The Influence of Carboxylic Acid in Packaging Materials

Abstract: Tetra Pak delivers food packing services world-wide and uses multilayer packaging materials to protect the food product from being affected by the environment. Loss of adhesion between the polymer and the aluminium layer in Tetra Paks packaging materials has proven to be a serious problem casing the package to lose its integrity and the food product to spoil. The problem is relatively unstudied but has shown to be connected to fatty and acidic products indicating a connection to the presence of carboxylic acids. This master thesis was conducted to identify the core mechanism of the delamination phenomenon and to create a base from which further studies can be conducted. Two basic hypothesis were provided by Tetra Pak for further study and evaluation. One is connected to the surface chemistry of the interface and the other is connected to changes in the mechanical properties of the polymers at the interface. A literature study was conducted and the work was delimited to the most basic carboxylic acid; acetic acid as well as the most commonly used polymer; low density polyethylene. The study indicated a connection between the presence of water and the delamination phenomena in agreement with the aluminium-water system. Three experiments where performed using two similar packaging materials produced at different times. These were exposed to food simulants containing different amounts of water to study the impact this would have on the delamination. The different experiments were then analysed using various methods to determine the mode of failure and connect these to the hypothesis. Results from peel testing showed a greater, or at least faster, loss in adhesion for the packaging materials that were exposed to food stimulants with high concentration of water. The FTIR measurements revealed interesting results for the simulant with average concentration of water showing a thin layer of polymer on the al surface at the highest peel force. This correlates with the hypothesis that the mechanics of the polymer would have changed. The FTIR measurements at the lowest peel force for the simulant with highest concentration of water however showed lower amounts of LDPE, supporting the theory that the delamination is due to surface chemistry. The FTIR findings were further supported by microscopic surface studies which showed a clearer fracture surface with less LDPE for the delaminated samples. The conclusion of this thesis shows that the setup used behaves according to the mechanism of the Al-water system which was strongly indicated by the formation of hydrogen gas and the lacking amount of polymer on the aluminium surface. The Al-water system in turn correlates with the hypothesis provided by Tetra Pak that the delamination phenomenon is due to surface chemistry.