Determination of Metal Adhesion Strength of Metallised Films by ...

Determination of Metal Adhesion Strength of Metallised Films by Peel Tests Esra Kucukpinar1, Marius Jesdinszki1, Norbert Rodler1, Carolin Struller1, Klaus Noller1, David Blondin2, Valerio Cassio2 and Horst-Christian Langowski1 1

Fraunhofer Institute for Process Engineering and Packaging, Giggenhauser Strasse 35, 85354

Freising, Germany 2

MET-LUX Vacuum Metallizing, B.P. 28, Site du P.E.D., L-4801 Rodange, Luxembourg

Abstract One major quality indicator of metallized films is the metal adhesion strength between the inorganic layer and the substrate. The weaknesses of ethylene-acrylic acid (EAA)-peel test for better adhering Aluminum layers are shown. A mechanically stronger film than EAA is required for more accurate measurements. There is still a need for further research on the improvement of available adhesion tests relevant to packaging and packaging materials. The packaging industry would then be able to prove superior adhesion and optimize their quality control.

Metallized (Vacuum Web Coated) Polymer Films for Packaging The metallized (vacuum web coated) polymer films are an important and steadily growing sector in the packaging industry. The worldwide metallized film production reached 2.4 x 1010 m2 / year and about 85% of the metallized films produced are used in flexible packaging [1]. The metallized films used for packaging applications are mostly used in laminate structures, which consist of a heat sealable polymeric top layer (e.g. poly(ethylene)), laminated to the metallic layer using a suitable adhesive (Figure 1). The metallization provides the polymeric substrate film several special functionalities at a higher productivity with low-cost production such as: 1) Oxygen and water vapor barrier, leading to a longer shelf life of the packed product, 2) Light barrier, 3) Decorative characteristics.

Sealable top layer (e.g. PE) Substrate film (e.g. PET)

Adhesive to metal layer Deposited metal layer (e.g. Aluminum)

Figure 1. Metallized film laminates for packaging or encapsulation

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One of the performance indicators of such metallized films is the adhesion strength between the metallized layer and the polymeric substrate film [2]. Delamination between the layers in a metalized film laminate structure might cause functional problems such as the reduction of the barrier performance. Therefore, in most cases, metal adhesion problems can be the main reason for disagreements between the customer and supplier in the process chain for production and usage of metallized films and laminates [3]. The metallized film producers and their customers need a reliable and an applicable method for the assessment of Aluminum metal adhesion before the subsequent conversion processes.

Peel Test Procedures for Metal Adhesion The peel test is widely used for the assessment of Aluminum metal adhesion on the polymeric substrate, mainly for the quantitative evaluation of the metal adhesion. In most cases, ethylene acrylic acid copolymer (EAA) film is heat sealed onto the metal layer and peeled off under defined conditions using a tensile testing device [4].

Laminated-film: EAA (used in peel tests) Metal layer Substrate film (e.g. PET) Peel direction Double-sided adhesive tape

Al-plate (1 mm thick)

Peel-off angle: . 180o Peel distance

Fixed in the bottom clamp Figure 2. Schematic representation of the 180o peel adhesion test [2] (edited)

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Figure 2 is a schematic representation of the 180o EAA peel adhesion test. It has to be stressed here that the peel strength measured by the peel test method is a combination of the basic adhesion and other additional factors [5, 6]. These other factors such as intrinsic stresses, the elastic and plastic deformations, the metallic layer width and thickness, the peel angle and rate, the failure mode during the peel test, etc. contribute to the final measured peel forces [5, 7, 8]. In addition, EAA film thickness and its thermal sealing conditions onto the metal layer have also a clear effect on the results. Therefore, the forces measured by peel tests are only comparable, when everything else is equal [9-11].

Weaknesses of EAA-peel Test One of the important weaknesses of EAA-peel test is that the heat laminated EAA film has a low cohesive strength. When a tensile force is applied on EAA, the measured tensile strength is low in both the elastic and the plastic regions. This low strength of EAA becomes a major problem, especially for the cases where the adhesion strength of the metal layer to the polymeric substrate is high. In such cases, the EAA film starts to overstretch during the peel test, and therefore the measured force is determined by the stretching action of the EAA.

The weaknesses of the EAA-peel test have been recently published by our group [2]. In this work, we used three different metallised PET films with metal adhesion strengths at different levels (i.e. low adhesion strength, medium adhesion strength, and high adhesion strength). Figure 3, 4 and 5 is for the sample with the lowest, medium and highest metal adhesion strength, respectively. The figures show the force recorded during the EAA-peel test versus measurement distance curves obtained for some representative test specimens of each sample.

Even if it is possible to compare the metal adhesion strengths of the three samples qualitatively (Sample A < Sample B