Coatings are typically applied to the interior of metal food and beverage containers to prevent the contents from contacting the metal surface of the container. Contact with certain foods, particularly acidic products, can cause the metal container to corrode. This corrosion results in contamination and deterioration in the appearance and taste of the food or beverage product.
Many of the coating compositions for food and beverage containers are based on epoxy resins that are the polyglycidyl ethers of bisphenol A. Bisphenol A in packaging coatings can be either bisphenol A itself (BPA) or derivatives thereof, such as diglycidyl ethers of bisphenol A (BADGE) and epoxy novolak resins. These coatings have excellent flexibility and adhesion to metal substrates. However, bisphenol A and derivatives thereof are problematic. Although the balance of scientific evidence available to date indicates that small trace amounts of BPA or BADGE that might be released from existing coatings does not pose health risks to humans, these compounds are nevertheless perceived by some as being harmful to human health. Consequently, there is a strong desire to eliminate these compounds from coatings for food and beverage containers.
Replacement coating compositions are typically based on hydroxyl group-containing polymers and aminoplast or phenolplast curing agents. Although these coatings have good adhesion, they lack the flexibility required to prevent corrosion that occurs when the coating undergoes microcracking due to severe deformation of the metal. In 2-piece food cans, for example, microcracking occurs in the headspace due to the double-seam process that affixes the can end after filling. This is commonly known as “pressure ridge” failure. Corrosive foodstuffs infiltrate the coating through the microcracks and corrode the metal surface causing corrosion, delamination of the coating and eventual perforation of the can which spoils the food. Because of the susceptibility of acrylic-based coatings to pressure-ridge failure, they are currently inferior to industry standard BPA-containing coatings in the packaging coatings industry because they cannot hold strongly corrosive foodstuffs such as tomato-based foodstuffs.