| Popis: |
A beverage or food can is very much a functionalized product, the overall performance characteristics being achieved by the use of several materials each of which provides a specific property. Schematically, the metal substrate provides the mechanical and barrier properties, whereby the chemical resistance is provided by specific surface treatments to the metal surface and the application of a thin organic coating. Between about 4–15 μm in thickness, this organic coating has a double protective role, as it must protect the substrate from the foodstuff (corrosion) and the foodstuff from the substrate (taste..) over the required shelflife of the product. To give an idea of the industrial importance of this application, over 100 billion beverage cans per year are produced worldwide, each being individually sprayed with a protective organic layer. To perform correctly these coatings need to possess the following characteristics: —ability to be applied in thin, homogeneous layers without macroscopic or microscopic defects, —sufficient adhesion with the substrate and possess considerable interface stability —mechanical properties sufficient to withstand the can forming operations —intrinsic diffusion barrier properties necessary to prevent significant interaction with the substrate —sufficient chemical resistance to withstand any significant modification of the coating structure and hence intrinsic properties induced by the foodstuff Whereas a considerable amount of scientific attention has been applied to ‘‘bulk’’ systems, such as the mechanical properties of epoxies used for composite materials, diffusion in polymer packaging..., little published work is available concerning the specific properties of these thin coatings. The task is not helped by the commercial nature of the resin formulations used, the need to adapt these formulations to the multitude of industrial operations and the physical size of the coatings. The above coating properties will be discussed in relation to the ability to understand the underlying mechanisms involved, to measure the required properties and in the long term predict coating performance. |
