), wherein the coating comprises a composite material, the process comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) applying layer (B) (formed in step a)) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (B) (formed in step b)) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Step a) is suitably carried out by extrusion, by extrusion film blowing, by hot pressing or by solution / dispersion casting; in particular by solution / dispersion casting. Step b) is suitably carried out by hot pressing. Step c) is suitably carried out by solution / dispersion casting. An example of this embodiment is described in Example 7. 06 11 23 Where a layer is formed by solution / dispersion casting, suitable processes are described in General Method W. Suitable solvents for solution / dispersion casting include esters (ethyl acetate, ethyl propanoate, ethyl butylate, propyl propanoate, butyl butylate, isobutyl butylate, 2-ethoxyethylacetate), ketones (acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 4-heptanone), ethers (tetrahydrofuran, 2-methyl-tetrahydrofuran, ethylene glycol dimethyl ether, methyl tert-butyl ether), aromatics (benzene, toluene, xylenes, propylbenzene, p-cymene, pyridine), chlorinated (dichloromethane, chloroform), water, DMSO, N-methyl morpholine and mixtures thereof. The solution / dispersion can be applied to the substrate / already formed layer or composite material by any suitable means including by doctor blade coating, bar coating, spiral wire bar coating, brush coating, spray coating, dip coating, curtain coating, spin coating or by roll coating, and subsequent evaporation of the solvent. Further embodiments of the invention: Clause 1. A composite material comprising: at least one layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and at least one layer (B) comprising a second polysaccharide which is un-derivatised. Clause 2. The composite material according to clause 1, wherein the first polysaccharide is selected from the group consisting of cellulose, starch, agar, carrageenan, alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), fucoidan, laminarin, agarose, agaropectin, ulvan, xanthan gum and pectin. Clause 3. The composite material according to clause 2, wherein the first polysaccharide is derived from seaweed and is selected from the group consisting of agar, carrageenan, alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), fucoidan, laminarin, agarose, agaropectin and ulvan. Clause 4. The composite material according to clause 2, wherein the first polysaccharide is selected from the group consisting of cellulose, starch and agar, and in particular is agar. Clause 5. The composite material according to any one of clauses 1 to 4, wherein the fatty acid ester moieties are of formula: -C(0)0(CH2)io-isCH3. 06 11 23 Clause 6. The composite material according to clause 5, wherein the fatty acid ester moieties are -C(O)O(CH2)i4CH3 (“C16” e.g. derived from activated palmitic acid) or -C(O)O(CH2)isCH3 or-C(O)O(CH2)i6CH3(“C18” e.g. derived from activated stearic acid). Clause 7. The composite material according to any one of clauses 1 to 6, wherein layer (A) comprises an additive selected from the group consisting of a plasticiser, a filler, a surfactant and an antioxidant. Clause 8. The composite material according to clause 7, wherein the plasticiser is selected from the group consisting a vegetable oil (such as sunflower oil, palm oil, rapeseed oil, coconut oil, linseed oil, castor oil, peanut oil, tung oil or soybean oil) or a derivative thereof (such as epoxidized linseed oil, epoxidized tung oil, epoxidized castor oil, acetylated castor oil, epoxidized soybean oil, an epoxidized soybean oil fatty ester or methyl epoxy soyate), diisononyl-phthalate, mineral oil, limonene, tributyl citrate, diethyl adipate, dibutyl sebacate, acetyl tributyl citrate, triethyl citrate, glycerol, glycerol triacetate, bis(2-ethylhexyl)adipate, glycerol diacetate, poly(ethylene glycol) monolaurate, poly(ethyleneglycol), 1,4-butanediol, dimethyl phthalate, diethyl phthalate, di-(2-ethylhexyl)phthalate, di-isodecyl phthalate, and any combination thereof; and is suitably a vegetable oil (such as sunflower oil, palm oil, rapeseed oil, coconut oil, linseed oil, castor oil, peanut oil, tung oil or soybean oil) or a derivative thereof (such as epoxidized linseed oil, epoxidized tung oil, epoxidized castor oil, acetylated castor oil, epoxidized soybean oil, an epoxidized soybean oil fatty ester or methyl epoxy soyate). Clause 9. The composite material according to clause 7, wherein the filler is selected from the group consisting of microcrystalline cellulose, nanocrystalline cellulose and a mineral clay such as Montmorillonite clay. Clause 10. The composite material according to clause 7, wherein the surfactant is selected from the group consisting of polysorbate 20, polysorbate 40, polysorbate 80, sorbitan monopalmitate, sorbitan monolaurate, sorbitan monooleate, Triton X-100, monolaurin, and combinations thereof. Clause 11. The composite material according to clause 7, wherein the antioxidant is selected from the group consisting of phenolic compounds such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), alpha-tocopherol, alpha-tocopherol acetate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1 H,3H,5H)-trione, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis(2,4-di-tert-butylphenyl) 4,4'- 06 11 23 biphenyldiphosphonite, tris (2,4-ditert-butylphenyl) phosphite and bis-(2,4-di-tert.-butylphenol)pentaerythritol diphosphite. Clause 12. The composite material according to any one of clauses 1 to 11, wherein the second polysaccharide is selected from the group consisting of agar, alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), carrageenan, starch and xanthan gum. Clause 13. The composite material according to clause 12, wherein the second polysaccharide is selected from the group consisting of alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), carrageenan and starch, and in particular is alginic acid or an alginate salt (such as sodium alginate). Clause 14. The composite material according to any one of clauses 1 to 13, wherein at least a portion of a surface of layer (A) is in contact with at least a portion of a surface of a layer (B). Clause 15. The composite material according to clause 14, wherein substantially all or all of a surface of layer (A) is in contact with substantially all or all of a surface of layer (B). Clause 16. The composite material according to any one of clauses 1 to 15, wherein at least a portion of a surface of layer (A) is bonded to at least a portion of a surface of layer (B) using an adhesive. Clause 17. The composite material according to clause 16, wherein substantially all or all of a surface of layer (A) is bonded to substantially all or all of a surface of layer (B) using an adhesive. Clause 18. The composite material according to clause 16 or clause 17, wherein the adhesive is selected from the group consisting of a starch, a starch derivative, a starch ester, casein, protein, cellulose, natural rubber, a polyamide, a polylactide, an acrylate, a styreneacrylate, a cyanoacrylate, an epoxy resin, a phenol, a polyurethane, a polyvinyl acetate, a polyvinyl alcohol, vinyl acetate ethylene, a urea-formaldehyde and a styrene-butadiene dispersion. 06 11 23 Clause 19. The composite material according to any one of clauses 1 to 18, wherein layer (B) comprises an additive selected from the group consisting of a plasticiser, a filler, a surfactant and an antioxidant. Clause 20. The composite material according to clause 19, wherein the plasticiser is selected from the group consisting of water, glycerol, sorbitol, mannitol, citric acid, polyethylene glycol, 1,4-butanediol, 1-butanol, tributyl citrate, diethyl adipate, dibutyl sebacate, acetyl tributyl citrate, triethyl citrate, glycerol triacetate, bis(2-ethylhexyl)adipate, glycerol diacetate, poly(ethylene glycol) monolaurate, xylitol, sucrose, glucose and fructose or any combination thereof; and is suitably selected from the group consisting water and glycerol. Clause 21. The composite material according to clause 19, wherein the filler is selected from the group consisting of microcrystalline cellulose, nanocrystalline cellulose and a mineral clay such as Montmorillonite clay. Clause 22. The composite material according to clause 19, wherein the surfactant is selected from the group consisting of polysorbate 20, polysorbate 40, polysorbate 80, sorbitan monopalmitate, sorbitan monolaurate, sorbitan monooleate, Triton X-100, monolaurin, and combinations thereof. Clause 23. The composite material according to clause 19, wherein the antioxidant is selected from the group consisting of phenolic compounds such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), alpha-tocopherol, alpha-tocopherol acetate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1 H,3H,5H)-trione, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenyldiphosphonite, tris (2,4-ditert-butylphenyl) phosphite and bis-(2,4-di-tert.-butylphenol)pentaerythritol diphosphite. Clause 24. The composite material according to any one of clauses 1 to 23, comprising or consisting of three layers in the arrangement ABA, optionally comprising a layer of adhesive between one or more of the layers. Clause 25. The composite material according to any one of clauses 1 to 23, comprising or consisting of three layers in the arrangement BAB, optionally comprising a layer of adhesive between one or more of the layers. 06 11 23 Clause 26. The composite material according to any one of clauses 1 to 23, comprising or consisting of four layers in the arrangement ABAB, optionally comprising a layer of adhesive between one or more of the layers. Clause 27. The composite material according to any one of clauses 1 to 23, comprising or consisting of five layers in the arrangement ABABA, optionally comprising a layer of adhesive between one or more of the layers. Clause 28. The composite material according to any one of clauses 1 to 23, comprising or consisting of five layers in the arrangement BABAB, optionally comprising a layer of adhesive between one or more of the layers. Clause 29. The composite material according to any one of clauses 1 to 23, comprising or consisting of six layers in the arrangement ABABAB, optionally comprising a layer of adhesive between one or more of the layers. Clause 30. The composite material according to any one of clauses 1 to 23, comprising or consisting of: a single layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and a single layer (B) comprising: a second polysaccharide which is un-derivatised. Clause 31. The composite material according to clause 30, wherein at least a portion of (and preferably substantially all or all of) a surface of the layer (A) is in contact with at least a portion of (and preferably substantially all or all of) a surface of layer (B), Clause 32. The composite material according to any one of clauses 1 to 23, comprising or consisting of: a first layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; a second layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and a single layer (B) comprising: a second polysaccharide which is un-derivatised; wherein the layers are arranged such that single layer (B) is positioned between the first layer (A) and the second layer (A). 06 11 23 Clause 33. The composite material according to clause 32, wherein at least a portion of (and preferably substantially all or all of) a surface of the first layer (A) is in contact with at least a portion of (and preferably substantially all or all of) a first surface of layer (B), and at least a portion of (and preferably substantially all or all of) a surface of the second layer (A) is in contact with at least a portion of (and substantially all or all of) a second surface of layer (B). Clause 34. The composite material according to any one of clauses 1 to 33, which is biodegradable. Clause 35. A substrate with a surface having a coating comprising the composite material according to any one of clauses 1 to 34. Clause 36. The substrate with a surface having a coating according to clause 35, wherein the substrate is a packaging material. Clause 37. The substrate with a surface having a coating according to clause 35 or clause 36, wherein the substrate is carbohydrate-based. Clause 38. The substrate with a surface having a coating according to any one of clauses 35 to 37, wherein the substrate is in sheet form. Clause 39. The substrate with a surface having a coating according to any one of clauses 35 to 38, wherein the substrate is in 3D form, such as in the form of a container (e.g. a tray, bowl or cup). Clause 40. The substrate with a surface having a coating according to any one of clauses 35 to 39, wherein the substrate is selected from the group consisting of paper, cardboard, corrugated board, paperboard, carton board, fibre and fabric. Clause 41. The substrate with a surface having a coating according to any one of clauses 35 to 40, which is biodegradable. Clause 42. A process for preparing a composite material comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; c) applying layer (B) to layer (A), then applying heat and / or pressure. 06 11 23 Clause 43. A process for preparing a composite material, comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; c) applying layer (A) to layer (B), then applying heat and / or pressure. Clause 44. A process for preparing a composite material, comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; and b) treating at least a portion of a surface of layer (A) to form a layer (B) comprising a second polysaccharide which is un-derivatised. Clause 45. A process for preparing a composite material, comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; and b) treating at least a portion of a surface of layer (B) to form a layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Clause 46. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) preparing a composite material according to any one of clauses 1 to 34; and b) applying the composite material to at least a portion of a surface of the substate using pressure and optionally heat. Clause 47. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a’) treating at least a portion of a surface of the substrate to form a first layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b’) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. Clause 48. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: 06 11 23 a’) treating at least a portion of a surface of the substrate to form a first layer (B) comprising: a second polysaccharide which is un-derivatised; b’) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Clause 49. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; b) applying layer (A) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised. Clause 50. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of: a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; b) applying layer (B) to at least a portion of a surface of the substate using pressure and optionally heat; c) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties. Clause 51. The substrate with a surface having a coating according to any one of clauses 35 to 41, wherein the substrate is a packaging material. Clause 52. The substrate with a surface having a coating according to any one of clauses 35 to 41, or clause 51, wherein the substrate is carbohydrate-based. Clause 53. The substrate with a surface having a coating according to any one of clauses 35 to 41, or clauses 51 or clause 52, wherein the substrate is in sheet form. Clause 54. The substrate with a surface having a coating according to any one of clauses 35 to 41, or any one of clauses 51 to 53, wherein the substrate is in the form of a container. Clause 55. The substrate with a surface having a coating according to any one of clauses 35 to 41, or any one of clauses 51 to 54, wherein the substrate is selected from the group consisting of paper, cardboard, corrugated board, paperboard, carton board, fibre and fabric. 06 11 23 Clause 56. The substrate with a surface having a coating according to any one of clauses 35 to 41, or any one of clauses 51 to 55, which is biodegradable. ADVANTAGES Composite materials of the invention and substrates coated with composite materials of the invention are, at least in some embodiments, expected to have one or more of the following merits or advantages: • good water and vapour barrier properties e.g. as measured using liquid water permeability and water vapour transmission rate, as set out in the Evaluation Methods below (Liquid water permeability (LWP) method, Water vapour transmission rate (WVTR) and Cobb test; • good oxygen barrier properties e.g. as measured using oxygen transmission rate, as set out in the Evaluation Methods below (Oxygen transmission rate (OTR) method); • providing a thin coating e.g. as measured using the layer and material thickness method in the Evaluation Methods below; • enhanced biodegradability e.g. as measured using Degradability in soil environment method and Anaerobic digestion methods set out in Evaluation Methods below; • sustainable starting materials; • oil and grease resistance; and • acid resistance. Combinations of the above properties are desirable, and a composite material or coating may be comparatively poor in one aspect, but have excellent properties in another aspect. For example, a material or coating may have only adequate water / oxygen barrier properties, but is able to be formed into a very thin material or coating, making it particularly advantageous for certain applications. It should be noted that in the context of the present application, when referring to a range of between about “AA” and about “BB”, the point values of AA and BB are intended to be included as possible values in the range. The word “comprise”, and variations such as “comprises” and “comprising” as used herein should be understood to mean the inclusion of the stated integer, step, group of integers or group of steps, but not to the exclusion of any other integer, step, groups of integers or group of steps. The word “consisting of” as used herein limits the scope of the integer, step, group of integers or group of steps to the specified integer, step, groups of integers or group of steps. The word “consisting essentially of’ as used herein limits the scope of the integer, step, group of integers or group of steps, and further integers, steps, groups of integers or groups of steps that do not materially affect the basic and novel characteristics of the invention. The invention embraces all combinations of indicated integers, steps, groups of integers or groups of steps recited above. All patents and patent applications referred to herein are incorporated by reference in their entirety. 06 11 23 ABBREVIATIONS 06 11 23 BHA butylated hydroxyanisole BHT butylated hydroxytoluene Da dalton DMSO dimethyl sulfoxide EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide FTIR Fourier-transform infrared LDPE low-density polyethylene LWP liquid water permeability OTR oxygen transmission rate PET polyethylene terephthalate PLA polylactic acid PP polypropylene PTFE polytetrafluoroethylene TPS thermoplastic starch WVTR water vapour transmission rate EXAMPLES Chemicals Agar (technical grade and biological grade), sodium alginate, pyridine and glycerol were purchased from Fisher Scientific. Acyl chlorides used were purchased form Tokyo Chemicals Industry UK Ltd (TCI) or Fisher Scientific. Soybean oil was purchased from Sigma Aldrich. Evaluation methods Laver and composite material thickness The individual films (i.e. layers (A) and (B)) are generated and then the thickness of each film is measured using a micrometer screw gauge. The films are then layered and adhered together to form the composite material, and afterwards the total thickness is measured again using a micrometer screw gauge. For coatings, the substrate thickness is measured beforehand using a micrometer screw gauge. The coating is then applied to the substrate and the total thickness of the coated substrate is measured using a micrometer screw gauge. The coating thickness (i.e the thickness of the composite material or individual film) is calculated as the difference between the total thickness and the substrate thickness. 06 11 23 Liquid water permeability (LWP) The liquid water permeability is tested by placing a drop of water on the material being tested, and visually checking if any water is permeating through over time (at least 72 hours). Water vapour transmission rate (WVTR) Water vapour transmission rate describes the rate at which water vapour can pass through a material such as a film or a coating and is determined using an in-house method according to standard ASTM E96. Whereby, a vial is half-filled with deionised water and the film or coating is affixed over the open end of the vial. The assembly is weighed at the start of the test, and periodically throughout the test period. Over the course of the test the assembly is stored in a controlled environment to maintain a constant water vapour gradient. Loss in mass (g) is plotted as a function of time (hours), and a straight line of least- squares regression is drawn through the points. WVTR can be calculated as g / m2 / d (wherein d = day i.e. 24 hours). Oxygen transmission rate (OTR) Oxygen transmission rate describes the rate at which oxygen can pass through a material such as a film or coating and is determined using an in-house method according to according to standard D3985 - 17. Whereby, a circular test sample is loaded into a membrane permeability test cell, and residual gas is removed from the test rig by a vacuum pump. Testrig pressure is recorded by data logging software and an electronic pressure transmitter. Once the baseline vacuum pressure is maintained, oxygen is introduced at a fixed pressure on one side of the test article. The pressure on the other side is logged with time. From the linear rate of pressure increase the OTR can be determined as cm3 / m2 / d (wherein d = day i.e. 24 hours). Cobb test This method describes a procedure for determining the quantity of water absorbed by non-bibulous or coated paper-based materials (Cobb value g / m2). Whereby, a circular test sample is weighed (g) and affixed to a Payne cup containing 1 cm of water. The Payne cup is inverted, and the test sample is uniformly wetted for 180 seconds. After 180 seconds the test specimen is removed and blotted to remove surface water. The specimen is immediately weighed again. The Cobb value is calculated as the weight of the water absorbed in g / m2. Degradability in soil environment Individual films (of layer (A), layer (B) or composite material), with or without additives such as plasticizers, were placed in a non-degradable PE plastic frame. The framed films were photographed and weighed, and then buried in a soil environment. The framed films were dug up every 2 weeks to measure their weight loss and check their visual appearance by taking 06 11 23 photographs. Full degradability is determined as more than 90% of the film area having disappeared by visual comparison with the starting material. Anaerobic digestion Anaerobic digestion is carried out as described in the protocols of Angelidaki et al. (2009), Holliger et al. (2016), and Shrestha et al. (2020). Briefly, substrate (test polymer material or control material) is assessed for volatile solids content (VS%). 1 litre bottles are used as reactor vessels and filled to maximum of 400 mL where a ratio of >4:1 in terms of VS in the inoculum (AD sludge collected from local authority site) and substrate is respected to avoid problems of media acidification from the decomposition of organic matter. Comparison across different substrates is made by respecting the same total initial VS% in each vessel, which is determined to be between 20 - 60 gvs% (-mixture ■ FTIR Infrared (FTIR) spectra were recorded on a Perkin Elmer Frontier FT-IR equipped with a zinc selenide crystal ATR module. Each spectrum was recorded with 8 scans between 4,000 and 400 cm1, with a resolution of 4 cm1. General Method A - Preparation of a fatty acid ester functionalised polysaccharide using a fatty acid acyl chloride The first polysaccharide is suspended in room temperature pyridine at approximately 6 % w / v in a round-bottomed flask with mechanical stirring. A predetermined molar equivalent (3-7 equivalents relative to first polysaccharide repeat unit) of a selected fatty acid acyl chloride is added to the first polysaccharide-pyridine mixture, preferably by dropwise addition to control the exotherm. An air condenser is fitted and the temperature of the reaction mixture is raised to 80-115 °C. Stirring of the reaction mixture is maintained for 1-6 hr. After completion of the reaction, the reaction mixture is decanted into 5 volumes of room temperature ethanol to quench the reaction. The fatty acid ester functionalised polysaccharide solid composition is recovered by filtration and washed with hot ethanol to remove unreacted fatty acid acyl chloride and fatty acid by-product. The fatty acid ester functionalised polysaccharide solid composition is subsequently dried in a laboratory oven overnight, then recovered and stored in a glass vial for later analysis. Functionalisation and purity were confirmed by FTIR analysis of the fatty acid ester functionalised polysaccharide solid composition, as set out in Evaluation Methods. 06 11 23 General Method B-1 - Forming layer (A) by extrusion Fatty acid ester functionalised first polysaccharide (e.g. obtained using General Method A) is optionally mixed with one or more additives (such as a plasticiser) to provide a fatty acid ester functionalised polysaccharide composition (layer (A)). If used, the fatty acid ester functionalised polysaccharide composition may be mixed with selected one or more additives using a Haake Minilab II micro-compounding twin-screw extruder (20-150 rpm, 20 40-150 °C). The fatty acid ester functionalised polysaccharide composition and additive(s) are internally recirculated within the extruder to ensure thorough mixing. Prior to loading any materials into the extruder, a zero-load calibration is performed at operating temperature to normalise the screw torque measurements with respect to the frictional drag, at low (20 rpm) and high (170 rpm) rotation speeds. After completion of the mixing phase with additive, where required, key extruder operating parameters (extruder screw motor torque, screw rotational speed, heating block temperature, inlet and outlet pressure of the internal recirculation channel) are recorded at 0.5 second intervals by use of serial data acquisition script written in Python. Motive power (the power required to convey the material within the extruder) is calculated: P ~ 1' x x n 60 Where P is the motive power (Watts), T is the screw torque (Nm), <w is the angular velocity (radians s~1), and n is the screw rotation speed (rpm). General Method B-2- Forming layer (A) by hot pressing Fatty acid ester functionalised first polysaccharide (e.g. obtained using General Method A) is optionally mixed with one or more plasticisers or additives to provide a fatty acid ester functionalised polysaccharide composition (layer (A)), optionally with a desired amount of additive (such as a plasticiser). The mixing process is conducted by grinding together the functionalised first polysaccharide and the additive with a pestle and mortar. The mixture is then thinly spread between two PTFE sheets which are placed between two stainless steel plates. The plates are placed in a thermopress and pressed at a suitable temperature (e.g. around 180 °C) and pressure (e.g. around 10 bar). 06 11 23 General method Y-1 - Preparing a solution of material (B) for solution casting The un-derivatised second polysaccharide is dissolved in deionised water; the mixture is heated (e.g. at around 70 °C) under vigorous stirring until homogeneous and transparent. Quantities are adjusted to obtain the desired concentration (e.g. 0.5 to 4 % w / v). Optionally, a desired amount of additive is added to the solution at this stage. General method Y-2 - Preparing a dispersion of material (A) for dispersion casting The fatty acid ester functionalised first polysaccharide (e.g. obtained using General Method A) and a suitable solvent are vigorously mixed by means of magnetic or mechanical stirring at room temperature for an appropriate length of time (e.g. 1-24 hrs). Suitable solvents and / or solvent mixtures may be selected among esters (ethyl acetate, ethyl propanoate, ethyl butylate, propyl propanoate, butyl butylate, isobutyl butylate, 2-ethoxyethylacetate), ketones (acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 4-heptanone), ethers (tetrahydrofuran, 2-methyl-tetrahydrofuran, ethylene glycol dimethyl ether, methyl tertbutyl ether), aromatics (benzene, toluene, xylenes, propylbenzene, p-cymene, pyridine), chlorinated (dichloromethane, chloroform), water, DMSO and N-methyl morpholine, or a mixture thereof. An additive (such as a plasticiser) may be added if required. The concentration of material A in the final mixture is selected depending on the solvent(s) chosen and the intended application; typically 5 to 40 % w / v is used. The mixture is stored in a sealed container and vigorously stirred again before being used. General Method C-1 - Forming layer (B) by solution casting or layer (A) by dispersion casting The material B solution, prepared according to General method Y-1, or the material A solution, prepared according to General method Y-2, is poured into an evaporation dish and the solvent was evaporated in the oven (e.g. at 50 °C) over night. The thickness of the formed material can be adjusted by the concentration of polysaccharide in the solvent and by the size of the evaporation dish. General Method C-2 - Forming layer (B) via further conversion of an alginate salt film Film formation using an alginate salt such as sodium alginate, calcium alginate or potassium alginate is carried out according to General Method C-1. Further conversion of these films to alginic acid films is carried out by placing them into an aqueous acid solution, e.g. 0.1 M HCI, for 5 minutes. After treatment the films are rinsed with deionised water, patted dry with blue roll and then left to dry at ambient conditions. Further conversion of a sodium alginate film to a calcium alginate film is carried out by placing it into an aqueous calcium chloride solution, e.g. 3 %w / v, for 5 minutes. After treatment the films are rinsed with deionised water, patted 06 11 23 dry with blue roll and then left to dry at ambient conditions. Optionally the films can then be further infused with additives (such as a plasticiser) by placing them in a solution of additive for 5 minutes. After treatment the films are patted dry with blue roll and left to dry at ambient conditions. General Method W - Solution / dispersion casting for composite and coating production A solution / dispersion prepared according to General method Y-1 or Y-2 is prepared, ready for coating a second material. The second material used for such composite may be a selfstanding layer (e.g. a film) produced from material A or material B (according to General Methods B-1, B-2, C1 or C2, respectively), to form for example composite material AB / BA; or a composite of said materials to form a multilayer composite, for example composite material ABA, BAB, ABAB etc. Alternatively, the second material can be a substrate such as a carbohydrate-based support (S), for forming for example coated substrates SB and SA. If the substrate already comprises one or more coating layers, substrates coated with composite material can be formed, for example SBA, SAB, BSB, ASA, SBABA (forming further coating layers in situ). The second material is coated with the solution / dispersion by means of doctor blade coating, bar coating, spiral wire bar coating, brush coating or spray coating. One or multiple layers of the same mixture may be applied. The composite material is then left to dry in a well ventilated area at a suitable temperature (e.g. fume cupboard, laboratory oven). General Method D-1 - Forming a composite material with arrangement ABA by hot pressing Formation of composite films is carried out by layering the individual films, which were produced according to General Methods B-1, B-2, Y-1, Y-2, C-1 and / or C-2. on top of each other forming an ABA structure. The layers are then placed between two PTFE sheets which are placed between two stainless steel plates. The plates are placed in a thermopress and pressed at a suitable (e.g. around 120 °C) and pressure (e.g. around 10 bar). This method is suitable for alternative arrangements of composite material, e.g. AB, BAB, ABAB etc. Layering of the films can also be carried out by placing an adhesive layer between the individual film layers. General method D-2 - Forming a composite material with arrangement AB - in situ A self-standing material B film, manufactured according to General Method C-1 or C-2, is coated with a layer of material A by means of solvent casting according to General Method W, using a solution / dispersion of material A prepared according to General Method Y-2. 06 11 23 General Method X-1 - Forming a coated substrate with arrangement SB in situ A carbohydrate-based substrate S is coated with a layer of material B by means of solvent casting according to General Method W, using a solution of material B prepared according to General Method Y-1. Optionally, if the un-derivatized polysaccharide layer B is sodium alginate, further conversion to alginic acid can be achieved by applying an aqueous acid solution, e.g. 0.1 M HCI, onto the B layer by means of brushing, dipping or spraying. After the acid treatment, the B layer side of the coated substrate is rinsed with deionised water, patted dry with blue roll and then left to dry in at suitable temperatures (e.g. room temperature). General Method X-2 - Forming a coated substrate with arrangement SA in situ A carbohydrate-based substrate S is coated with a layer of material A by means of solvent casting according to General method W, using a solution / dispersion of material A prepared according to General method Y-2. General method X-3- Forming a coated substrate with arrangement SA by hot pressing - using preformed layer A A carbohydrate-based substrate S is coated with a layer of material A which has been prepared according to General method B-2. Layer A and the substrate are placed two PTFE sheets which are placed between two stainless steel plates. The plates are placed in a thermopress and pressed at a suitable temperature (e.g. around 120 °C) and pressure (e.g. around 5 bar). General Method E-1 - Forming a coated substrate with arrangement SABA by hot pressing - using preformed composite material ABA A carbohydrate-based substrate S is coated with a composite material (a film) to form a coated substrate with arrangement SABA. Firstly, an ABA composite film is produced according to General method D-1. The ABA film and the substrate are placed two PTFE sheets which are placed between two stainless steel plates. The plates are placed in a thermopress and pressed at a suitable temperature (e.g. around 120 °C) and pressure (e.g. around 5 bar). General Method E-2 - Forming a coated substrate with arrangement SBA - in situ A composite material with arrangement SBA is manufactured in situ by firstly producing an SB-type composite in accordance to General Method X-1, and secondly by coating that with a layer of material A by means of solvent casting according to General Method W, using a solution / dispersion of material A prepared according to General Method Y-2. 06 11 23 General Method E-3 - Forming a coated substrate with arrangement SBA - mixed in situ formation of SB following by hot pressing to form SBA A coated substrate with arrangement SBA is manufactured with a mixed method. Firstly, an SB-type composite is produced according to General Method X-1 (where layer B is formed in situ). Secondly, a preformed film of material A (produced according to General Method B-1 or B-2) is layered on top of layer B, the substrate is sandwiched between two PTFE sheets. These are placed between two stainless steel plates and pressed in a thermopress at a suitable temperature (e.g. around 120 °C) and pressure (e.g. around 10 bar). Alternative application methods of coatings are discussed in the description, and include co-extrusion and roll coating. Alternatively, layering can be carried out by placing an adhesive layer between the substrate / material layers. Example 1: Preparation of agar functionalised with palmitic acid chains according to General Method A Pre-dried (50 °C, overnight) agar (50 g) is loaded into a round bottom flask and suspended in pyridine (750 mL) at room temperature by means of mechanical stirring. Palmitoyl chloride (5.0 molar equivalents vs. polymer repeat unit, 250 mL) is slowly added under vigorous stirring, then the mixture is brought to a temperature of 105 °C and left to react for 3 hours under stirring. The mixture is then precipitated into ethanol (5 volumes), filtered under reduced pressure, and the solid washed with hot ethanol until running clear. The resulting solid is dried in an oven at 40-60 °C overnight. FTIR analysis is used to confirm the formation of the desired ester product (new peak at ca. 1742 cm-1) and its purity (absence of palmitoyl chloride and palmitic acid peaks at ca. 1800 and 1698 cm1, respectively) (Figure 8). Figure 8 shows the FTIR spectrum of agar palmitate obtained as per Example 1, overlayed with spectra for agar and palmitic acid Example 2a: Preparation of layer (A) comprising agar functionalised with palmitic acid using hot pressing according to General Method B-2 The functionalised polysaccharide (agar functionalised with palmitic acid) prepared according to Example 1 was thinly spread between two PTFE sheets which are placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 180 °C and 10 bar for 10 minutes. Example 2b: Preparation of layer (A) comprising agar functionalised with palmitic acid and soybean oil additive using hot pressing according to General Method B-2 The functionalised polysaccharide (agar functionalised with palmitic acid) prepared according to Example 1 and 20 wt% soybean oil were mixed by grinding them together in a pestle and 06 11 23 mortar. The mixture was then thinly spread between two PTFE sheets which are placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 180 °C and 10 bar for 10 minutes. Example 3a: Preparation of layer (B) comprising sodium alginate using solution casting according to General Methods Y-1 and C-1 Deionised water (100 mL) was placed in an Erlenmeyer flask and sodium alginate (1 g) was added under vigorous stirring. The mixture was heated to 70 °C for 30 minutes until full dissolution of sodium alginate was achieved. The solution was then poured into an evaporation dish and the water was evaporated in the oven at 50 °C over night. Example 3b: Preparation of layer (B) comprising sodium alginate and glycerol using solution casting according to General Methods Y-1 and C-1 Deionised water (100 mL) was placed in an Erlenmeyer flask, sodium alginate (1 g) and glycerol (0.8 g) were added under vigorous stirring. The mixture was heated to 70 °C for 30 minutes until full dissolution of sodium alginate was achieved. The solution was then poured into an evaporation dish and the water was evaporated in the oven at 50 °C over night. Example 3c: Preparation of layer (B) comprising alginic acid and glycerol using solution casting according to General Methods Y-1 and C-2 Deionised water (100 mL) was placed in an Erlenmeyer flask, sodium alginate (1 g) and glycerol (0.4 g) were added under vigorous stirring. The mixture was heated to 70 °C for 30 minutes until full dissolution of sodium alginate was achieved. The solution was then poured into an evaporation dish and the water was evaporated in the oven at 50 °C over night. The obtained film was then placed into a 0.1 M HCI solution for 5 minutes. The film was rinsed with deionised water, patted dry with blue roll and then left to dry at ambient conditions. The film was then placed in glycerol for 5 minutes. After treatment the film was patted dry with blue roll and left to dry at ambient conditions. Example 3d: Preparation of layer (B) comprising alginic acid using solution casting according to General Methods Y-1 and C-2 Deionised water (100 mL) was placed in an Erlenmeyer flask, sodium alginate (1 g) was added under vigorous stirring. The mixture was heated to 70 °C for 30 minutes until full dissolution of sodium alginate was achieved. The solution was then poured into an evaporation dish and the water was evaporated in the oven at 50 °C over night. The obtained film was then placed into a 0.1 M HCI solution for 5 minutes. The film was rinsed with deionised water, patted dry with blue roll and then left to dry at ambient conditions. 06 11 23 Example 4: Preparation of composite film with arrangement ABA using hot pressing according to General Method D-1 Two individual films (A), prepared according to Example 2b, and one individual film (B), prepared according to Example 3c, were placed on top of each other forming an ABA structure. The layers were then placed between two PTFE sheets which were placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 120 °C and 5 bar pressure for 5 minutes. Example 4a - Forming a composite film with arrangement AB in situ according to General Method D-2 Individual film (B) was prepared according to Example 3c. A dispersion of the palmitate-functionalised agar was produced by loading palmitate-functionalised agar (12.5 g; prepared according to Example 1) and toluene (100 mL) into a glass vial, which was sealed and magnetically stirred at room temperature overnight. The functionalised polysaccharide dispersion was applied to film (B) by brushing on the dispersion with a brush and allowing to dry at room temperature in a fume hood. Example 4b - Forming a coated substrate with arrangement SB in situ according to General Method X-1 Deionised water (100 mL) was placed in an Erlenmeyer flask and sodium alginate (1 g) was added with vigorous stirring. The mixture was heated to 70 °C for 30 minutes until full dissolution of sodium alginate was achieved. The sodium alginate solution was applied to a paper or card substrate by brushing it on with a brush. The coated substrate was allowed to dry under ambient conditions. Once dry, 0.1 M HCI was applied to the coating using a transfer pipette and rinsed off with deionised water and allowed to dry under ambient conditions. The process of applying sodium alginate and subsequent treatment with acid was then repeated to ensure complete coverage of the substrate. Example 4c - Forming a coated substrate with arrangement SA in situ according to General Method X-2 (using doctor blade coating) Palmitate-functionalised agar (12.5 g, prepared according to Example 1) and toluene (100 mL) were loaded into a glass vial, which was sealed and magnetically stirred at room temperature overnight. The dispersion was used to coat a paper or card substrate by means of a motorised blade coater instrument using a micrometer-adjustable doctor blade. The substrate was laid flat onto the instrument, the blade gap was adjusted to 600 pm, the agar palmitate dispersion 06 11 23 was poured onto the substrate, and the motor was activated at 30 mm / s speed. The coated substrate was then allowed to dry in a fume cupboard at room temperature. Example 4d - Forming a coated substrate with arrangement SA in situ according to General Method X-2 (using brush coating) Palmitate-functionalised agar (12.5 g, prepared according to Example 1) and toluene (100 mL) were loaded into a glass vial, which was sealed and magnetically stirred at room temperature overnight. The dispersion was used to coat a paper or card substrate by brushing it on with a brush until the surface was entirely covered. The coated substrate was then allowed to dry in a fume cupboard at room temperature. Example 4e - Forming a coated substrate with arrangement SA in situ according to General Method X-2 (using spray coating) Palmitate-functionalised agar (12.5 g, prepared according to Example 1) and toluene (100 mL) were loaded into a glass vial, which was sealed and magnetically stirred at room temperature overnight. The dispersion was used to coat a paper or card substrate by placing it in a spray bottle and spraying in onto the surface until the surface is entirely covered. The coated substrate was then allowed to dry in a fume cupboard at room temperature. Example 4f - Forming a coated substrate with arrangement SA using hot pressing according to General Method X-3 Individual film (A) prepared according to Example 2a and a paper or card substrate were placed on top of each other to forming an SA structure. The layers were then placed between two PTFE sheets which were placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 120 °C and 5 bar pressure for 5 minutes. Example 5: Preparation of coated substrate with coating of composite film ABA using hot pressing (using preformed ABA) Individual ABA film prepared according to Example 4, and substrate S (paper or card) were placed on top of each other forming a SABA structure. The layers were then placed between two PTFE sheets which were placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 120 °C and 5 bar pressure for 5 minutes. Example 6: Preparation of coated substrate with coating of composite film BA (in situ) Firstly, paper or card substrate S was coated with a solution of B according to Example 4b to yield a material with the structure SB. A dispersion of Palmitate-functionalised agar (12.5 g, 06 11 23 prepared according to Example 1) and toluene (100 mL) were loaded into a glass vial, which was sealed and magnetically stirred at room temperature overnight. Once thoroughly combined, dispersion A was applied to the substrate SB by brushing it on with a brush, forming the structure SBA. The coating was allowed to dry at room temperature in a fume hood. Example 7: Preparation of coated substate with coating of composite film BA (mixed) Firstly, paper or card substrate S was coated with a solution of B according to Example 4b to yield a material with the structure SB. Individual film (A) was prepared according to Example 2a. The layers SB and film A were positioned on top of each other to yield the structure SBA. The layers were then placed between two PTFE sheets which were placed between two stainless steel plates. The plates were placed in a thermopress and pressed at 120 °C and 5 bar pressure for 5 minutes. Example 8: Comparative evaluation of composite material ABA, and coated substate with SBA structure Thickness, liquid water permeability, water vapour transmission, oxygen transmission rate and Cobb value for composite film ABA (prepared according to Example 4) and coated paper substrate SBA (prepared according to Example 7) were evaluated as set out in the Evaluation Methods. For comparative purposes, individual layers A (prepared according to Example 2a) and B (prepared according to Example 3a) were also evaluated, and substrates coated separately with layer A (SA with the substrate being paper, prepared according to Example 4f) and with layer B (SB with the substrate being paper, prepared according to Example 4b). The results are summarised in Table 1 below. Table 1: Parameter evaluation for individual layers (A) and (B), composite film ABA, paper coated SA substrate, paper coated SB substrate and paper coated SBA substrate (paper thickness 0.100 mm in each case). Material Thickness including substrate (mm) Liquid water permeability WVTR (g / m2 / d) OTR (cm3 / m2 / d) Cobb (g / m2) Layer A 0.200 None 19 166 N / A Layer B 0.020 7 min breakthrough time 607 31 N / A ABA 0.313 None 14 11 N / A SA 0.220 None 80 62 2.4 Material Thickness including substrate (mm) Liquid water permeability WVTR (g / m2 / d) OTR (cm3 / m2 / d) Cobb (g / m2) SB 0.107 7 min breakthrough time 699 183 68 SBA 0.481 none 49 54 1.5 06 11 23 Comparing individual layers (A) and (B), it can be seen that layer (A) exhibited excellent water barrier properties (as evidenced by having no liquid water permeability, and a relatively low WVTR value) but relatively poor oxygen barrier properties (as evidenced by the relatively high OTR value). Conversely, layer (B) exhibited excellent oxygen barrier properties, but relatively poor water barrier properties. The composite film ABA exhibited improved water and oxygen barrier properties, compared to the layers individually. Comparing the paper substrate coated with layer (A) (SA) with the paper substrate coated with layer (B) (SB), material SA outperformed SB in terms of both water and oxygen barrier properties. However, paper coated with a composite layer BA outperformed material SA, exhibiting even lower WVTR and OTR values. By varying the thickness of the materials a further reduction of WVTR and OTR values can be achieved as these values are directly correlated to the thickness of the material. Example 9: Degradability study of layers (A) and (B) The degradability of individual layers (A) and (B) (prepared according to Examples 2b and 3c, respectively) was tested using the method set out in Evaluation Methods above. The results are shown in Figure 6, where it can be seen that both layers exhibited significant degradation over a relatively short time period. Example 10: Anaerobic digestion study Individual layers (A) and (B), with and without plasticiser (prepared according to Example 2a, 2b, 3c, 3d, with 20 wt% soybean oil as the plasticizer for layer (A) and 20 wt% glycerol for layer B) and composite film ABA (prepared according to Example 4) were subjected to anaerobic digestion as described in the protocols of Angelidaki et al. (2009), Holliger et al. (2016), and Shrestha et al. (2020). Briefly, substrate (test polymer material or control material) is assessed for volatile solids content (VS%). 1 litre bottles are used as reactor vessels and filled to maximum of 400 mL where a ratio of >4:1 in terms of VS in the inoculum (AD sludge collected from local authority site) and substrate is respected to avoid problems of media acidification from the decomposition of organic matter. Comparison across different substrates 06 11 23 is made by respecting the same total initial VS% in each vessel, which is determined to be between 20 - 60 gvs% Lmixture ■ Known petroleum-derived plastics PE (polyethylene), and common bio-derived compostable polymers PLA (polylactic acid) and TPS (thermoplastic starch) were also tested as comparators. The results are shown in Figure 7, where it can be seen that comparators PE, PLA and TPS produced no, or negative gas volumes, indicating no degradation. Layers (A) and (B) alone exhibited some degradation, which was enhanced when a plasticizer was included in the layer, as evidenced by the greater volumes of gas released. Interestingly, layered film ABA exhibited higher levels of anaerobic digestion that either of individual layers (A) and (B), which was not expected. Example 11: Further examples of composite films ABA with varying thickness Layer (A) in each case was formed using the method set out in Example 2b and contained agar functionalised with palmitic acid together with soybean oil (20wt.% forABA1-6 andABA8, 30 wt% for ABA7, 10 wt% for ABA9 and 5 wt% for ABA10) additive. Layer (B) in each case was formed using the method set out in Example 3c and contained alginic acid, together with glycerol (40 wt.%) additive. The layering of the individual films to form the ABA structure was carried out according to Example 4. In each case, thickness, liquid water permeability, water vapour transmission and oxygen transmission rate were evaluated as set out in the Evaluation Methods. The results are summarised in Table 2 below. Table 2: Parameter evaluation for composite films ABA of varying thickness Composite film ABA Thickness layer A (mm) Thickness layer B (mm) Total film thickness (mm) LWP* WVTR (g / m2 / d) OTR (cm3 / m2 / d) ABA1 0.124,0.158 = 0.282 (total) 0.03 0.301 None 14.19 35.52 ABA2 0.154,0.145 — 0.299 (total) 0.056 0.267 None 11.59 27.53 ABA3 0.145,0.153 = 0.298 (total) 0.094 0.313 None 14.20 13.19 ABA4 0.146, 0.146 = 0.292 (total) 0.059 0.232 None 22.20 31.39 ABA5 0.289,0.292 = 0.581 (total) 0.059 0.420 None 13.08 42.92 ABA6 0.405,0.405 = 0.81 (total) 0.06 0.732 None 7.25 30.72 ABA7 0.121,0.132 = 0.253 (total) 0.067 0.238 None 35.42 41.00 ABA8 0.152,0.155 = 0.307 (total) 0.063 0.294 None 19.29 15.00 ABA9 0.140,0.153 = 0.293 (total) 0.062 0.311 None 14.99 19.00 ABA10 0.154,0.159 = 0.313 (total) 0.062 0.331 None 14.33 11.00 * Liquid water permeability - breakthrough time All composite films exhibited good water barrier properties, with no water permeability being observed over the time period monitored. It can be observed that the values for the WVTR decrease with increasing thickness of layer (A), confirming that the modified polysaccharide layer (A) provides the main barrier against moisture. Lowering the amount of soybean oil plasticiser in film (A) does also result in a reduction in WVTR. The OTR values decrease with increasing thickness of film (B), confirming that the unmodified polysaccharides provide the main oxygen barrier. Furthermore, a reduction in plasticiser loading in film (A) also facilitates a reduction in OTR. 06 11 23 Example 12: Further examples of coated substate with SBA structure with variations in application of layer (A) and variations of composition of (B) layer Coated card samples (0.400 mm thickness) with a coating of composite film (SBA) were prepared as set out in Example 6 (in situ) and Example 7 (mixed). Layer (A) in each case contained agar functionalised with palmitic acid. Layer (B) consisted of either sodium alginate or alginic acid. Table 3: Parameter evaluation for composite SBA substrates with variations with variations in application of layer (A) (brush coated vs. hot pressed) and variations of composition of (B) layer (sodium alginate vs. alginic acid) Coated substrate Application of Layer A Layer B Application of Layer B Total coating thickness (mm) LWP* WVTR (g / m2 / d) OTR (cm3 / m2 / d) SBA1 brush coated sodium alginate brush coated 0.025 None 360.00 36.02 SBA2 brush coated alginic acid brush coated 0.020 None 369.05 NT* SBA3 hot pressed sodium alginate brush coated 0.151 None 28.41 27.73 SBA4 hot pressed alginic acid brush coated 0.240 None 5.50 29.49 *Not tested When comparing SBA1 with SBA2 it can be observed that the WVTR values are similar for sodium alginate as (B) layer and alginic acid as (B) layer. This is due to the moisture barrier 06 11 23 stemming from layer (A) which is of similar thickness for both samples as the same application method and thickness was chosen. Hot pressing as an application method for layer (A) leads to thicker coating layers of (A) and consecutively lower WVTR values compared to coated substrates where layer (A) has been applied via brush coating. OTR values are similar for all samples as layer (B), which is the main oxygen barrier, is applied via brush coating for all samples leading to with similar thicknesses of layer (B) of around 0.007 mm. OVERALL CONCLUSION All ABA composite films and SBA coated substrates exhibited good water barrier properties, with no water permeability being observed over the time period monitored and low Cobb values. For applications where WVTR needs to be low, an increase in the thickness of layer (A), as demonstrated film ABA6 (Table 2) can achieve WVTR values as low as 7.25 g / m2 / d. The same was observed for SBA coated substrates where thicker (A) layers applied via hot pressing led to lower WVTR (see SBA4) values compared to thinner (A) layers applied via brush coating. For applications where the OTR values need to be low, composite films with a thick (B) layer, such as ABA3 (Table 2), are preferred. Composite material ABA10 (Table 2) gives the best performance taking both WVTR and OTR values into account. REFERENCES The following publications cited in this application are herein incorporated by reference in their entirety. Abdullah et al. Front. Nutr. 2022, 9:1000116. Angelidaki et al. Water Science and Technology 2009, 59, no. 5, 927-34. Cazon et al. Food Hydrocolloids 2017, 68, 136-148. Diaz-Montez Polysaccharides 2022, 3, 480-501. Ghiasi et al. Industrial Crops &Products 2020, 154, 112679. Holliger et al. Water Science and Technology 2016, 74, no. 11, 2515-22. Khalil et al. J. AppL Polym. Sci. 2019, 136, 47251. Kibar Journal of Food Process Engineering 2017, 40, e12382. Shrestha etal. Sustainability 2020, 12, no. 10 4231.

Claims

06 11 231. A composite material comprising:at least one layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; andat least one layer (B) comprising a second polysaccharide which is un-derivatised.

2. The composite material according to claim 1, wherein the first polysaccharide is selected from the group consisting of cellulose, starch, agar, carrageenan, alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), fucoidan, laminarin, agarose, agaropectin, ulvan, xanthan gum and pectin.

3. The composite material according to claim 1 or claim 2, wherein the fatty acid ester moieties are of formula: -C(0)0(CH2)io-isCH3.; and in particular are-C(O)O(CH2)i4CH3(“C16” e.g. derived from activated palmitic acid) or-C(O)O(CH2)isCH3, or -C(O)O(CH2)i6CH3 (“C18” e.g. derived from activated stearic acid).

4. The composite material according to any one of claims 1 to 3, wherein the second polysaccharide is selected from the group consisting of agar, alginic acid, an alginate salt (such as sodium alginate, calcium alginate or potassium alginate), carrageenan, starch and xanthan gum.

5. The composite material according to any one of claims 1 to 4, wherein substantially all or all of a surface of layer (A) is in contact with substantially all or all of a surface of layer (B).

6. The composite material according to any one of claims 1 to 4, wherein at least a portion of a surface of layer (A) is bonded to at least a portion of a surface of layer (B) using an adhesive.

7. The composite material according to any one of claims 1 to 6, which is biodegradable.

8. A substrate with a surface having a coating comprising the composite materialaccording to any one of claims 1 to 7.

9. The substrate with a surface having a coating according to claim 8, wherein the substrate is a packaging material, which is suitably carbohydrate-based.06 11 2310. The substrate with a surface having a coating according to claim 8 or claim 9, wherein the substrate is in sheet form or wherein the substrate is in 3D form, such as in the form of a container (e.g. a tray, bowl or cup).

11. The substrate with a surface having a coating according to claim 9 or claim 10, wherein the substrate is selected from the group consisting of paper, cardboard, corrugated board, paperboard, carton board, fibre and fabric.

12. A process for preparing a composite material according to any one of claims 1 to 11, comprising the steps of:a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties;b) forming a layer (B) comprising: a second polysaccharide which is un-derivatised;c) applying layer (B) to layer (A), then applying heat and / or pressure;or,a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised;b) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties;c) applying layer (A) to layer (B), then applying heat and / or pressure.

13. A process for preparing a composite material according to any one of claims 1 to 11, comprising the steps of:a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties; andb) treating at least a portion of a surface of layer (A) to form a layer (B) comprising a second polysaccharide which is un-derivatised;or,a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised; andb) treating at least a portion of a surface of layer (B) to form a layer (A) comprising a first polysaccharide which has been derivatised to comprise fatty acid ester moieties.

14. A process for preparing a substrate having a coating, wherein the coating comprises a composite material, the process comprising the steps of:a) preparing a composite material according to any one of claims 1 to 7; andb) applying the composite material to at least a portion of a surface of the substate using pressure and optionally heat.06 11 2315. A process for preparing a substrate having a coating, wherein the coating comprises a composite material according to any one of claims 1 to 11, the process comprising the steps of:a’) treating at least a portion of a surface of the substrate to form a first layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties;b’) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised;or,a’) treating at least a portion of a surface of the substrate to form a first layer (B) comprising: a second polysaccharide which is un-derivatised;b’) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties.

16. A process for preparing a substrate having a coating, wherein the coating comprises a composite material according to any one of claims 1 to 11, the process comprising the steps of:a) forming a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties;b) applying layer (A) to at least a portion of a surface of the substate using pressure and optionally heat;c) treating at least a portion of the surface of layer (A) to form a layer (B) comprising: a second polysaccharide which is un-derivatised;or,a) forming a layer (B) comprising: a second polysaccharide which is un-derivatised;b) applying layer (B) to at least a portion of a surface of the substate using pressure and optionally heat;c) treating at least a portion of the surface of layer (B) to form a layer (A) comprising: a first polysaccharide which has been derivatised to comprise fatty acid ester moieties.

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