Evaluation of adhesion strength between thin aluminum layer and poly(ethylene terephthalate) substrate by peel tests - A practical approach for the packaging industry

Metallized films consisting of thin, vacuum-deposited inorganic layers are used for a wide range of packaging applications for foods, pharmaceuticals and other technical purposes. They are made as laminates and consist of a polymeric film (substrate), an inorganic layer, mostly aluminum (Al), and atop layer, laminated to the inorganic layer using a suitable adhesive. One major quality indicator in such flexible packaging materials is the adhesion strength between the inorganic layer and the substrate. In order to measure the adhesion strength of thin Al layers deposited on a substrate, the following procedure is often used: Ethylene acrylic acid (EAA)-films are thermally sealed to the Al layers. In a subsequent peel test, the EAA-film is peeled-off at 180 peel angle, delaminating the Al layer from the substrate. This method shows weaknesses in cases of high bond strength: The sealed EAA-film is elongated or even torn during the measurements, whereby it is difficult to obtain reproducible and repeatable results. In this study two alternative approaches have been tested to overcome the weaknesses of EAA-peel test. One of them is to use thermally sealable polymeric films, such as amorphous poly(ethylene terephthalate) and amorphous polyamide (both having a high mechanical strength), instead of the EAA film. Although the adhesion forces might have been weakened during the heat lamination of these selected films onto the Al surface, a quantitative comparison between the three different types of metallized films (with low, medium and high adhesion strength) is found to be promising by this approach. The other approach is to perform the peel tests with the laminates of the metallized films. The laminates are produced by laminating a low density polyethylene film (PE-LD) on top of the metallized film using an adhesive via a bench lamination process. The laminated PE-LD film in this case replaces the EAA-film. In this approach, the laminate structure is similar to the final product in the end-use. The metal adhesion strength is found to be in good agreement with the strength measured for similar structures produced at pilot scale.