Retortable standup pouches PET/AL/PA/PP are usually laminated with 2-component polyester polyurethane adhesives.
During the retorting of the filled pouches, water vapour can pass through the plastic layers, but this normally has a positive impact on adhesion: the silanol groups of silane-functionalised adhesives, formed by hydrolysis react with the hydroxylated oxide surface progressively. Here the hydroxylation step of the oxide is substantial, because exclusively in this state the oxide reacts with the silanol groups and adhesion is improved. During retorting, however, bubbles may form between the aluminum foil and adhesive, and these are often accompanied by strong oxide growth.
During retorting, the water vapour and oxygen permeability through the plastics can increase by some decimal digits. If there are dry spots with no adhesion on the aluminum, because the foil packaging adhesive has been applied on the printing ink, oxide growth can start in this area neighbouring precipitations. Depending on the availability of electrolyte and oxygen, the bubble can expand and is usually accompanied and favoured by the formation of hydrogen.
