A coating for a substrate

Abstract

A coating comprises a first biodegradable polymer, a second biodegradable polymer and a compatibilizer. The first biodegradable polymer is an elastomer (preferably natural rubber) and is the most abundant component of the coating. The second biodegradable polymer is preferably poly lactic acid, while the compatibilizer is preferably clay.

Description

TECHNOLOGICAL FIELD Embodiments of the present disclosure relate to a coating for a substrate, and in particular to a coating comprising biodegradable polymers. BACKGROUND Coatings for substrates, such as textiles, typically comprise polyurethanes (PUs) or other elastomers which render the substrate and / or coating difficult or impossible to recycle. The manufacture of polyurethanes in particular, typically requires the use of harmful chemicals, such as toluene di-isocyanate at some stage of their synthesis. The application of polyurethane coatings is typically carried out via the use of organic solvents having environmental and / or health hazards. The production and / or disposal of such coated substrates or coatings can cause damage to the environment in the form of plastic pollution and is therefore unsustainable. There is, therefore, a need to mitigate these drawbacks. BRIEF SUMMARY According to various, but not necessarily all, embodiments there is provided a coating for a substrate, wherein the coating comprises: a first biodegradable polymer, wherein the first biodegradable polymer is an elastomer; a second biodegradable polymer; and a compatibilizer, wherein the elastomer is the most abundant component of the coating. Possibly, the first biodegradable polymer and the second biodegradable polymer are immiscible without a compatibilizer. The ratio of the elastomer to the second biodegradable polymer may be from 50:1 to 2:1. The ratio of the elastomer to the second biodegradable polymer may be from 20:1 to 2:1. The ratio of the elastomer to the second biodegradable polymer may be from 10:1 to 2:1. Preferably, the ratio of the elastomer to the second biodegradable polymer may be from 7:1 to 2:1. Most preferably, the ratio of the elastomer to the second biodegradable polymer may be from 4:1 to 2:1. The ratio of the elastomer to the second biodegradable polymer may be from 17:3 to 13:5. The ratio of the elastomer to the second biodegradable polymer may be from 17:3 to 13:7. Alternatively, the ratio of the elastomer to the second biodegradable polymer may be from 50:1 to 45:1. The elastomer may be derived from a biological source. The elastomer may comprise rubber. The elastomer may consist of rubber. The rubber may comprise natural rubber, synthetic rubber, or combinations thereof. The rubber may comprise natural rubber. The rubber may consist of natural rubber. In other examples, the elastomer may comprise synthetic rubber. The second biodegradable polymer may be derived from a biological source. The second biodegradable polymer may comprise: polylactic acid (PLA), cellulose acetate, polyvinyl alcohol (PVA), poly(butylene adipate co terephthalate) (PBAT), polybutylene succinate (PBSA), polyhydroxyalkanoates (PHA), cellulose, polysaccharide, a starch based polymer, or a casein polymer. The second biodegradable polymer may be selected from the group consisting of: polylactic acid (PLA), cellulose acetate, polyvinyl alcohol (PVA), poly(butylene adipate co terephthalate) (PBAT), polybutylene succinate (PBSA), polyhydroxyalkanoates (PHA), cellulose, polysaccharide, a starch based polymer, and a casein polymer. The second biodegradable polymer may comprise: polylactic acid (PLA), cellulose acetate, polyvinyl alcohol (PVA), poly(butylene adipate co terephthalate) (PBAT), polybutylene succinate (PBSA), or polyhydroxyalkanoates (PHA). The second biodegradable polymer may be selected from the group consisting of: polylactic acid (PLA), cellulose acetate, polyvinyl alcohol (PVA), poly(butylene adipate co terephthalate) (PBAT), polybutylene succinate (PBSA), and polyhydroxyalkanoates (PHA). The second biodegradable polymer may comprise polylactic acid (PLA). The second biodegradable polymer may consist of polylactic acid (PLA). The term “polylactic acid” refers to a polymer constituted with a monomer unit derived from L-lactic acid and / or D-lactic acid. Other monomer units which are not derived from L-lactic acid or D-lactic acid may also be included. The molecular weight preferably falls within the range of 50,000 to 300,000 in terms of the weight average molecular weight. The compatibilizer may be derived from a biological source. The compatibilizer may comprise clay, microfibrillated cellulose (MFC), or carbon fibres. The compatibilizer may be selected from a group consisting of: clay, microfibrillated cellulose (MFC), and carbon fibres. The compatibilizer may comprise clay or microfibrillated cellulose (MFC). The compatibilizer may comprise clay. The compatibilizer may consist of clay. The compatibilizer may consist of microfibrillated cellulose (MFC). The clay may comprise bentonite clay. The clay may comprise organically modified nanoclay. The elastomer may have an elongation to break greater than the elongation to break of the second biodegradable polymer. The elastomer may have an elongation to break greater than 300%. The elastomer may have an elongation to break greater than 350%. The elastomer may have an elongation to break greater than 400%. The elastomer may have an elongation to break greater than 450%. The elastomer may have an elongation to break greater than 500%. The second biodegradable polymer may have an elongation to break less than 50%. The second biodegradable polymer may have an elongation to break less than 40%. The second biodegradable polymer may have an elongation to break less than 30%. The second biodegradable polymer may have an elongation to break less than 20%. The second biodegradable polymer may have an elongation to break less than 10%. The second biodegradable polymer may have a hardness greater than the hardness of the elastomer. The elastomer may have a Young's modulus less than the Young's modulus of the second biodegradable polymer. Possibly, the coating does not comprise polyurethanes. Possibly, the manufacture of the coating does not use harmful chemicals. Possibly, the coating does not comprise harmful chemicals. Possibly, the coating is biodegradable. Possibly, the coating does not comprise vulcanised rubber. Possibly, the coating does not comprise vulcanised natural rubber. The coating may be an elastomeric material. The coating may be a physical mixture. The coating may comprise a surfactant. The coating may comprise one or more additives, which additives may comprise dyes, pigments, colourants, and / or fillers. The coating may comprise from 65 to 95 wt.% of the elastomer. Preferably, the coating may comprise from 75 to 85 wt.% of the elastomer. Alternatively, the coating may comprise from 85 to 95 wt.% of the elastomer. The coating may comprise from 1 to 29 wt.% of the second biodegradable polymer. Preferably, the coating may comprise from 13 to 22 wt.% of the second biodegradable polymer. Alternatively, the coating may comprise from 1 to 13 wt.% of the second biodegradable polymer. The coating may comprise from 1 to 6 wt.% of the compatibilizer. Preferably, the coating may comprise from 1 to 3 wt.% of the compatibilizer. Alternatively, the coating may comprise from 3 to 5 wt.% of the compatibilizer. The coating may comprise less than 6 wt.% of the compatibilizer. The coating may comprise a surfactant. The coating may comprise from 1 to 5 wt.% of the surfactant. Preferably, the coating may comprise from 1 to 3 wt.% of the surfactant. The coating may be formed from a mixture comprising the elastomer, the second biodegradable polymer, and the compatibilizer. The mixture may comprise water. The mixture may comprise water as a solvent. The mixture may be an aqueous mixture. The mixture may be a solution, dispersion, or emulsion. The mixture may comprise a surfactant. The elastomer comprised in the mixture may be in the form of latex. The latex may be a waterborne latex. The elastomer comprised in the mixture may be natural rubber latex. The elastomer comprised in the mixture may consist of natural rubber latex. The second biodegradable polymer comprised in the mixture may be in the form of an emulsion. The second biodegradable polymer comprised in the mixture may be poly lactic acid (PLA) emulsion. The second biodegradable polymer comprised in the mixture may consist of poly lactic acid (PLA) emulsion. The mixture may have a viscosity of from 5000 to 20000 cPs. According to various, but not necessarily all, embodiments there is provided a substrate having a coating, wherein the coating comprises: a first biodegradable polymer, wherein the first biodegradable polymer is an elastomer; a second biodegradable polymer; and a compatibilizer, wherein the elastomer is the most abundant component of the coating. Possibly, the substrate is biodegradable. Possibly, the coated substrate is biodegradable. Possibly, the substrate is a sheet material comprising a body of fibres. The body of fibres may comprise synthetic fibres, natural fibres, or naturally derived fibres, or may comprise a blending of synthetic fibres, natural fibres and / or naturally derived fibres. The body of fibres may comprise leather fibres. The body of fibres may comprise predominantly leather fibres. The body of fibres may comprise leather fibres and nonleather fibres. The non-leather fibres may be natural or synthetic. Possibly, natural fibres may include: wool, cotton, and flax. A non-exhaustive list of example naturally derived fibres includes leather, soybean, viscose and bamboo fibre. Possibly, synthetic fibres may include: nylon, polyester, acrylic. The synthetic fibres may be bi-composite fibres. In some examples, the body of fibres may comprise synthetic and natural fibres in combination. The body of fibres may have a non-woven arrangement. Alternatively, the body of fibres may have a woven arrangement. The sheet material may be a textile comprising the body of fibres. The sheet material may be a composite material comprising the body of fibres. The sheet material may be a fabric comprising the body of fibres. The body of fibres may comprise fibres interlocked together by entanglement. The body of fibres may be interlocked together by hydroentanglement by subjecting an arrangement comprising a web of fibres to high pressure jets of liquid to interlock the fibres together by entanglement. The body of fibres may be mechanically bonded to a reinforcing structure during hydroentanglement to provide a substrate, such as a sheet material, with improved mechanical properties, such as greater tensile strength. The substrate may comprise a non-woven fabric. The substrate may comprise a textile. The substrate may comprise a fabric. The substrate may comprise leather fibres. The substrate may comprise predominantly leather fibres. The substrate may be a sheet material comprising leather fibres. The leather fibres may have a non-woven arrangement. The leather fibres may be interlocked together by entanglement. The leather fibres may be interlocked together by hydroentanglement by subjecting an arrangement comprising a web comprising leather fibres to high pressure jets of liquid to interlock the fibres together by entanglement. The leather fibres may be derived from waste leather. The substrate may have a topcoat overlying the coating. The topcoat may be a haptic modifier layer. The topcoat may be used as a haptic modifier layer. The topcoat can provide additional durability, abrasion, scratch resistance, UV protection, or other surface property such as stain resistance, water repellency, hydrophobicity, control gloss, The substrate may be an engineered leather product or used to form an engineered leather product. According to various, but not necessarily all, embodiments there are provided clothing, footwear, accessories or upholstery comprising a substrate according to any of the preceding paragraphs. According to various, but not necessarily all, embodiments there is provided a method of coating a substrate, wherein the method comprises: mixing a first biodegradable polymer, a second biodegradable polymer, a compatibilizer, and water to provide an aqueous mixture, wherein the first biodegradable polymer is an elastomer; applying the aqueous mixture to the substrate; and forming a coating from the aqueous mixture, wherein the elastomer is the most abundant component of the coating. The method may comprise mixing the elastomer, the second biodegradable polymer, the compatibilizer, the water, and a surfactant to provide the aqueous mixture. According to various, but not necessarily all, embodiments there are provided examples as claimed in the appended claims. BRIEF DESCRIPTION For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only. DETAILED DESCRIPTION The disclosure provides a coating for a substrate. The coating comprises a first biodegradable polymer, a second biodegradable polymer and a compatibilizer. Accordingly, the coating comprises at least a first biodegradable polymer, a second biodegradable polymer and a compatibilizer. The first biodegradable polymer is an elastomer. The elastomer is the most abundant component of the coating. The coating therefore comprises at least two different biodegradable polymers, one of which biodegradable polymers is an elastomer. The coating may comprise a number of additional different layers, which may or may not be distinguishable by optical microscopy. These additional different layers may include adhesive, skin and / or topcoat. The first biodegradable polymer and the second biodegradable polymer may be immiscible. In some examples, the coating may comprise only two different biodegradable polymers, wherein one of the two biodegradable polymers is an elastomer. In other examples, the coating may comprise more than two different biodegradable polymers, wherein at least one of the biodegradable polymers is an elastomer. Accordingly, the coating may comprise three, four, five or more biodegradable polymers. In such examples, at least one of the biodegradable polymers is an elastomer. In such examples, more than one of the biodegradable polymers may be an elastomer. The elastomer is the most abundant component of the coating. Accordingly, the amount of elastomer in the coating is greater than the amount of the second biodegradable polymer in the coating. The coating therefore comprises more of the elastomer than the second biodegradable polymer. The elastomer is the predominant component of the coating. The elastomer is therefore the major component of the coating. In some examples, the coating may optionally comprise a surfactant. The coating may optionally comprise a filler, a dye, a pigment and / or another colourant. The disclosure also provides a substrate having the coating (i.e., a coated substrate), and clothing, footwear, accessories or upholstery comprising the substrate. Optionally, the coating may be provided as an outermost layer on at least one surface of the 9 substrate. In some examples, the coated substrate is useable in various applications where leather or coated textiles are used, such as consumer goods, seating, automotive applications, and footwear. In some examples, the elastomer is natural rubber, the second biodegradable polymer is polylactic acid (PLA), and the compatibilizer is clay. Natural rubber and clay are derived from respective biological sources. PLA is derived from renewable and / or organic sources, such as corn starch or sugar cane. Accordingly, PLA is also derived from a biological source. In some examples, the immiscibility of the elastomer (e.g., natural rubber) and the second biodegradable polymer (e.g., PLA) means that these materials do not mix well due to a lack of interfacial adhesion between the materials. For example, natural rubber is typically very nonpolar, and PLA is very polar. Without being bound by theory, the compatibilizer (e.g., clay) sits at the interface between the immiscible biodegradable polymers improving their compatibility and allowing a blend to be obtained which has the desired attributes from each of the biodegradable polymers, and therefore the required properties. In other examples, a compatibilizer (e.g., clay) may further improve the compatibility of miscible biodegradable polymers. In examples of the disclosure, the elastomer (e.g., natural rubber) has a relatively high elongation to break and a relatively low hardness and the second biodegradable polymer (e.g., PLA) has a relatively low elongation to break and a relatively high hardness. Accordingly, the elastomer has an elongation to break greater than the elongation to break of the second biodegradable polymer and the second biodegradable polymer has a hardness greater than the hardness of the elastomer. Coatings according to examples of the disclosure have an optimum balance of the properties of elongation to break and hardness based on the respective attributes of the elastomer and the second biodegradable polymer in the coating. Tables 1 and 2 below provide details of example coatings E1 to E17 and a comparative example C1. The comparative example does not include a compatibilizer. All proportions in Tables 1 and 2 are expressed as parts. Table 3 shows selected properties of a coated substrate having example coatings E2, E3, E5 and E6 compared to a coated substrate having the example comparative coating C1. At least in relation to the properties measured (as described below) examples of the disclosure show an improvement in the balance of these properties. Without a compatibilizer (example C1), an optimum balance of properties is not obtained because the materials are too different and / or insufficiently compatible. As detailed in Tables 1 and 2, example coatings comprise the elastomer, the second biodegradable polymer, the compatibilizer, and optionally a surfactant. In some examples, the coatings may further comprise additives, such as pigments, dyes and / or fillers. In some examples, the elastomer comprised in a mixture for forming the coating is in the form of latex, for example, natural rubber latex. In some examples, the second biodegradable polymer comprised in a mixture for forming the coating is in the form of an emulsion, for example, PLA emulsion. In some examples, the substrate comprises leather fibres. The substrate may be a sheet material comprising leather fibres. The leather fibres can have a non-woven arrangement. The leather fibres can be interlocked together by entanglement. The leather fibres can be interlocked together by hydroentanglement by subjecting an arrangement comprising a web of leather fibres to high pressure jets of liquid to interlock the fibres together by entanglement. The web in such examples may also comprise non-leather fibres and / or additives or other materials. The leather fibres may be derived from waste leather. For illustrative purposes only, the fibre length resulting from disintegrating of waste leather in textile reclaiming equipment ranges from less than 1 mm with occasional fibres up to 20 mm. The fibre structure of natural leather before disintegration consists of closely interwoven bundles of collagen fibres, which in turn consist of even finer fibrils, many of which become separated during the mechanical action. This results in a range of fibre diameters from about 100 microns for the bundles to very fine fibres below 1 micron for individual fibrils. In some examples, the substrate has a topcoat overlying the coating. The topcoat may be a haptic modifier layer, e.g., lacquer. A haptic modifier layer may be applied to the coating to meet end user expectations, or for additional durability. In this specification the term “elastomer” means a natural or synthetic polymer having elastic properties, such as rubber (natural or synthetic). Elastomers are typically materials which have high elongation to break values under strain. Elongation to break values are typically several hundred percent. For example, some elastomers according to the disclosure can be stretched to at least about 150% or more of their original dimension, and which then recover to no more than about 120% of their original dimension in the direction of the applied stretching force. Elastomers are elastomeric materials. Elastomeric materials are both stretchable and recoverable. "Stretchable" and "recoverable" are descriptive terms used to describe the elastomeric properties of a material. "Stretchable" means that the material can be extended by a pulling force to a specified dimension significantly greater than its initial dimension without breaking. For example, a material that is 10 cm long that can be extended to about 13 cm long without breaking under a pulling force could be described as stretchable. "Recoverable" means that a material which is extended by a pulling force to a certain dimension significantly greater than its initial dimension without breaking will return to its initial dimension or a specified dimension that is adequately close to the initial dimension when the pulling force is released. For example, a material that is 10 cm long that can be extended to about 13 cm long without breaking under a pulling force, and which returns to about 10 cm long or to a specified length that is adequately close to 10 cm could be described as recoverable. In some examples, elastomeric materials may be extensible. "Extensible" refers to materials that can be stretched at least about 130% of their original dimension without breaking, but which either do not recover significantly or recover to greater than about 120% of their original dimension and therefore are not elastomeric as defined above. For example, an extensible film that is 10 cm long should stretch to at least about 13 cm under a stretching force, then either remain about 13 cm long or recover to a length more than about 12 cm when the stretching force is removed. Extensible materials are stretchable, but not recoverable. In this specification, elongation to break is defined as the length at breaking point of a material expressed as a percentage of its original length (i.e., length at rest). In this specification the term “natural rubber” includes all materials made from or containing natural latex. In other words, it refers to natural rubber latex, dry natural rubber, and synthetic latex or synthetic rubber that contains natural rubber in its formulation. Natural rubber latex is a milky fluid that comprises extremely small particles of rubber obtained from plants, mainly from the rubber tree, dispersed in an aqueous medium. Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. In this specification the term “biodegradable” means that a material is capable of being decomposed by bacteria or other living organisms and thereby avoiding pollution. In examples of the disclosure, a biodegradable material will meet the requirements set out under one of standards for the biodegradability of plastics in the ISO discussed under TC61 / SC5 / WG22. These include ISO 14855-1 and ISO 14855-2 (biodegradation tests in a compost), ISO 17088 (Specifications for compostable packaging) and / or ISO 16929 (Plastics-Determination of the degree of disintegration of plastic materials under defined compositing conditions in a pilot-scale test). In this specification, the term “compatibilizer” means an agent for stabilizing at least two polymers. Compatibilizer according to examples of the disclosure can be agents for stabilizing at least two immiscible polymers in a physical mixture. A “physical mixture” is a mixture in which the constituent substances are not chemically combined though they may be so intimately mingled as to be impossible to separate by simple mechanical means. EXAMPLES Materials Used Elastomers PE1001 - natural rubber latex, Corrie-MacColl Deutschland GmbH LA-TZ - natural rubber latex, Corrie-MacColl Deutschland GmbH Second biodegradable polymers Y225 - 50% PLA emulsion, CHUKYO YUSHI CO., LTD. W465 - polybutylene succinate adipate (PBSA), CHUKYO YUSHI CO., LTD. Esacol ED 5 - polysaccharide, Lamberti Chitosan - polysaccharide, Plater Group Compatibilizers Cloisite Na+ - bentonite clay, Byk Cloisite Ca+ - bentonite clay, Byk Ceratosil FG - bentonite clay, Clariant Ceratosil R - bentonite clay, Clariant Ceratosil R - bentonite clay, Clariant Ceratosfix XXMG - bentonite clay, Clariant Lanco 1955 SF - carnuba wax, Lubrizol Arbocell BE 600 - microfibrillated cellulose, Rettenmaier UK Ltd Surfactants Prifac 8944 - rapeseed fatty acid, Croda Crodamol GTCC - mixed decanoyl and octanoyl glycerides, Croda Table 1: Example coatings E1 to E9 Generalisation Specific example of ingredient used E1 parts E2 parts E3 parts E4 parts E5 parts E6 parts E7 parts E8 parts E9 parts First biodegradable polymer (elastomer) PE1001 75 65 85 65 85 75 75 75 75 LA-TZ - - - - - - - - - Second biodegradable polymer Resem Y225 22 33 13 29 14 22 22 22 22 W465 Esacol ED5 - - - - - - - - - Chitosan - - - - - - - - - Compatibilizer Cloisite Na 3 2 3 6 1 3 Cloisite Ca+ - - - - - - 3 - - CERATOSIL FG - - - - - - - 3 - CERATOSIL R - - - - - - - - 3 CERATOFIX XXMG Lanco 1955 SF - - - - - - - - - CERATOSIL XXMG - - - - - - - - - Arbocell BE 600 Surfactant Prifac 8944 1 1 1 1 1 1 1 - - Crodamol GTCC - - - - - - - - - Table 2: Example coatings E10 to E17 and comparative coating C1 Generalisation Specific example of ingredient used E10 parts E11 parts E12 parts E13 parts E14 parts E15 parts E16 parts E17 parts C1 parts First biodegradable polymer (elastomer) PE1001 75 75 - - - 94 - - 75 LA-TZ 73 75 75 94 90 Second biodegradable polymer Resem Y225 22 21 21 21 - - - - 25 W465 - - - - 21 - - - - Esacol ED5 - - - - - 2 - 2 - Chitosan - - - - - - 2 - - Compatibilizer Cloisite Na - - - - - 3 3 5 - Cloisite Ca+ - - 3 - - - - - - CERATOSIL FG - - - - - - - - - CERATOSIL R - - - - - - - - - CERATOFIX XXMG 3 Lanco 1955 SF - 1 - - - - - - - CERATOSIL XXMG - 3 - - - - - - - Arbocell BE 600 3 3 Surfactant Prifac 8944 - - - - - 1 1 3 1 Crodamol GTCC - - 3 - - - - - - In the above examples, the elastomer is the most abundant component of the coating. Table 3: Properties of a coated substrate having example coatings E2, E3, E5 and E6 5 compared to a coated substrate having the example comparative coating C1. ID Veslic dry - rubs to breakthrough Veslic wet - rubs to breakthrough Bally Flex Average C1 90 60 20,000 E2 200 30 15,000 E3 500 40 55,000 E5 500 400 32,000 E6 500 200 40,000 Veslic is the name of a rub test (based on ISO11640) that repeatedly moves a felt pad over a material looking for signs of damage. It can be carried out with various pad 10 conditions, such as dry or wet. The failure point has been defined as wearing away the coating to the point the substrate below is visible. Bally Flex is a test to assess the flexibility of a material (based on, ASTM D6182 Standard Test Method for Flexibility and Adhesion of Finish on Leather or BS EN ISO 15 5402-1:2017 Leather — Determination of flex resistance) samples are repeatedly flexed in a holder of fixed dimensions and the samples are assessed looking for signs of cracks or damage. The failure point has been defined as the coating broken to the point the substrate is visible. The coating of example C1 does not comprise a compatibilizer (see Table 2 above) and is not therefore according to the invention as defined in the claims. For C1, the veslic numbers are low, i.e., <100, and bally flex only achieving 20,000 flexes before the coating breaks fully to the substrate. Accordingly, C1 has low flex resistance and is easily rubbed off in wear testing. In examples of the disclosure, for instance E2, E3, E5 and E6, the veslic and / or flex tends to improve. The ratio of elastomer to the second biodegradable polymer can be important in some examples. For instance, E6, which has a ratio of elastomer to second biodegradable polymer of about 3:1, has an excellent balance of tested properties. Example methods In some examples, the coating may be formed from a mixture of the elastomer (e.g., natural rubber latex), the second biodegradable polymer (e.g., PI_A), the compatibilizer (e.g., clay or MFC), and optionally a surfactant all combined in solution or in a dispersion state using water as a solvent. Two options for this methodology are described below. Option 1 Clay (or MFC) is pre-dispersed using high shear in a PLA emulsion to ensure full dispersion. This mixture is then combined with natural rubber latex and again mixed to provide a blend. The blend is applied to a substrate either via transfer / release paper or applied directly to substrate by knife over roll / air, gravure or spray and dried in an oven at 100-130°C typically for about 2 minutes. Further layers can be applied either via a spray or gravure topcoat, drying or curing in an oven around 100-130°C for 1-3 minutes. Alternatively, the surface haptic response can be adjusted using a thin layer on the release paper. Option 2 Clay (or MFC) is pre-dispersed using high shear in a PLA emulsion with natural rubber latex to ensure full dispersion and to provide a blend. The blend is applied to a substrate either via transfer / release paper or applied directly to substrate by knife over roll / air, gravure or spray. Further layers can be applied either via a spray or gravure topcoat. Alternatively, the surface haptic response can be adjusted using a thin layer on the release paper. In some examples, for mixtures to be useable they must be able to be pumped, and flow during application with low shear rates. Typically, mixtures have a viscosity of about 5000-20000cPs at Spindle 63, speed 12. In other examples, the coating may be formed from a mixture of the elastomer (e.g., natural rubber latex), the second biodegradable polymer (e.g., PLA), the compatibilizer (e.g., clay or MFC), and optionally a surfactant all combined in a solid or melt state (rather than in a solution or in a dispersion liquid state), such as compounding by milling, or extruding, injection moulding, or casting depending on the choice of materials and desired output. Examples of the disclosure include transfer coating and direct coating of substrates. Examples of the disclosure also provide a method of coating a substrate. In some examples, the method comprises mixing a first biodegradable polymer, a second biodegradable polymer, a compatibilizer, and water to provide an aqueous mixture, wherein the first biodegradable polymer is an elastomer. The method further comprises applying the aqueous mixture to the substrate. The method further comprises forming a coating from the aqueous mixture. The elastomer is the most abundant component of the coating. In some examples, the method may comprise mixing the elastomer, the second biodegradable polymer, the compatibilizer, the water, and a surfactant to provide the aqueous mixture. The method may comprise drying the coating (e.g., in an oven), for example, at about 100 to 130°C for a short period of time, for instance, for about 2 minutes. In some examples of the disclosure, the component materials of the mixture for forming the coating do not react. The coating itself may therefore be a solid physical mixture. In such examples, the water borne mixture can be applied to a release paper or a directly to a substrate and then dried into a film (without a reaction occurring). Accordingly, any functional groups present on these component materials would not react, and no initiators nor any cross linkers are added. In other examples, a cross linker may be included in mixtures for forming the coating to improve the performance. In such examples, a curing reaction is expected. Accordingly, a resulting coating would be the reaction product of the mixture subjected to a curing (or hardening) reaction. In such examples, the cross linker is selected such that sustainability and / or biodegradability of the coating and / or coated substrate is not compromised. There is thus described a coating, a substrate, a method, and clothing, footwear, accessories or upholstery comprising the substrate with a number of advantages as detailed above and as follows. Coatings and coated substrates according to examples of the disclosure can be readily recycled and / or readily biodegrade at end of life. The production and / or disposal of coatings and coated substrates according to examples of the disclosure does not cause damage to the environment and is therefore sustainable. In some examples, the described coatings and / or coated substrates are made from materials derived from nature and are intended to return to nature at the end of their useful life. The inclusion of a compatibilizer may also increase the colour density compared to without it. In some examples, materials for forming coatings can be waterborne dispersions or latex, for instance PLA and natural rubber latex. These materials do not require high temperatures to process as they are flowable at room temperature. This avoids the requirement to melt materials and / or use organic solvent which can cause environmental damage and / or health problems. Growing awareness of environmental issues have resulted in a desire to move away from fossil derived materials, such as fossil derived polyurethanes (PUs). These are known to be harmful and have end of life issues. For example, it is difficult to separate polyurethane from a substrate and flexible polyurethanes are virtually impossible to fully recycle. Most polyurethanes do not degrade in nature and are not therefore biodegradable. Whilst biobased polyurethanes are more sustainable in their feedstocks than their fossil derived counterparts, they do not overcome end of life issues i.e., they are not necessarily biodegradable. Coatings according to some examples of the disclosure do not comprise polyurethanes, i.e., coatings may be free from polyurethanes and are therefore more sustainable. Natural rubber is commonly vulcanised and is heavily used in the car and tyre industry. This vulcanisation, or curing, step limits the ability of the rubber to biodegrade causing plastic pollution. Coatings according to some examples of the disclosure do not comprise vulcanised natural rubber (or vulcanised or cured rubber), i.e., coatings may be free from vulcanised natural rubber (or vulcanised or cured rubber). Vulcanisation is typically carried out using sulfur as curing agent and often with zinc oxide, or with peroxide as curing initiator agent. In examples of the disclosure, no sulfur or peroxide is added to the mixture. Accordingly, example coatings may be free from vulcanised natural rubber (or vulcanised or cured rubber). Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, the coatings and / or substrates may comprise additives and / or topcoats. In such examples, materials are selected such that sustainability and / or biodegradability is not compromised. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one” or by using “consisting”. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’ or ‘the’ with an exclusive meaning, then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon.

Claims

1. A coating for a substrate, wherein the coating comprises:a first biodegradable polymer, wherein the first biodegradable polymer is an elastomer;a second biodegradable polymer; anda compatibilizer, wherein the elastomer is the most abundant component of the coating.

2. A coating according to claim 1, wherein the elastomer comprises rubber.

3. A coating according to claim 2, wherein the rubber comprises natural rubber.

4. A coating according to any of the preceding claims, wherein the second biodegradable polymer comprises: poly lactic acid (PLA), cellulose acetate, polyvinyl alcohol (PVA), poly(butylene adipate co terephthalate) (PBAT), polybutylene succinate (PBSA), polyhydroxyalkanoates (PHA), cellulose, polysaccharide, a starch based polymer, or a casein polymer.

5. A coating according to any of the preceding claims, wherein the second biodegradable polymer comprises polylactic acid (PLA).

6. A coating according to any of the preceding claims, wherein the compatibilizer comprises clay, microfibrillated cellulose (MFC), or carbon fibres.

7. A coating according to any of the preceding claims, wherein the compatibilizer comprises clay.

8. A coating according to any of the preceding claims, wherein the elastomer has an elongation to break greater than the elongation to break of the second biodegradable polymer.

9. A coating according to claim 8, wherein the elastomer has an elongation to break greater than 300%.

10. A coating according to claim 8 or 9, wherein the second biodegradable polymer has an elongation to break less than 50%.

11. A coating according to any of the preceding claims, wherein the second biodegradable polymer has a hardness greater than the hardness of the elastomer.

12. A coating according to any of the preceding claims, wherein the coating does not comprise polyurethanes.

13. A coating according to any of the preceding claims, wherein the coating does not comprise vulcanised rubber.

14. A coating according to any of the preceding claims, wherein the coating comprises from 65 to 95 wt.% of the elastomer.

15. A coating according to any of the preceding claims, wherein the coating comprises from 1 to 29 wt.% of the second biodegradable polymer.

16. A coating according to any of the preceding claims, wherein the coating comprises from 1 to 6 wt.% of the compatibilizer.

17. A coating according to any of the preceding claims, wherein the coating is formed from a mixture comprising the elastomer, the second biodegradable polymer, and the compatibilizer.

18. A substrate having a coating, wherein the coating comprises:a first biodegradable polymer, wherein the first biodegradable polymer is an elastomer;a second biodegradable polymer; anda compatibilizer, wherein the elastomer is the most abundant component of the coating.

19. A substrate according to claim 18, wherein the substrate comprises leather fibres.

20. A substrate according to claim 18 or 19, wherein the substrate is a sheet material comprising leather fibres.

21. A substrate according to claim 19 or 20, wherein the leather fibres have a nonwoven arrangement.

22. A substrate according to any of claims 19 to 21, wherein the leather fibres are interlocked together by entanglement.

23. A substrate according to any of claims 19 to 22, wherein the leather fibres are derived from waste leather.

24. Clothing, footwear, accessories or upholstery comprising a substrate according to any of claims 19 to 23.

25. A method of coating a substrate, wherein the method comprises:mixing a first biodegradable polymer, a second biodegradable polymer, a compatibilizer, and water to provide an aqueous mixture, wherein the first biodegradable polymer is an elastomer;applying the aqueous mixture to the substrate; andforming a coating from the aqueous mixture, wherein the elastomer is the most abundant component of the coating.