Improving coating resistance to acetic acid sterilisation: An EIS approach

In situ EIS spectroscopy was used to evaluate the performance of the inside coating of a deep-drawn polymer-coated steel can during sterilisation and pasteurisation with a 1% acetic acid solution. Acetic acid is a very common ingredient in filling goods and is a particularly aggressive substance for packaging materials. The in situ coating performance was correlated with polymer properties, like glass transition and thermal expansion.Three systems were tested: a PET coating, an improved PET coating, and a PP coating. The PET coating, which performed poorly and showed a relatively high postmortem corrosion rate of the substrate, showed a clear change in properties when heated above the polymer glass transition temperature (Tg) and subsequently cooled down below Tg. Above Tg the coating resistance decreased with temperature and corrosion products were formed beneath the coating. On subsequent cooling below Tg, the coating develops microscopic cracks, enabling transfer of ionic species to the solution.Based on these observations two improvements were made to the PET coating. In a first approach, the PET coating was modified by blending with PEN, a polymer which is structurally related to PET but with enhanced properties such as a higher Tg, higher strength and toughness and improved barrier properties. The addition of PEN is found to suppress micro-cracking phenomena during cooling after sterilisation and thus prevents ion transfer to the solution. In a second approach, PET is replaced by PP, a polymer that has a much lower Tg and is more hydrophobic than PET on account of its apolar and flexible chain structure. Using this coating, the formation of corrosion products under the coating decreased and ion transfer to the solution was prevented. A clear relation between coating composition, in situ EIS behaviour and postmortem observations is shown in all three cases.