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Coatings are layers deposed in substrates with functional or decorative purpose. The application of organic coating on metallic substrates is a well consolidate technique that meets both protective and decorative needs. Decorative function responds to the needs of imprint logos and information on branded products and is demanded to coatings applied generally on external surfaces of containers. The inner side of those containers must, instead, possess protective properties ensuring protection of metal substrate from chemical attack by products contained, and vice versa, for the long storage periods. In any case also the coatings of external surfaces must own protective properties to guarantee resistance to environmental conditions20_.

Basic coating systems consist generally of two layers for the inner covering and up to six layers when applied for decorative purpose (e.g. in automotive industries)126_.

Coating formulation are generally solutions or dispersions of resinous materials made liquid by addition of appropriate solvents. Coating process needs so a step following the application of the resinous layer, in which it is dried buy solvent removal, oxidation and/or polymerization. When further coating layers are needed, each coat will be dried/heated before the application of the subsequent (except in cases in which the application wet on wet is specifically required 20_.

For the specific application in food can coating, precise properties are required: resistance to scratching, stream processes and fabrication conditions (e.g. welding). Moreover, they have to guarantee that no substance is extracted, above all toxic-ones, during cans lifecycle and does not have to affect flavor of products in them contained20_.

Canning food and beverages has the function of protect and preserve them taste and nutritional values for moths to year127_.

To can coatings are requested a variety of different technical and legal requirements127_:

o ideally, they should be flexible enough to avoid defections during fabrication, which would result in acid attack on exposed metal,

o be universally applicable to all different food types,

o resist a wide temperature and pressure range deriving by food processing, o do not transfer constituents to food in quantities that can affect human health o do not peel off during can production, shelf-life and after non- intentional

deformation of the cans

o resist to the chemistry of aggressive food types (e.g. acidic foods) and protect the metal of the can from corrosion

o preserve the flavor and appearance of food and maintain its organoleptic properties o be stable over several years

Basic methods used for the application coating on metallic surfaces are roller coating and spraying20_{. The majority of food and beverage cans, coatings are applied to}

both sides of planar metal sheets or coils by roller coating before the cans are formed. Spray coating are mainly applied in the inner side of cans127_{. Resins are generally applied as}

suspensions in organic or aqueous solvents, dried by solvent removal, oxidation or heat polymerization, which is usually achieved by heating and/or UV radiation. In some cases powder coatings are applied under the direction of an electrostatic field and cured by heat127_.

Coating composition depends also by food that will be contained in those. When acid colored food are wrapped more attention is posed on chemical resistance of coating that must protect the metallic substrate from reaction with food pigments (inducing both substrate corrosion and food color changes), more than resist to high mechanical and thermal treatments. Vegetables, instead, are generally made of sulphur-containing proteins that can migrate from food during hot packaging: sulphur reacts with substrate generating tin sulphide or iron sulphide layer, not toxic but affecting the appearance of the container. Opposite are operating conditions of meat-containers: strong sterilization processes are required so that coating must possess great thermal resistance20_{. In specific}

cases tin cans can be also produced without an internal organic layer; it is the case of tin cans containing light colored, acidic juices and fruits. Tin is more easily oxidized than the food, thus preventing darkening and flavor changes caused by oxidation of the food127_.

Early coatings were made oleoresins, basically blends of natural resins and gums with drying oils. Their use largely stopped with the invention of epoxy coatings but oleoresins were rediscovered as BPA-free alternatives even if their properties are not competitive. In facts, they do not adhere well to metallic substrates and even if can be easily applied, need long curing time, do not guarantee a good protection to metal corrosion and can change organoleptic properties of food127_.

Over years, new coating materials have been developed, with improved properties (but not ever suitable for food cans lining applications), differing for their main chemical composition , production processes, costs and properties.

Epoxy-based resins are the most commonly used class of coatings for aluminum and steel cans and 55% of epoxy resins production is intended for coating application. They protect the metal from corrosion, withstand a wide range of foods, resist to heat and acidic conditions, have good flexibility and adhesion properties. 127 _{Epoxy resins are usually more}

final cured resin. Processing epoxy resins is convenient since is possible to formulate compositions with the required rheological properties (such as low viscosity), that can be cured at room temperature as well as at elevated temperature, thanks to the wide range of possible hardeners. Moreover, no volatiles are produced during cure and resins show a low shrinkage 128

Vinyl coating, made by vinyl chloride and vinyl acetate, are really flexible but do not adhere on metals and are commonly utilized as top coat that is a second coating layer. Generally mixed with oleoresins, need the addition of stabilizers and plasticizes. They can endure to acidic environments but not to high temperatures

Alkyds are widely used for decorative applications thanks to low price and the easily modification for specific purposes; however they cannot be used in contact with food possessing themselves a taste that can interfere with food taste. 20

Phenolic and acrylic resins are really good in corrosion protection but possess very low adhesion properties, low flexibility and in contact with food can adulterate their organoleptic properties. Polyesters resins, obtained by condensation of polyacids with polyalcohol or epoxides, have, instead, good adhesion properties but do not resist to acidic conditions and have low resistance to corrosion.

Organic coating composition may be a complex formulation of a variety of components, each of them having a specific function. Generally each formulation is composed by three components, with a specific function, that are binders, particles and pigments. Binders are basically the resinous material, have the task of stick pigments and impart the chemical and physical properties of the coating. Additives modify the properties of coating in fluid states while pigments contribute to the color of the resin and can accomplish to further functions. 126