Why tin plate is coated with a lacquer?

Tinplate Lacquering and decorating

Electrolytic tinplate containers are frequently lacquered to provide additional protection and/or to enable thinner tin coatings to be used.For internal can surfaces, a lacquer must provide an inert barrier between the metal of the container and its contents, and must not convey any odour or taint to the food and drink coming into contact with it. Important properties required in all lacquers include an ability to spread evenly, completely over tinplate surfaces, which have received no special pre-treatment, and to adhere to the surfaces under what may be adverse conditions.

For processed food cans, any external coating or decoration system must be capable of withstanding subjection to steam / water mixtures for periods up to approximately one hour at temperatures in the range 120 &#; 132°C . Historically virtually all lacquers have been solventbased, containing up to 80% solvent in the wet state. Over recent years, many of these have been replaced by water-based materials, which contain 10 &#; 15% solvent and are therefore more environmentally friendly. Many types of lacquer are now in general use and new or modified formulations are continually being introduced.

• Oleoresinous

Oleoresinous types are still much used in the U.S.A. for cans for packaging fruit and vegetables, but are not widely employed in Europe.They are based on natural products, so that the composition may vary somewhat, but they are cheap.They are not very effective against sulphide staining unless pigmented with zinc oxide. They dry by oxidation and by thermal polymerisation.

• Vinylic

Vinylic types are vinyl copolymers, reinforced by thermoplastic resins. Principal qualities include adherence, flexibility (useful in deep drawing applications) and lack of taint. This type is not very temperature resistant; it dries by evaporation of solvent.Vinylic lacquers are often used as second coats, for example on beer and beverage cans or biscuit tins, where lack of odour or taint is important. They cure at 170 &#; 175°C , to avoid thermal degradation in contact with steel at pores.

• Phenolic

Phenolic resins give good coverage and have excellent chemical resistance. They are impermeable to sulphide ions and will resist the heat of soldering. A limitation is the sensitivity of their adherence properties to the surface condition of the tinplate and their low flexibility in coatings thicker than 3-4µm. Phenolics cannot be heated above 190°C without degradation; drying occurs by heating. Vinyls and phenolics are often used in combination for optimum properties.

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  Organosols

Organosols are vinylic resins of high molecular weight dispersed in solvent mixtures, to give, typically, 55% dry solids. By fusion, at the time of heating the resins form a thick (10 &#; 20µm) continuous film. They are often used inside cans, for bases of aerosols or on beer cans or DRD cans. Acrylics are particularly used where high heat resistance and colour retention is needed. They are used for exterior decoration and increasingly in recent years as a white coating for food cans, where they give an attractive ceramic appearance.

• Epxoy phenolics lacquers

Epxoy phenolics lacquers are the most widely used types. They have good resistance to acid products and the epoxy component confers adherence and flexibility. They are cured at about 200°C . A wide range of formulations are commercially available, differing in ratios of the two resins.They are used in cans for packing soups, meats, fish, fruits and fruit juices, and internally and externally on crown corks and other closures. They are suitable for deep drawing. Certain formulations resist the action of polyphosphates which can reduce the adherence of phenolics, and they can be pigmented with aluminium or with zinc carbonate to counteract sulphide staining.

Other epoxy based materials have had a major impact in the market place in recent years. Concern over Volatile Organic C ompound (VOC ) emissions has been a driving force to reduce the level of organic solvents present in coatings systems and this has led to the introduction of water based epoxy acrylate emulsions which now play a major role within the beer and beverage market. More recent developments in terms of improved flexibility have extended the use of these materials into the food can market. Requirements for improved performance white internal coatings have led to the development of solvent based epoxy anhydride based systems.

However, recent concerns over the environmental impact of epoxy based systems, including epoxy phenolics, has led to pressures within the industry to review the use of bisphenol A and bisphenol F based materials. Although no definite evidence supporting these concerns has materialised, it is likely that polyester and acrylic based polymer systems may well become more widely used in the future as replacements for epoxy materials in certain applications.

Except for D&I cans, virtually all lacquering and decorating of tinplate is applied to flat sheets, prior to the forming operations. The coatings are applied via a roller system, as described below, and then enter a stoving oven. They are taken on a continuous conveyor which carries the sheets vertically on edge through a tunnel oven, which has a series of temperature controlled zones. The actual temperatures and total stoving times depend on the curing requirements of the particular coating.Temperatures must be below the melting point of tin, and for this reason a maximum of 205°C is normally never exceeded. Time at peak metal temperature is of the order of 10 minutes. At the exit of the stoving oven the cured sheets are stacked on pallets. They may require several stages of lacquering and/or decorating, necessitating several stoving treatments. Some plants have installed lines for continuous lacquering of tinplate coil: on these high-speed lines the melting point of tin may be momentarily exceeded.

The decoration of tinplate requires the printing of coloured designs on the surface.Tinplate has an excellent surface for printing and in this respect is superior to many high quality papers. However, there are certain fundamental differences between printing on tinplate and printing on paper. In the first place tinplate is non-absorbent, so that ink or lacquer remains on the surface and has to be dried by the application of heat or other form of energy.

Secondly, the metal has a hard inflexible surface compared to paper, so that direct contact between printing plate and the tinplate surface is not practicable. Thirdly, in terms of its spectral response, tinplate can be considered as being essentially black: this means that if reproduction similar to that achieved on white paper is required, a white base must first be applied to the metal surface. However, for special decorative effects, use may be made of the surface appearance of the tinplate itself. Another important fact to be borne in mind is the functional nature of the printed tinplate which is subsequently formed into various shapes, to be used as containers or other items. Coatings must remain adherent and undamaged even after forming operations which may involve quite severe deformation of the surface.

The tinplate surface makes possible a wide diversity of colour effects when used in conjunction with the range of opaque and transparent printing inks available and the printing process itself has evolved to allow the precise registration of fine detail at high line speeds.The decorating process consists of two operations; roller coating and printing. A number of coatings may be applied to tinplate before printing and a final overvarnish coating may also be applied after printing.

Printing on tinplate is not only for decorative purposes, but the creative and appealing design of a packaged product is vitally important for its sales potential and this is especially true for the large range of foods, beverages and household products marketed in cans. Although many food cans use separately applied paper labels, the use of labels printed directly onto the can is increasing and is already universal for beer and beverage cans. Printed tinplate has a variety of other uses including battery cases, trays, large container tins (for sweets, biscuits, etc.), bottle caps, ash-trays, and advertising signs.

• Source: The International Tin Association (formerly ITRI Ltd)

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