Why tin plate is coated with a lacquer?

Recent searches on the query "Why tin plate is coated with a lacquer?" indicate that lacquer is essential in protecting tinplate and enhancing its suitability for packaging, especially for food and beverages. The lacquer provides an inert barrier, preventing any interaction between the metal and the contents while ensuring the safety and quality of the packaged items.

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 the ability to spread evenly over tinplate surfaces, which have received no special pre-treatment, and to adhere to the surfaces under potentially adverse conditions.

Durability Against Environment

For processed food cans, any external coating or decoration system must be capable of withstanding steam and water mixtures for periods of up to approximately one hour at temperatures ranging from 120 to 132°C. Historically, almost all lacquers have been solvent-based, containing up to 80% solvent in the wet state. Over recent years, many of these have transitioned to water-based materials, which contain 10 to 15% solvent, making them more environmentally friendly. A variety of lacquer types are now in use, and new formulations are continually being introduced.

Types of Lacquer Used

• Oleoresinous Lacquers

  The oleoresinous types are well-utilized in the U.S. for packaging fruits and vegetables. Being based on natural products, their composition may vary, but they are economical. However, they are not very effective against sulphide staining unless pigmented with zinc oxide.

• Vinylic Lacquers

  Vinylic types are vinyl copolymers reinforced with thermoplastic resins. They offer adherence, flexibility, and lack of taint, making them suitable for deep drawing applications. Vinylic lacquers are often used as second coats on beverages to ensure no odour is imparted.

• Phenolic Lacquers

  Phenolic resins are known for their good coverage and excellent chemical resistance. They are impermeable to sulphide ions; however, their adherence properties can be sensitive to surface conditions, and they are less flexible when applied in thicker coatings.

• Organosols

  Organosols consist of high molecular weight vinylic resins dispersed in solvent mixtures. These are often used within cans and for aerosol bases, providing an attractive ceramic appearance.

• Epoxy Phenolic Lacquers

  These lacquers are prevalent in the market due to their good resistance to acidic products and the combinations of resins that ensure adherence and flexibility. They are commonly used in cans for various food items.

Coating Processes

Except for D&I cans, lacquering and decorating of tinplate are primarily applied to flat sheets before forming operations. The application of coatings involves a roller system, followed by curing in a stoving oven at controlled temperatures. The drying and curing processes are essential since they must not exceed the melting point of tin, typically capped at 205°C.

Printing on Tinplate

Printing designs on tinplate is crucial for decorative purposes, improving the sales potential of packaged products, especially food and beverages. Although labels are traditionally applied, direct printing on cans has gained popularity for its aesthetic and functional advantages.

Conclusion

In summary, the lacquer coating of tinplate serves vital roles in protecting contents, ensuring food safety, and enhancing visual appeal, thus driving the success of packaged products.

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